The ozone layer in the stratosphere is a natural protective layer that safeguards human health and the environment from the harmful ultraviolet radiation from Sun. Total column ozone defined as the total amount of ozone in a column extending vertically from the Earth’s surface to the top of the atmosphere is measured in Dobson Unit (DU). The ozone hole (defined as an area in which total column ozone amounts are less than 220 DU) occurs every year in Antarctica between the months of September and November, which is the South Pole’s springtime.
As winter arrives in each hemisphere, a vortex of winds develops around the poles and isolates the polar stratosphere. Without milder air flowing in from the lower latitudes and in the absence of sunlight, air within the vortex becomes very cold. The Antarctic continent is a very large land mass surrounded by oceans. This symmetrical condition produces very low stratospheric temperatures within a meteorologically isolated region called polar vortex. When temperatures drop below -78°C, clouds of ice, nitric acid, and sulphuric acid mixtures known as Polar Stratospheric Clouds (PSCs) form. These clouds provide surfaces that promote production of forms of chlorine and bromine that are chemically active and can rapidly destroy ozone. The conditions that maintain elevated levels of chemically active chlorine and bromine persist into September and October in Antarctica, when sunlight returns over the region to initiate ozone depletion. The destruction continues rapidly until the available ozone is depleted. As the atmosphere slowly warms in the spring, the vortex dissipates and warmer temperatures prevent further PSCs from being formed.
The winter meteorological conditions also lead to formation of Arctic Polar Vortex, just like the Antarctic Polar Vortex. However, the geographic symmetry about the North Pole is less than about the South Pole. As a result, large-scale weather systems disturb the wind flow, making it less stable over the Arctic region than over the Antarctic continent. These disturbances prevent the temperature in the Arctic stratosphere from being as cold as in the Antarctic stratosphere, and fewer polar stratospheric clouds are therefore formed. Nevertheless, chemically active chlorine and bromine compounds are also formed over the Arctic, as they are over Antarctica, from reactions at the surface of the clouds. But the cold conditions rarely persist into March, when sufficient sunlight is available to initiate large ozone depletion.
Smallest Ozone Hole over Antarctica in 2019 since its discovery
The unusual weather system in the upper atmosphere over Antarctica in 2019 restricted ozone depletion leading to the smallest mean ozone hole area since its discovery in 1985. The ozone hole reached its peak extent of 16. 4 million square kilometers on September 8 and then shrank to less than 10 million square kilometers during the remainder of September and October. The mean ozone hole area (for 07 Sep-13 Oct) was 9.3 million sq.km, lowest observed since 1985, the year when ozone hole was discovered. The observed mean minimum total ozone was 167 DU, the highest since 1985.
At an altitude of about 20 km, temperatures during September were 16oC warmer than average, the warmest in the 40-year historical record for September. The strength of the polar vortex normally determines the extent of ozone depletion. The “sudden stratospheric warming” events during 2019 weakened the Antarctic polar vortex which reduced the strong September jet stream around Antarctica from a mean speed of 260 kmph to a speed of 108 kmph. This slowing vortex rotation allowed air to sink in the lower stratosphere where ozone depletion occurs. The sinking motion warmed the Antarctic lower stratosphere, minimizing the formation and persistence of the polar stratospheric clouds that are a main ingredient in the ozone-destroying process. These weather system led to unusually higher than normal ozone levels over Antarctica compared to ozone hole conditions usually present since the 1985.
The 2019 ozone hole over Antarctic reached a peak extent of 16.4 million square km on Sept 8, 2019. The mean ozone hole area (for 07 Sep-13 Oct) was 9.3 million sq.km, lowest observed since 1985 the year when ozone hole was discovered. (Credit: NASA Goddard and NASA Ozone Watch)
Largest Arctic Ozone Depletion
The satellite data, showed significant and unusual ozone layer depletion in 2020 over North Pole. The lowest ozone values observed over the Arctic in March are at least 240 DU. But as a result of unusual atmospheric conditions, ozone concentrations above the Arctic reached a record low for the month of March, decreasing to 205 Dobson units. The last time similarly strong ozone depletion was observed over the Arctic was during spring 2011, and ozone depletion in 2020 was even stronger. The temperatures as cold as Antarctic are rare in the Arctic. However, this year, the extreme low temperatures in the North Pole trapped cold air in a ‘polar vortex’ within which high altitude polar stratospheric clouds formed, catalyzing the ozone destroying reactions leading to exceptional. This winter (2019-2020) the severity of the Arctic ozone depletion was also supported by unusually weak upper atmospheric wave events. These waves drive masses of air through the upper atmosphere travelling upward from the lower atmosphere in middle latitudes which disturb the vortex around the Arctic and bring ozone rich air from other parts of the stratosphere. The Northern Hemisphere polar vortex reaches an average size of about 14 million square kilometres in late January or early February. Throughout March it drops rapidly, and in an average year, it has disappeared altogether by April 1. This winter, it peaked at close to 19 million square kilometres in mid-February and remained at more than 15 million square kilometres through the middle of April. The ozone hole closed in April with an increase in stratospheric temperatures which culminated in an influx of ozone-rich air from the lower atmosphere. The exceptional ozone depletion in Arctic during 2020 shows that we have to remain observant and continuous monitoring is required.
Arctic stratospheric ozone reached its record low level of 205 DU on March 12, 2020. (Credit: NASA Goddard and NASA Ozone Watch)
The Ozone Layer Recovery
The 16 September is International Day for the Preservation of the Ozone Layer, commemorating the date, in 1987, on which the Montreal Protocol on Substances that Deplete the Ozone Layer was signed. This year, we celebrate 35 years of the Vienna Convention and 35 years of global ozone layer protection. The 2020 World Ozone Day theme and tagline is Ozone for Life – 35 Years of Ozone Layer Protection.
Through the Montreal Protocol, production of major ozone depleting substances (ODSs) has been ceased for developed countries in 1996, and for developing countries in 2010. Prospects for the long-term recovery of the ozone layer are good. It is anticipated that the recovery of the ozone layer will gradually diminish the large ozone holes that occur over both the Antarctic and Arctic. Scientific assessment of ozone depletion shows that actions taken under the Montreal Protocol have led to decreases in the atmospheric abundance of controlled ODSs and the start of the recovery of stratospheric ozone. The Antarctic ozone layer is recovering, although continues to occur every year. Outside the Polar Regions, upper stratospheric ozone has increased by 1–3% per decade since 2000. Northern Hemisphere mid-latitude total column ozone is expected to return to 1980 abundances in the 2030s, and Southern Hemisphere mid-latitude ozone to return around mid-century.
India Meteorological Department maintains observatories at Indian Antarctic stations Maitri and Bharati. Apart from weather observations, Ozone is being monitored continuously and data contributed to WMO for assessment of ozone variation.
(This blog has been written by Dr V. K. Soni, Scientist-F, Email: firstname.lastname@example.org)
The weather is one of the main factors affecting the air quality. Weather can help to clear away pollutants from atmosphere to improve air quality, or it can make air pollution extremely worse by helping to form pockets of highly polluted regions or dispersepollutants from source.Rain washes out water-soluble pollutant gases and particulate matter helping to improve the quality of the air. The rain can scavenge the air pollutants by the process called Wet deposition or ‘precipitation scavenging’. Wet deposition can further be divided in to Rainout and Washout.
Rainout is a process in which very small pollutant particles become nuclei for the formation of rain droplets that grow and eventually fall as precipitation, whereas washout is the collection of particulate matter by falling hydrometeors. For gases, the mechanism is the same in both processes: dissolution in liquid water droplets. Keep in mind that the effect of rain might be somewhat different in different locations as some cities will have a higher percentage of large particles or small particles. In general, the amount of the air pollutant removed by wet deposition process can be affected by the amount, duration and intensity of rainfall. Increased concentrations of pollutants in the atmosphere results in more wet deposition of pollutants, with negative effects on human health, crop yields, and land and marine ecosystems. Acid rain is an example of wet deposition of pollutants.
Observational studies by several researchers from different regions suggest that rainfall can effectively remove atmospheric particulate pollutants, andthe removal rate of PM10 is greater than the removal rate of PM2.5. CO and O3 are much less soluble than SO2 or NO2 and many other air pollutants. The reduction of SO2 may be more substantial during the occurrence of a rain event. Tropospheric O3 is produced during photochemical oxidation of volatile organic carbons and CO in the presence of NOx. However, the O3 concentration tends to increase somewhat under rainy conditions due to the vertical mixing of the stratospheric and tropospheric O3 concentrations during convective rain activity and thunderstorms. The enhancement ofNO2concentrations is observed probablydue to convection and lightning. Lightning-generated NO2at the surface can be suppressed by heavy rainfallbecause of its washout on NO2.
Keep looking our blog for more interesting topics on Understanding Weather & Air Quality.
Compiled by: Dr VK Soni, Scientist E, India Meteorological Department.
It is well known that thunderstorms are accompanied by lightning. However, the damaging cloud to ground lightning occurs at the beginning phase of a thunderstorm lifetime, that too for a short duration. This may occur for 30 minutes to 1 hour duration over a place. The thunderstorm and lightning warning system is an emerging science in the world. While there has been remarkable progress in recent years in the developing countries like USA, UK, Japan and Korea. India is one such developing country which has state of art thunderstorm and associated weather like wind, rainfall and lightning detection and warning system in recent years. India provides thunderstorm and associated weather forecast over a potential zone like meteorological sub-division and districts one to five days ahead. It further provides thunderstorm and lightning detection information at a place using lightning detection network blended with satellite and Doppler weather Radar observations every 10 minutes. It provides lightning warning at different locations (878 places) as well as districts (739 districts) every three hourly valid for next three hours using all the above observations and numerical high resolution models. Considering the short life of lightning, the lightning warning is communicated with fastest communication means like SMS, Mobile App, Whatsapp, Social media like Twitter and public media like TV and Radio.
The developed countries like USA provides forecast for thunderstorm and associated precipitation, high winds and tornadoes forecast 4-8 days ahead, 3 days ahead, 2 days ahead, 1 day ahead and upto 12 hours ahead in 4 hour slots. It does not provide lightning warning. Japan provides city specific thunderstorm forecasts for one week ahead and pictorial forecasts for thunderstorms upto 24 hours ahead at three hour intervals. It also provides nowcasts up to 60 minutes ahead based on Composite weather radar echoes. Korea also has the system to detect lightning with accuracy and efficiency. The detection data such as time of occurrence, location, intensity, and polarity of lightning strokes are processed into images to be distributed to the public on websites and mobile apps. The UK produces a routine delivered service upto six days, which blends observations and Atmospheric High Resolution model to provide thunderstorm forecast. It generates “Number of flashes of lightning seen per second at a point” at 15 minute intervals upto six hours.
Thus, While the National Meteorological Service of most countries do not issue lightning warnings, on account of short lifetimes, there are a few countries, who provide lightning strikes and their short range nowcast (about half an hour to 3 to 6 hrs ahead.
Numerical weather prediction (NWP) is a method of weather forecasting that employs a set of Hydro-dynamical equations that describe the motion in the atmosphere and oceans to predict the future weather condition based on current weather conditions.
Current weather observations serve as input (initial condition) to the NWP models through a process known as data assimilation to produce weather forecast of temperature, wind, precipitation, and other meteorological parameters.
What are the major steps involved in the NWP modelling system?
There are 3 steps involved in the NWP modelling system,
NWP model processing
In the pre-processing, observed data, available at unevenly spaced observing station points, are subjected to different quality control checks, followed by interpolation techniques to prepare initial conditionthrough a process known as data assimilation. Data assimilation is the science of combining different sources of information to estimate possible states of a system.
With the above initial conditions, the dynamical equations are numerically integrated forward with time, in the model. After integration, future values of model variables at different grid points are generated, known as direct/raw model output.
These above raw model output may be of very little use for the weather forecasters rather, forecasters may like to have specific weather information, like, rainfall, visibility, divergence, vorticity, precipitable water content , different convective indices, etc, which are prepared by post processing of raw model output.
What is the reliability of NWPmodels forecasts in short to medium range time scale?
Due to the advances in observational technology, telecommunication system, computing powers and understanding of different physical processes, there is much needed progress in the field of NWP and the reliability of numerical model forecasts is much higher compared to that of a few decades earlier.
What are the numerical weather prediction (NWP) models run in IMD?
IMD runs a global model, viz., Global Forecasting System (GFS) model, a regional model, viz., weather research and forecasting (WRF) model and an ensemble forecasting system, viz., Global Ensemble Forecasting System (GEFS).In addition to these dynamical models, IMD also runs a cyclone prediction model, viz., Hurricane weather research and forecasting (HWRF) during Cyclone periods.
How many times are the models run in IMD?
The GFS and WRF models are run four times a day based on observation of 0000, 0600, 1200 and 1800 UTC respectively. The GEFS model is run two times a day based on 0000 and 1200 UTC observations.
What is UTC?
The UTC stands for Universal Time Coordinate, previously known as Greenwich Mean Time (GMT). Indian Standard Time (IST) is UTC (GMT) plus 05 hours 30 minutes.
At what time the forecast products from these models available in IMD website based on above runs?
GFS model output is available approximately 4 hours after model run time. For 00 UTC run, products will be available at 0400 UTC. The GEFS model output of 00UTC run is available at 0630 UTC and 12UTC run is at 1830 UTC. WRF model output is available approximately at 0500 UTC for 00 UTC and at 1700 UTC for 12UTC.
Why is it delayed by about 4 hours 30 minutes in case of GFS model 10 day forecast; 5 hours 30 minutes in case of WRF model and GEFS model?
The observational data cutoff time for the generation of initial condition to GFS is 2 hours 45 minutes and the global data assimilation system (GDAS) takes around 45 minutes to generate initial condition. Finally GFS model takes around 7 minutes for one day forecasts. As such it takes 4 hours 30 minutes to complete 10 days forecast. In the GEFS model, there are 21 members, so it needs more nodes & computing time to complete the forecast. After completion of 3 days forecasts of GFS model, WRF analysis and 3 days forecast together takes around 1.30 minutes. Hence WRF model output is available after five hours 30 minutes of model run at 0000 UTC or 1200 UTC.
What are the weather forecast parameters generated from these models and available in IMD website?
The forecasts of surface wind, surface pressure, visibility, cloud cover, precipitation and wind, temperature, relative humidity (RH) at different heights of the atmosphere are generated and presented in map form (called as weather charts) in the website. In addition to these basic parameters, IMD also generates different diagnostic parameters and location specific forecast called Meteogram.
Why are the values in the maps/charts equally spaced? Is spacing between any two values remain same in any chart? Whether this spacing is same for all the models chart?
All the model charts are plotted with using square grid. The square grid is the most commonly used grid in the NWP models/charts over tropical regions. The resolution of the model is defined as the horizontal distance between the two adjacent grid points. The grid spacing is not same for all the model charts. The value of grid spacing is mentioned in the model charts for reference based on the resolution of the model.
What T1534 is as mentioned in GFS model?
GFS is a spectral global model. In spectral models, the horizontal resolution is designated by a “T” number. The ` T1534 ` indicates 1534 number of waves used by a spectral model. The T1534 mentioned in the GFS model indicates the horizontal resolution of GFS model. The approximate grid spacing can then be represented as
∆X=360°/ (3T+1) = 12 km in case of GFS model.
(“T “stands for triangular truncation ); for example: T254, T382, T574 and T1534, etc.
What the meanings of “based on — and valid for —-“in the NWP products.
In general, the “based on “in each NWP model charts indicates the time of model run i.e. initial condition of either 00 ,06,12 or 18 UTC and “ valid for” indicates the forecast validity time which is mentioned in the charts. For daily rainfall (24 hour accumulated), the validity period is always from 03 UTC (8.30 IST) of previous day to 03 UTC (8.30 IST) of that validity day.
In India, the rainfall accumulation is from 8.30 IST of the previous day to 8.30 IST of the validity day.
For Example, 72 hour rainfall (Day-3) forecast based on 00UTC of 04-07-2020 valid for 03UTC of 07-07-2020 means the 72 hour forecast is generated based on model run at 5.30 IST of 04-07-2020 and valid from 8.30 IST of 06-07-2020 to 8.30 IST of 07-07-2020.
Is there any difference between GFS/WRF Rainfall and Rainfall-India products quantitatively?
No, both are displaying the 24 hour accumulated rainfall in mm except at different domains and with metrological sub-divisions map in the latter case.
What do the symbols on the NWP model wind forecast charts indicate?
These symbols are called “wind barbs”, and they indicate both the wind speed and the wind direction (at different heights starting from at a height of 10m above sea level to 100 hPa (~at 15 km) in pressure levels. The wind direction is indicated by the angle of the barb, and the wind speed is represented by the number of lines on the tail. Each full line equals 10 knots (nautical mile per hour) a smaller half-line is 5 knots. A solid black triangle is equal to 50 knots. Every item should be added together to determine the speed of the wind.
These wind barbs show average wind speeds over the grid. Wind gusts may be up to 40% stronger.
What is kt? How does it differ from kmph?
KT means Knot. Knot is a unit of speed equal to one nautical mile per hour, exactly 1 knot equals to 1.852 km/h (approximately 0.514 m/s).
What is wind gust?
Gust is defined as, a sudden increase in wind speed above the average wind speed. More specifically, wind speed must temporarily peak above 16 knots (about 30 km per hour) after accelerating by at least 9–10 knots (about 17–19 km per hour) to qualify as a gust.
A gust is briefer than a squall and usually lasts 20 seconds or less. Air turbulence around an obstacle causes gusts; they occur frequently over buildings and irregular ground and are less frequent over water.
How do you estimate gustiness of wind from the model?
The gustiness is calculated by comparing the model output wind at each time step. The time step is the time duration for each computation on the model. It is uniform and fixed for a model run. For example, in case of GFS model, it is 450 seconds. Thus the gustiness reported in model output is the maximum 7- 8 minute average during forecast period. The forecasted values correspond to the maximum wind gusts at the given time.
What is average wind speed in the model? What is the averaging period?
The wind speed represented in the model output is average wind speed computed for a period of a few minutes. Hence while comparing the NWP model wind with the actual wind reported by a station, we should keep in mind that model wind is averaged over the period of a few minutes and over an area equal to size of the grid (12 x12 km in case of GFS model).
Hence it is not truly the gusty wind as defined in Q FAQ No. 13.
What is hPa? Why are the charts generated in hPa levels? How does it relate with the actual height of the atmosphere?
Here “hPa” stands for hecta-pascal. The pascal (symbol: Pa) is the SI unit of pressure. Common multiple units of the Pascal is the hecta-pascal (1 hPa =100 Pa), which was previously called as millibar.
Meteorological forecasts and observations report atmospheric pressure in hectopascal (hPa) as per the recommendation of the World Meteorological Organization (WMO).so, model analysis and forecast charts are generated in constant pressure level. The most common are the 1000 hPa, 925 hPa, 850 hPa, 700 hPa, 600 hPa,500 hPa, 400 hPa,300 hPa, 200 hPa and 100 hPa levels. These are called as standard isobaric levels of atmosphere. Every location on the charts has the same pressure, however, heights will vary.
The actual height of the atmosphere is calculated by using the hydrostatic equation. I.e. P = 𝝆 x g x h; where, P=Pressure; 𝝆=density of fluid; g=acceleration due to gravity; h=height from main sea level (MSL).
As we know that the density of air is heavier near earth’s surface. It becomes lighter as we go to higher altitudes. Thus, at lower altitudes, density being higher, atmospheric pressure is high. At higher altitudes, density being lower, atmospheric pressure is low. Thus, Atmospheric pressure decreases with altitude.It must be noted that atmospheric pressure is measured at the station level and converted in to mean sea level (MSL). This is due to the fact that, sea is at constant level throughout the earth’s surface and taken as a reference point.
What is total forecast period of different models? Why are they different?
The total forecast period of GFS/GEFS model is 10 days, while WRF model is only 3 days. The different total forecast period is due to its application in the forecast. GFS/GEFS is a hydrostatic global model meant for forecasting synoptic scale weather systems i.e. Western Disturbance, Monsoon Low Pressure system, etc.; whereas WRF is a meso-scale non-hydrostatic model meant for forecasting convective meso scale systems i.e. Thunderstorm, cloud burst, etc.
What is the time interval in forecast charts for a given run of the model?
The time interval in forecast charts of models are given below,
GFS model output is hourly up to 36 hours for some products used in aviation & power sector, 3 hourly up to 5 days for all the products and 6 hourly up to 10 days for main forecast (wind, rainfall) products.
GEFS model forecast products are available at 6 hourly intervals up to 10 days for probabilistic forecast (wind, rainfall, temperature, etc).
WRF model forecast is available at hourly up to 72 hours for some products used in aviation & power sectors, 3 hourly up to 3 days for all the products and 6 hourly up to 3 days for main forecasts (wind, rainfall, etc) products.
What is GPM height? How can it be interpreted?
The unit of the Geopotential height is the Geopotential meter (GPM). Geopotential height approximates the actual height of a pressure surface above mean sea-level.
With the analysis of Geopotential height chart, forecasters identifies the regions of high and low GPM height .High GPM region normally corresponds to fair weather and low GPM regions corresponds to adverse weather.
What are diagnostic products from GFS model, WRF model and GEFS model?
There are two kinds of NWP model products namely; (a) direct model products and (b) diagnostic/derived model products. Directly available as model outputs are called direct model products, they are wind, temperature, pressure, Geopotential-height and humidity. The outputs such as vorticity, divergence, vertical wind shear, moisture flux; CAPE, CINE, Thunderstorm indices, Lifted Index, etc. are called diagnostic/derived model products. These derived products from GFS and WRF models are used by the forecasters for diagnosing the weather systems predicted by the model.
What do you mean by ensemble prediction system?
As the atmosphere is a chaotic system, very small errors in its initial state can lead to large errors in the forecast. This means that we can never create a perfect forecast system because we can never observe every detail of the atmosphere’s initial state. Tiny errors in the initial state will be amplified, so there is always a limit to how far ahead we can predict any detail. To test how these small differences in the initial conditions may affect the outcome of the forecast, an ensemble system can be used to produce many forecasts.
Instead of running just a single model forecast, the computer model is run a number of times from slightly different initial conditions. The complete set of forecasts is referred to as the ensemble, and individual forecasts within it as ensemble members.
How many members are there in GEFS ensemble model?
The Global Ensemble Forecast System (GEFS) is a weather forecast model made up of 21 ensemble members (separate forecasts) to address the nature of uncertainty in weather observations.
What do you mean by control, ensemble mean, ensemble spread?
The mean forecast of 21 ensemble members is known as ensemble mean. The ensemble spread is a measure of the difference among the ensemble members and is represented by the standard deviation with respect to the ensemble mean (EM). The control is same as the deterministic model output. If the spread is more, the uncertainty in forecast is more and vice versa. If the difference between control and ensemble mean is more, the spread is more and hence uncertainty is more and vice versa.
How is probability rainfall calculated in GEFS model?
The probability of rainfall amount is typically derived by using forecast of 21 ensemble members. GEFS does not directly produce probability distribution of any variables for all possible events and it is calculated by using the simple probability of occurrence of the event at a grid point based on 21 forecasts from 21 ensemble members at that grid point.
What is the difference between rainfall forecast from GFS and GEFS?
The GEFS is a weather forecast model made up of 21 separate forecasts, or ensemble members. GEFS provide Ensemble mean (average of 21 members) and probability forecasts for an event from GEFS’s 21 ensemble members, whereas GFS provide only deterministic rainfall forecast (single value forecast)
What is the difference between GPM height based on GFS and GEFS?
GEFS provide Ensemble mean (average of 21 members) values of GPM height from GEFS’s 21 ensemble members along with ensemble spread, whereas GFS provide only single value deterministic GPM height forecast.
What is the difference in wind forecast by GFS and GEFS in different heights of atmosphere?
GEFS provide Ensemble mean (average of 21 members) values of wind along with ensemble spread (magnitude difference of wind speed among the ensemble members (21) with respect to and ensemble mean). GFS provide only single value deterministic wind forecast.
What does the shading indicates in different forecast products of GFS, WRF and GEFS?
The information about shading in all the forecast products of GFS, WRF and GEFS is mentioned in the legend of the respective charts. In the wind charts of GFS/WRF models, the shading indicates the magnitude of wind speed, whereas in GEFS model, the shading indicates the ensemble spread from ensemble mean position.
What do you mean by Isobars, Isotach, streamlines and contours?
Isobar is a line of constant pressure. Isobars are found only on surface charts. They connect lines of equal pressure in the units of hPa.
Isotach is a line of equal wind speed.
Streamlines are Lines of equal wind direction. Streamlines are used primarily in tropical regions since the pressure gradient is weak. They show areas of convergence, divergence and pressure circulation. The Geopotential height is normally displayed as contour a line of constant GPM values.
Whether forecast is available for any given location?
Meteogram is a graphical presentation of one or more meteorological variables with respect to time, whether observed or forecast, for a particular location. Where forecast data is used, the Meteogram will typically be generated directly from a weather forecasting model based on the longitude, latitude and elevation of the location, but it can also be corrected by a meteorologist.
In the Meteogram, time is plotted along the X axis, while the values of the different weather parameters are plotted along the Y axis. The most common weather parameters in a Meteogram are precipitation, temperature, air pressure, cloud cover, wind speed and wind direction. Meteogram based on GFS &WRF is available in the link.
This panel has a blue background to show the cloud-free areas. The panel is divided into three horizontal layers for the display of low, middle, and high cloud cover, which are drawn as white bars. If the white bar covers the full height of its layer, that is 100% cloudiness. The white bars have no gap between them to better simulate the appearance of cloudiness in the panel.
How can I get forecast for my city and town from IMD GFS model?
The GFS model based weather forecasts are available in the web site link http://nwp.imd.gov.in/gfs_meteograms.phpfor most of the city/town. If you don’t find your city/town in the above link provided and want to get forecast from GFS model, then you can contact IMD/GFS at. email@example.com
What is the all India monthly and seasonal rainfall?
All India monthly rainfall is the amount of accumulated rainfall received over India for a particular month. For example, All India monthly rainfall of June 2018 is 155.7mm. Similarly All India seasonal rainfall is the amount of accumulated rainfall received over India for a particular season. e.g. All India seasonal rainfall of South-West monsoon (JJAS) of 2018 is 804.1mm. These quantities are not constant; they vary from year to year.
What do we mean by long period average (LPA) of rainfall?
LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season)average over a long period like 30years, 50-years etc. It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. For example,LPA of south west monsoon rainfall over Kerala for the months June, July, August and September are 556mm, 659mm, 427mm and 252mm respectively. Current LPA of all India south west monsoon rainfallbased on the average rainfall over the period 1961 -2010 is 880.6mm.
What islarge excess, excess, normal, deficient, large deficient rainfall?
These are categories of rainfall used to describe realised rainfall averaged over various temporal scales like daily, weekly, monthly etcfor spatial scales like districts, states operationally. Accordingly, when the realised rainfall is ≥60%, 20% to 59%, -19% to +19%, -59% to -20%, -99% to -60% of long period average (LPA), the rainfall is categorized as large excess, excess, normal, deficient, large deficient respectively.
What is below normal, normal and above normal rainfall for the country as a whole?
If ‘m” is the mean and “d” is the standard deviation of a long time series of any climate variable like rainfall, temperature etc.Assuming the time series is normally distributed, 68% of the observations fall within +/- 1 standard deviation (d) from the mean (d). Therefore, when a realised value of the variable falls between m-d to m+d, it is categories as normal. When the realised value is <(m-d), it is categorised as below normal and when the realised value is > (m+d), it is categorised as above normal.
In case of monsoon season (June to September) rainfall over India as a whole, the long period average (LPA) is 88 cm and standard deviation is 9cm (about 10% of mean value).Therefore, when the rainfall averaged over the country as a whole is within ±10% from its long period average (LPA) or 90% to 110% of LPA, the rainfall is said to be “normal” and when the rainfall is <90% (>110%) of LPA, the rainfall is said to be “below (above) normal”.
What is the role of monsoon trough?
Monsoon Trough is an elongated low-pressure area which extends from heat low over Pakistan to Head Bay of Bengal. This is one of semi-permanent feature of monsoon circulation. Monsoon trough may be a characteristic of east west orientation of Himalayan ranges and north south orientation of Khasi-Jaintia Hills. Generally eastern side of monsoon trough oscillates, sometimes southwards and sometimes northwards.
Southward migration results in active/vigorous monsoon over major part of India. In contrast, the northward migration of this trough leads to break monsoon condition over major part of India and heavy rains along foothills of Himalayas and sometimes floods in Brahmaputra river.
What is heat low? What are its impacts on monsoon rainfall?
During the northward march of sun in northern hemisphere, the continent surrounding the Arabian Sea begin to receive large amounts of heat; not only in the form of radiationfrom sun, but also flux of heat from the earth’s surface into atmosphere (160 Watts/m2 for month of June over the arid zones of northwest India, Pakistan and middle eastern countries). As a result of this large input of power, trough of low pressure forms over this region. It is a semi-permanent feature of monsoon over India. The heat low is very shallow (extending up to 850 hPa(1.5 KM) leveland there exist a well-marked ridge above heat low. In spite of occurrence of cloudiness, the precipitation is very small. Intense heat low (pressure departure is below normal) acts as suction devise for moist air along the monsoon trough and to some extend related to good monsoon over India.During weak heat low (pressure departure is above normal) monsoon rainfall over India is greatly affected and results in deficient or scanty rainfall over vast area of country (eg. In 1987, central pressure over heat low area was mostly above normal, which proved to be drought year). Satellite measured estimates of longwave radiation indicates that tropical /subtropical deserts are heat sinks.
What is monsoon low, how does it influence monsoon?
An area with pressure at the centre lowest one, closed in shape with winds blowing around in anti-clockwise direction in Northern Hemisphere is Low Pressure Area (LPA). The LPA is associated with a whirling motion of air, convergence and upward motion of air. In the low usually clouds and rainfall are present. LPA which seen during monsoon is monsoon low.
The monsoon lows may be intensified into monsoon depressions. The monsoon lows and depressions are the principal rain bearing systems of the south west monsoon period over India.Substantial amounts of rainfall are generated by the westward passage of monsoon depressions forming in the Bay of Bengal. These are low pressure areas having wind speeds between 17 and 33 knots in their circulation.
On the average, 2 depressions form in each of the monsoon months (June-September). However, year to year variation in their number is quite large. Those that form in early June are responsible for the advance of the southwest monsoon, and are not strictly monsoon depressions. In July and August they usually form north of 18°N in the northwest Bay, and the site of genesis shifts in September southward in the Central Bay.
What is Tibetan High? How is it related with monsoon rainfall?
Tibetan High is a warm anticyclone (in this wind are changing in a clock-wise direction in the Northern Hemisphere and it will have always outflow of winds) located over Tibetan Plateau (centre latitude at 28ºN, longitude 98ºE) in the middle/upper troposphere during monsoon period. It is marked at 300 hPalevel with centre 30ºN, 90ºE and extends 70ºE-110ºE.The outflow of winds from Tibetan High as the easterly flow concentrates into jet stream centred near about the latitude of Chennai at 150 hPa in July. The jet stream runs from the east coast of Vietnam to the west coast of Africa. Thusthe location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall.Shifting its position east or west causes variation of monsoon activity over India. The Tibetan ‘High’ may sometimes shift much to the west of its usual position. In such a situation, the monsoon may extend further westward into Pakistan and in extreme cases into north Iran, though such a westward position of the Tibetan ‘High’ would be against its having origin in the heating effect ofthe Tibetan Plateau.
What is Mascarene high? How does it influence monsoon rainfall?
Mascarene High is a high-pressure area that is found around Mascarene Islands (in south Indian Ocean) during monsoon period. This is responsible for cross-equatorial flow through south Arabian Sea and it acts as southern hemispheric linkage. The variation in the intensity of High Pressure causes monsoon surges across equatorial flow. These surges are responsible for heavy rains along the west coast.
What is Somali jet?
Somali jet is low level (1 to 1.5 km asl) inter hemispheric cross equatorial flow of air, attains Jet speed at the west end of monsoon regime along the east coast of Africa. This Jet originates near Mauritius and northern part of Madagascar in the southern Hemisphere. This jet reaches Kenya coast (at about 3ºS) covers the plains of Kenya, Ethiopia and to Somali Coast at about 9ºN)
During May, it moves further into eastern Africa, then into Arabian sea and reaches west coast of India in June. It attains maximum strength in July. Short period (8-10 days) fluctuations are observed in Low Level Jet stream. Its strengthening gives rise to strong monsoon over peninsular India.
What is Tropical easterly Jet? How does it influence the rainfall?
South of the sub-tropical ridge over Asia, the easterly flow concentrates intojet stream centred near about the latitude of Chennai at 150 hPa in July. This is Tropical easterly jet. Thejet stream runs from the east coast of Vietnam to the west coast ofAfrica. Over Africa, the location is at 10° N. Normally, the jet is at an accelerating stage from the South China Sea to south India and decelerates thereafter.The location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall.TEJ weakens to less than 50 knots over India in September. During Breakmonsoon conditions TEJ moves northwards up to latitude 20ºN.
What is the difference between monsoon depression and depression forming in pre-monsoon season and post-monsoon season?
The depressions which form in the monsoon season are called the monsoon depressions. These are low pressure areas with two or three closed isobars (at 2 hPa interval), which cause most of monsoon rains. These can be of Bay origin, Land origin or Arabian Sea origin. Their shape is roughly elliptical and its horizontal extension is about 1000’s of Kms of surface. Its vertical extension is about 6-9 kms.
Monsoon depression is cold core system (central temperature colder than environment) over surface and in the lower levels and warm core in upper levels (central temperature warmer than environment). The Maximum wind strength and intensity can be noticed at the levels of 0.9km or 1.5 km. The monsoon depressions tilt southwards with height and if monsoon depression is moving westward, the heavy rainfall is mainly concentrated in the SW quadrant.Due to the high vertical wind shear present during South west monsoon season, monsoon depressions are generally do not intensify into cyclonic storms.
The depressions forming in pre-monsoon season and post-monsoon season intensify into a cyclonic storm.The average diameter of Post monsoon storms is about 1200 km whereas in pre-monsoon season it is about 800 km, however intensity does not depend on size. The cyclonic storm is a warm core phenomenon where the temperatures at the centre are warmer than the surrounding (areas) regions. The maximum warming occurs at the 300 hPa level.
Why don’t we get cyclones during main monsoon months like July and August?
Tropical cyclogenesis requires several favourable precursor environmental conditions. Warm Ocean waters (of at least 26.5℃ throughout sufficient depth at least on the order of 50 m). Relatively moist layers near the height of 5 km. Non-negligible amount of Coriolis force, pre-existing near surface disturbance. Low values of vertical wind shear between the surface and upper troposphere.
In July and August winds on the surface are westerly/south-westerly to the south of monsoon trough and south easterly/ easterly to its north and are generally stronger over the seas than the Land areas. The upper winds are westerly/south-westerly to the south and south easterly/ Easterly to the north of this trough region. Westerly winds increase with height and reach a maximum speed of 20-25 knots between 900 to 800 hPa levels. Easterly winds strengthen with height from 200 hPa reaching a maximum at 100 hPa. Speeds are between 60 to 80 knots over peninsula at 150 /100 hPa level or even at lower height (around 200 hPa) in the southern latitude. This results in high values of vertical wind shear which is unfavourable for Tropical cyclogenesis. So, we don’t get cyclones during main monsoon months like July and August.
What is off shore trough?
During monsoon season a shallow trough of low pressure is observed (on sea level surface chart) along west coast of India. This is known as off-shore trough.This type of system quite frequently develops off the west coast of India, anywhere from north Kerala to south Gujarat, during the period of southwest monsoon, and is responsible for the strengthening of the monsoon in terms of rainfall, in the adjacent parts of the coastal belt.
What is off shore vortex?
West coast of India has an orographic barrier in the form of Western Ghats. These mountains are oriented in north south direction and approximately 1000 km in length and 200 km in breadth. When monsoon winds strikes the mountains, on many occasions they do not have enough energy to climb over Western Ghats. On such occasions they tend to be deflected round the mountains and return current forms the off-shore vortex. These vortices are responsible for the occurrence of heavy to very heavy rainfall over west coast during monsoon season.
How does rainfall vary during monsoon season? Is there any periodicity.
During monsoon, considerable variability in rainfall is seen with space and time. The following are reasons which contribute to this.
Onset, Advancement and Withdrawal of monsoon. They decide the duration, period of monsoon current at different places.
Position of monsoon trough: It can oscillate 5º to north and 5º to south within 24 hours. If this trough is in south of normal position, strong monsoon conditions are observed over India. If this trough is in north of normal position or if it runs to foothills of Himalayas or not seen at all, then break monsoon conditions are observed. Synoptic systems like cyclonic circulations, lows, depressions move along trough and contribute to rainfall.
Formation and movement of synoptic systems and number of days of systems:
Westward propagating biweekly oscillation of 14 to 15 days.
What is synoptic mode of variation? How does it influence rainfall?
Low frequency oscillations considerably change the rainfall distribution over different parts of India. Synoptic mode variation has a periodicity of 3-7 days. It is mainly due to the formation of low pressure systems and its movements over Indian land mass. Under its influence, central Indian region receives good amount of rainfall.
Westward propagating biweekly oscillation of 14 to 15 days.Trough lines and low-pressure systems, ridges and high-pressure systems propagate in sequence from east to west with a periodicity of 2 weeks (14 to 15 days). This is known as Quasi-biweekly oscillation. When a trough or low-pressure area propagates on a particular area that area will experience an enhanced precipitation and when ridge or high-pressure passes over particular area, it will lead to suppressed rainfall or no rainfall over a particular area.
What is Madden Julian Oscillation? How does it influence rainfall?
The Madden Julian Oscillation (MJO) is one of the most important atmosphere-ocean coupled phenomena in the tropics, which has profound influence on Indian Summer Monsoon. The MJO is the leading mode of tropical intra-seasonal climate variability and is characterized by organization on a global spatial scale with a period typically ranging from 30-60 days, which was discovered by Madden and Julian in 1971 in a published paper. It has the following characteristics: –
MJO is a massive weather event consisting of deep convection coupled with atmospheric circulation, moving slowly eastward over the Indian and Pacific Oceans.
MJO is an equatorial traveling pattern of anomalous rainfall that is planetary in scale.
Each cycle lasts approximately 30–60 days. Also known as the 30-60 day oscillation, 30-60 day wave, or intra-seasonal oscillation (ISO).
The MJO involves variations in wind, sea surface temperature (SST), cloudiness, and rainfall.
Based on the place of convective activity the period of MJO is divided into 1-8 phases with each phase roughly last for 7 to 8 days. Since the MJO is the most important mode of tropical intra-seasonal variability with potentially important influences on monsoon activity in the Asian regions on extended range time scale (beyond 7 days to on1 month), the capability of statistical or numerical models in capturing MJO signal is very crucial in capturing the active/break cycle of monsoon.
How does monsoon vary from year to year? Is there any periodicity?
Year to year variation of monsoon rainfall over the large number of years is known as the interannual variability of monsoon. Periodicity of monsoon is largely controlled by the global ocean atmospheric phenomena like El nino Southern oscillation (ENSO).
What are the main factors governing interannual variation of south west monsoon?
Interannual variations are the variations on the annual cycle of the monsoon producing anomalously wet or dry years. The major factors governing interannual variation of south west monsoon are El ninoSouthern oscillation(ENSO) and Indian Ocean Dipole (IOD). Other contributing factors are North Atlantic Oscillation (NAO) and Pacific Decadal Oscillations (PDO).
How do we monitor monsoon?
Monsoon is being monitored by IMD using various techniques as given below.
Continuous monitoring of surface and upper air meteorological observations
Real time monitoring of the monsoon using remote sensing techniques like satellite and Radars.
Analysis of the different meteorological charts.
Guidance from various national and international weather forecasting models at different spatio-temporal scales.
What are the observational tools used for monitoring monsoon?
The observational tools used for monitoring monsoon are:
Synoptic observations of various meteorological parameters plotted on appropriate charts viz., mean sea level pressure chart, wind observations at constant pressure levels, geopotential heights, temperature, etc.
Auxiliary charts prepared out of derived parameters, for example, dew point temperature, pressure tendency, anomaly charts of pressure, maximum & minimum temperatures etc.
Various derived products from satellite observations, viz. Cloud Top Temperatures, Cloud Motion Vector (CMV) winds, water vapour derived winds, Outgoing Longwave Radiation (OLR), Quantitative Precipitation Estimates (QPE), divergence-convergence patterns of lower & upper levels, wind shear tendency, etc.
The AWS plotted charts and other products of numerical weather prediction models available on the ftp server.
Some more products of numerical weather prediction models available on Internet from IMD and NCRMWF and other worldwide centres such as UKMO, ECMWF, COLA, NOAA, NOGAPS, JTWC, etc.
Ships and buoy observations
Weather Radar and Doppler Weather Radar Observations.
Current Weather Observations (CWOs), Aircraft Reports (AIREPs)
How do we predict monsoon in different spatial and temporal scale?
Prediction of monsoon is done by IMD in different spatial and temporal scales. It varies from country as a whole to district wise in spatial scales and from a seasonal forecast to nowcast in temporal scale.
Seasonal forecast for monsoon rainfallissued based on long range forecast (LRF)in the month of April for the entire country. This forecast update in the month of May for the entire country and also for its broad homogenous regions. LRF of monsoon will give a general picture of seasonal and monthly rainfall for the coming monsoon season.
Extended range forecast of IMD issued in every Thursday of the week give forecast for a period extending from about 10days to 30 days in advance for the entire country. This forecast helps in the forecasting of active-break cycle of monsoon, formation of monsoon lows and depressions.
IMD issues short to medium range forecast for 5 days based on various national and international weather forecasting model guidance and expertise from the scientists. This forecast is being used by various stakeholders for planning their routine activities.
Different meteorological centresissuenowcastfor heavy rainfall during monsoon season with validity up to 6 hours.
What is long range forecast of monsoon?
As per the World Meteorological Organization (WMO) definition, long range forecast is defined as the forecast from 30 days’ up to one season’s description of averaged weather parameters. The monthly and seasonal forecast comes under long range forecast.
What is extended range forecast of monsoon?
Extended range Forecast is a forecast for a period extending from about 10days to 30 days in advance. With regard to the extended range time scale it is the time scales between medium range (About a week in the tropics) and seasonal scale and extends up to a period of one month. The extended range time scale of monsoon is also called the intra-seasonal time scale or active-break cycle of monsoon. The synoptic scales systems and other oscillations like Madden Julian Oscillation (MJO) influence the monsoon in this time scale. The monsoon forecast in this time scale is most difficult as it is neither a complete initial value problem (like the short to medium range prediction) nor a complete boundary value problem (like the seasonal forecast) but perhaps the most important of all time scales for economic and agricultural sectors. It makes difficult to predict in this time scale since the time scale is sufficiently long so that much of the memory of the atmospheric initial conditions is lost, and it is probably too short so that the variability of the ocean is not large enough.
What is forecast accuracy of ERF?
With regard to the skill of monsoon prediction in this time scale based on the operational coupled models runs at IMD it shows very useful guidance about the monsoon onset, monsoon withdrawal, active and break phases of monsoon and active-break transitions reasonably well. With regard to the quantitative verification, on average, it shows significant skill up to about three weeks for all India rainfall. Over the homogeneous regions of Indian it shows significant skill up to 2 to 3 weeks over Central India, monsoon zone of India & Northwest India. Over the south peninsula and northeast India it shows significant skill up to 2 weeks. On smaller spatial domain like meteorological subdivision level forecast skill it shows useful skill up to 2 weeks.
What is short to medium range forecast of monsoon?
Short range forecast is forecast valid for a period up to 3 days and the medium range forecast is valid for 3 to 10 days. This forecast is useful for planning agricultural activities, disaster management, town planning etc.
What is forecast accuracy of short to medium range forecast, especially heavy rainfall?
Accuracy of the short to medium range forecast is quite good in capturing the rainfall distribution, heavy rainfall events and formation of the synoptic systems up to 3-4 days in advance. Accuracy of the forecast decreasing beyond 5 days due to error propagation in initial condition.
What is SEFS?
SEFS stands for Statistical Ensemble Forecasting system. It is a statistical model used by IMD for the long range forecasting of South west monsoon season rainfall over the country as a whole. Forthisa set of 8 predictors (given in below Table) that having stable and strong physical linkage with the Indian south-west monsoon rainfall is used.
Details of the 8 predictors used for SEFS
Used for forecasts in
Correlation Coefficient (1981-2010)
Europe Land Surface Air Temperature Anomaly (January)
Equatorial Pacific Warm Water Volume Anomaly (February + March)
SST Gradient Between Northwest Pacific and Northwest Atlantic (December +January)
April and June
Equatorial SE India Ocean SST (FEB)
April and June
East Asia MSLP (FEB + MAR)
April and June
NINO 3.4 SST (MAM+(MAM-DJF) Tendency)
North Atlantic MSLP (MAY)
North Central Pacific Zonal Wind Gradient 850 hPa (MAY)
The geographical domains of the predictors are shown in the above figure. For the April SEFS, first 5 predictors listed in the Table are used. For June SEFS, the last 6 predictors are used that include 3 predictors used for April forecast. The standard errors of the 5–parameter and 6- parameter SEFSs were taken as ±5% and ±4% respectively. According to this forecasting system, the forecast for the seasonal rainfall over the country as a whole is computed as the ensemble average of best few models out of all possible models constructed using two statistical methods; multiple regression (MR) technique and projection pursuit regression (PPR) – a nonlinear regression technique. In each case, models were constructed using all possible combination of predictors. Using ‘n’ predictors, it is possible to create (2n-1) combination of the predictors and therefore as many number of models. Thus with 5 (6) predictors respectively for April (June) SEFS, it is possible to construct 31 (63) models.
What is MMCFS?
MMCFS stands for Monsoon Mission Coupled Forecasting System. It is a coupled dynamical model developed under Monsoon Mission Project. The original model frame work of CFS was developed by the National Centers for Environmental Prediction (NCEP), USA. This model was modified to provide better forecast over Indian monsoon region for different spatial and temporal resolutions through mission mode research work by the Indian Institute of Tropical Meteorology (IITM), Pune in collaboration with various climate research centers from India and abroad. The latest high-resolution research version of the coupled model has been implemented in High Performance Computer (HPC) at Indian Institute of Tropical Meteorology (IITM), Pune. IMD uses Monsoon Mission Climate Forecast System (MMCFS) model for preparing operational forecast of rainfall and temperature along with statistical models. More model details are available at http://imdpune.gov.in/Clim_Pred_LRF_New/Models.html
What is forecast accuracy of LRF for the Southwest Monsoon Seasonal Rainfall over the country a whole?
The LRF for all India season rainfall was reintroduced in 1988 using 16 parameter power regression and parametric models. IMD introduced new state of art LRF models during 2003 & 2007 following review of old forecasting system in 2002. The performance of the operational long range forecast for the season rainfall over the country as whole for the period 1988-2019 is shown in Figure below. During the period the absolute error was ≥ 10% of LPA in 7 years with highest in 1994 (21%) followed by 2002 (20%).
The average absolute error (difference between forecast and actual rainfall) during the last 13 years (2007 -2019) during which forecast was prepared using new Statistical Ensemble Forecasting system (SEFS)was 6.25% of LPA compared to the average absolute error of 8.91% of LPA during the 13 years (1995 -2006) just prior to that period. During 1994-2006, the forecast within the ±8% of actual values during 8 years. Within these 8 years, forecast was within ±4% during 3 years. On the other hand, during 2007-2019, the forecast was within the ±8% of actual values during 8 years with forecast within ±4% during 5 years.This clearly indicates improvement made in the operational forecast system in the recent 13 years period compared to earlier 13 years period.It is not possible to have 100% success for forecasts based on statistical models. The problems with statistical models are inherent in this approach and are being faced by forecasters worldwide.
What is Monsoon Mission?
Monsoon Mission is a national programme launched by Ministry of Earth Sciences (MoES) with a vision to develop state-of-the-art dynamical prediction system for the monsoon rainfall in different time scales. The mission supports focused research by national and international research groups with definitive objectives and deliverables to improve models in the short, medium, extended and seasonal range scales through setting up of a framework for generating dynamical forecasts and improving skill of forecasts. It also supports observational programs which will helps the better understanding of the atmospheric processes related to monsoon. The main objectives of this mission are
To improve Seasonal and Intra-seasonal Monsoon Forecast
To improve Medium Range Forecast.
Indian Institute of Tropical Meteorology (IITM), Pune, India Meteorological Department (IMD), Indian National Centre for Ocean Information Services (INCOIS), Hyderabad and National Centre for Medium Range Weather Forecast (NCMRWF), Noida are the major participating institutions in Monsoon Mission.
How do we declare onset and advance of monsoon?
The onset of the broad-scale monsoon occurs in many stages and represents a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.
At present IMD uses a new-criteria adopted in 2016 for declaring the onset of monsoon over Kerala which was based on the daily rainfall of 14 stations over Kerala and neighbouring area along with wind field and Outgoing Longwave Radiation (OLR) over southeast Arabian Sea. The new criteria emphasize on the sharp increase in rainfall over Kerala along with setting up of large-scale monsoon flow and extension of westerlies up to 600 hPa. However, IMD declares monsoon onset/progress dates for other regions operationally in a subjective manner considering the sharp increase in rainfall and its characteristic sustenance for a few days and associated changes in the atmospheric circulation features.
The guidelines to be followed for declaring the onset of monsoon over Kerala and its further advance over the country are enlisted below:
If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy,
Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery,Kannur, Kasargode and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the 2nd day, provided the following criteria are also in concurrence.
b) Wind field
Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN and Long. 55ºE to 80ºE. The zonal wind speed over the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/satellite derived winds.
c) Outgoing Longwave Radiation (OLR)
INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE. Further Advance of Monsoon over the Country a) Further advance be declared based on the occurrence of rainfall over parts/sectors of the sub-divisions and maintaining the spatial continuity of the northern limit of monsoon, further advance be declared.
The following auxiliary features may also be looked into:
b) Along the west coast, position of maximum cloud zone, as inferred from the satellite imageries may be considered.
c) The satellite water vapour imageries may be monitored to assess the extent of moisture incursion.
Northern Limit of Monsoon (NLM)
Southwest monsoon normally sets in over Kerala around 1st June. It advances northwards, usually in surges, and covers the entire country around 8th July(More details are available at http://www.imdpune.gov.in/Clim_Pred_LRF_New/Reports.html). The NLM is the northern most limit of monsoon up to which it has advanced on any given day.
How do we define withdrawal of monsoon?
Like the onset criteria, the criteria for the withdrawal of monsoon have also undergone changes. The current operational criteria used by IMD for declaring the withdrawal from extreme north-western parts of the country was adopted in 2006 andconsist of the following major synoptic features which will be considered only after 1st September,
i) Cessation of rainfall activity over the area for continuous 5 days.
ii) Establishment of anticyclone in the lower troposphere (850 hPa and below)
iii) Considerable reduction in moisture content as inferred from satellite water vapour imageries and tephigrams.
Further withdrawal from the country is declared keeping the spatial continuity, reduction in moisture as seen in the water vapour imageries and prevalence of dry weather for 5 days. SW monsoon is withdrawn from the southern peninsula and hence from the entire country around15thOctober, when the circulation pattern indicates a change over from the south-westerly wind regime.More details are available at http://www.imdpune.gov.in/Clim_Pred_LRF_New/Reports.html.
Is there any impact of climate change on monsoon?
Yes. Several studies have attributed the rising trend in the frequency and magnitude of the extreme rainfall events and decreasing trend in moderate rainfall events during monsoon season over central Indian region to climate change and natural variability.
What is its impact on monsoon rainfall over different parts of the country?
Based on recent studies it has been observed that the summer monsoon precipitation (June to September) over India has declined by around 6% from last fifty years, with notable decreases over the Indo-Gangetic Plains and the Western Ghats. It is also observed that there has been a shift in the recent period toward more frequent dry spells and more intense wet spells during the summer monsoon season. Over central India, the frequency of daily precipitation extremes with rainfall intensities exceeding 150 mm per day increased by about 75% during the recent decades.
How Climate Change does influences heavy rainfall activity?
Temperature of the earth is increasing rapidly due to anthropogenic greenhouse gas emissions. Thermodynamically, warm air holds more moisture as compare the dry air. According to Clausius-Clapeyron equation, the capacity of air to hold moisture increases by 7% for each degree of warming. Studies indicate that, in a changing climate, heavy rainfall events are expected to rise due to abundance of the moisture due to warming.
What is its impact on monsoonal low-pressure systems?
Several studies have shown significant decreasing trend in the frequency of Monsoon Depressions over the east coast of India in the recent decades. Some studies showed significant increasing trend in the frequency and duration of monsoon lows, whereas the number of lows intensifying into depressions are observed to be decreasing.
What are future projections on monsoon rainfall?
Simulations of Earth’s climate in future decades (typically until 2100) based on assumed scenarios of the concentrations of greenhouse gases, aerosols, and other atmospheric constituents that affect the planet’s radiative balance is called climate projections. Studies show that all-India summer monsoon mean rainfall is likely to increase moderately in future. It is also projected that the lengthening of the season due to late withdrawal. On interannual timescales, it is speculated that severity and frequency of both floods and droughts might increase noticeably in future climate.
What are the vagaries of monsoon?
Monsoon brings relief to dry and parched land in the form of rain, and affects Indian agriculture in a very substantial measure. The impact of monsoon on Indian economy is more pronounced. The Indian farmer has to put up a temperamental nature a many occasion in past. Excessive rain leads to floods in certain areas, while little or no rain in other parts bringing drought and famine resulting in acute distress to millions. Such fluctuations in rainfall have engaged the attention of our people and to make considerable effort to avert these calamities. There are many legends in our land of worship to the rain God for averting famines, and prayers are offered for pacifying the turbulent rivers of India. The Indian poets have sung about the rainy season in prose and verse.
How do we define floods and droughts?
A great flow of water, especially, a body of water rise in, swelling and over flowing land usually thus covered. Generally, flood occurs due to heavy rainfall in the catchment area but some time it occurs due to upstream discharge/ dam failure.
A flood that occurs in a short time (Usually less than six hours) of heavy or excessive rainfall, dam or levee failure is called flash flood.
Drought is the consequence of a natural reduction in the amount of precipitation over an extended period of time, usually a season or more in length, often associated with other climatic factors (viz. high temperatures, high winds and low relative humidity) that can aggravate the severity of the drought event.
There are four types of droughts:
According to India Meteorological Department, meteorological drought over an area is defined as a situationwhen the seasonal rainfall received over the area is less than 75% of its long term average value. It is further classified as “moderate drought” if the rainfall deficit is between 26-50% and “severe drought” when the deficit exceeds 50% of the normal.
Hydrological Drought can be defined as a period during which the stream flowsare inadequate to supply established use of water under a givenwater management system.
It occurs when available soil moisture is inadequate for healthy crop growth and cause extreme stress and wilting.
Abnormal water shortage affects all aspects of established economy of a region. This in turn adversely affects the social fabric of the society creating unemployment, migration, discontent and various other problems in the society.
Thus, meteorological, hydrological and agricultural drought often leads to what is termed as Socio-economic drought.
Which areas are mainly affected by heavy rainfall and dry spells?
Densely populated urban areas are mainly affected by the heavy rainfall due to chances of urban flooding. Land slide prone hilly areas are also affected by heavy rainfall. On the other hand rainfed areas of the agriculture sector are strongly affected by the dry spells.
Whether lightning occurs during monsoon season?
Lightening is mainly associated with the convective clouds. Monsoon clouds are mainly stratiform. Therefore, lightening generally do not occur during active phase of the monsoon season. However, the convective activity during break spell of the monsoon may lead to formation of convective clouds and hence the lightening.
How IMD doessupport flood management?
IMD provides real time rainfall situation and intensity as well as rainfall forecast for different temporal and spatial scales to support flood management.
What is cloud burst? Can it be predicted? Which area mainly gets cloud burst?
If 10 cm rainfall is received at a station in one hour, the rain event is termed as cloud burst. It is very difficult to predict the cloud bursts due to its very small scale in space and time. To monitor or nowcast (forecasting few hours lead time) the cloud burst, we need to have dense radar network over the cloud burst prone areas or one need to have a very high resolution weather forecasting models to resolve the scale of cloud burst. Cloud bursts do occur at plains, however, mountainous regions are more prone to cloud bursts due to orography.
What is break and active monsoon spells?
After southwest monsoon gets established over Central India in July, copious rainfall is received over large areas of the country with maximum over central India. During the peak monsoon rainfall months (July and August) of the season, the monsoon trough shifts north and south about its normal positioncausing large scale rainfall variation over the country both in terms of spatial and temporal scales. The intervals of drymonsoon conditions during which the large-scale rainfallover the monsoon trough zone (the zone between which themonsoon trough fluctuates north and south wards) is interruptedfor several days in July and August are known as thebreaks. On the other hand, the intervals between spells of dry monsoon conditions when the rainfall is higher than normal are known as active spells. Break in the monsoon rainfall was defined as the situations when the trough of low pressure was not seen on the surface chart and the easterlies were practically absent in the lower tropospheric levels up to about 1.5 km above sea level for more than 2 days.
What are the criteria used to declare active and weak monsoon condition?
Criterion for declaring active monsoon condition over a meteorological sub division is
Rainfall 1 ½ to4 times the normal.
The rainfall in at least two stations should be 5 cm, if that sub-division is along the west coast and 3 cm, if it is elsewhere.
Rainfall in that sub-division should be fairly widespread to widespread. (over the land area)
Wind speed is between 23 to 32 knots(over the Sea)
Criteria for declaring weak monsoon condition over a meteorological sub division is
Rainfall less than half the normal (over the land area)Wind speed upto 12 knots (over the Sea)
What is rainstorm?
Rainstorm is a storm characterised by substantial heavy rainfall. It is an extreme rainfall event experienced over a particular area for a particular period, in association with various weather systems of different spatial scales (Monsoon, Thunderstorms, cyclonic storm etc.)A rainstorm of any considerable duration typically consists of spurts of high-intensity rain punctuated by variable periods of low-intensity rain.Many times it has been observed that rainstorms lead to floods and landslides.
What are the special forecasts provided for agriculture during monsoon season?
India Meteorological Department (IMD) in active collaboration with ICAR, State Agricultural Universities and other institutes is rendering the weather forecast based Agromet Advisory Services (AAS), under GraminKrishiMausamSewa (GKMS) scheme, to the farmers at district level.
Under this scheme, medium range weather forecast at district level is generated for eight weather parameters, viz., rainfall, maximum temperature, minimum temperature, morning and evening relative humidity, wind speed, wind direction and cloud cover.Based on this forecast, Agromet Advisories are prepared by the Agromet Field Units (AMFUs) located at State Agricultural Universities, institutes of ICAR and IIT etc., in collaboration with State Departments of Agriculture and communicated to the farmers to take decision on day-to-day agricultural operations.
Based on the past weather conditions and regular extended range forecasts (ERF), Agromet advisories are being prepared and issued on every Friday by ICAR-CRIDA in collaboration with IMD.
In addition to above, IMD monitors weather aberrations and issues alerts and warnings to the farmers from time to time under GKMS scheme. SMS-based alerts and warning for extreme weather events like cyclone, floods, hailstorm, delayed arrival of monsoon, long dry spells etc. along with suitable remedial measures are issued to take timely operations by the farmers. Such alerts and warnings are also shared with State Department of Agriculture at State level and also with respective districts in various States for the effective management of calamity.
What are the forecast products provided for river in flood management?
In flood management India Meteorological Department is providing QuantitativePrecipitation Forecast (QPF) for river sub basins of India for Day 1, Day 2 and Day 3.IMD also monitoring rainfall and volume of water for cumulative period of 1-week for101 river sub basins of India and using Extended Range Forecast providing rainfalland volume of water for 101 river sub basins for week 1, week 2, week 3 and week4.
What does IMD do for monitoring and forecasting of urban flooding?
IMD is providing real-time rainfall situation and rainfall intensity with its highlydense AWS/ARG network at major urban cities. The AWS/ARG network is beingincreased to include more urban cities. Also with Doppler Weather Radar andnowcasting it is providing expected rainfall intensities and warning if any in the majorcities of India to avoid urban flooding. In addition to existing services on urban flooding, IMD is starting Impact based forecast (IBF) for major cities from monsoon-2020 onwards. However proper urban drainage system is thekey issue in urban flood management.
What is IFLOWS? How does it work?
IFLOWS is a monitoring and flood warning system that will be able to relay alerts of possible flood-prone areas anywhere between six to 72 hours in advance. The primary source for the system is the amount of rainfall, but the system also factors in tidal waves and storm tides for its flood assessments.
The system has provisions to capture the urban drainage within the city and predict the areas of flooding. The system comprises seven modules- Data Assimilation, Flood, Inundation, Vulnerability, Risk, Dissemination Module and Decision Support System.
The system incorporates weather models from the National Centre for Medium Range Weather Forecasting (NCMRWF), India Meteorological Department (IMD), field data from the rain gauge network of 165 stations set up by Indian Institute of Tropical Meteorology (IITM), BMC and IMD. It has been launched for Mumbai city on 12th June 2020.
What are the gap areas in monsoon monitoring and forecasting?
Monsoon forecasting was a grand challenging problem for a very long time and significant progress is made after Monsoon Mission program was launched by Ministry of Earth Sciences, Government of India. Forecasts have improved significantly in recent decade withlong lead times (short range forecasts -3 to 5 days, Extended range forecasts up to 3 weeks and long range forecasts 2 to 4 months lead time). Major gap areas of monsoon forecasting are
1. Systematic biases in mean state of monsoon in present day weather/climate models (Dry and cold bias in Coupled models and wet bias in atmospheric models).
2. If present day models get mean state correctly then they miss to capture reasonable interannual variability vice versa.
3. Interconnections of Indian summer monsoon and Indian Ocean SST is not correct in present day models.
Monitoring of boundary layer and upper air is very much essential to address the above gap areas and at present we have very few observations in the boundary layer and upper air. Enhancing the sampling in these areas over India will significantly reduce the systematic biases in the present day models.Extension and development of observational network will helpful for the better monitoring of monsoon especially for extreme rainfall events.
What are the science issues?
How and why the interannual variability of monsoon is controlled beyond El Nino and Indian Ocean Dipole (Equinoo)?
How to improve the synoptic variability (first building block of interannual variability of monsoon) in present day climate models?
What is required to represent clouds accurately in the present weather forecasting and climate models?
What is the future plan of IMD for monsoon monitoring and forecasting?
Operationalising the Impact Based Forecast (IBF) for major cities.
Exploring the use of Artificial Intelligence and Machine Learning techniques(AIML) for weather services during monsoon season.
Increasing the number of Automatic weather stations (AWS) and Automatic rain gauge stations (ARG).
Enhanced and sustained observations in the monsoon region particularly in the boundary layer and upper air.
Weather/climate models will be using very high resolution coupled models (Short range: 5km globally and 1 km locally; Extended and long range forecast: 25 km) with improved physics constrained by observations to improve the accuracy of weather/climate predictions including cyclones. The models also will employ Artificial Intelligence and Machine learning techniques to improve the accuracy of the forecasts.
Multi-model ensemble forecasting for the short to medium range using various models of the MoES institutions.
Monitoring of monsoon for South-Asia region.
Operationalisation of Multi Model Ensemble (MME) technique for seasonal prediction.
Development of integrated mobile app for weather information.
Expansion of the Doppler Weather radar (DWR) network.
Enhancement of the in-house research activities related to monsoon.
Observed rainfall is a very essential parameter for the analysis of rainfall, day to day weather forecast and its validation. The observed rainfall data is only available from five observatories of IMD; while no rainfall data is available at various important locations in and around Delhi-NCR. However, the 24-hour rainfall data observed by Doppler Weather Radar (DWR) for entire Delhi and surrounding region (up to 150 km) is readily available in a pictorial form.
A recent research conducted at India Meteorological Department aimed at estimating the rainfall at desired locations using hydrological products of the Doppler Weather Radar located at IMD Headquarters in Lodhi Road, New Delhi. The study was conducted by Dr. Kuldeep Srivastava (Scientist-E & Head, RWFC, N.Delhi) and Mr. Ashish Nigam (Scientific Assistant, RWFC, N.Delhi)
In this research, firstly, the rainfall at desired locations was estimated from the precipitation accumulation product (PAC) of the DWR using image processing in Python language. After this, a linear regression model using the least square method was developed in R language. Estimated and observed rainfall data of year 2018 (July, August and September) was used to train the model. After this, the model was tested on rainfall data of year 2019 (July, August and September) and validated. The paper has been published in the Journal of Atmospheric Science Research and can be found at the link: https://ojs.bilpublishing.com/index.php/jasr/article/view/1859
Using the above, 24-hour accumulated rainfall is being estimated at 35 different locations in and around Delhi-NCR (including Noida, Greater Noida, Ghaziabad, Meerut, Gurugram, Faridabad, etc.) and it is being updated on RMC New Delhi website from 1 June 2020 onwards, daily at 09:30 AM. (Website: http://rmcnewdelhi.imd.gov.in/)
These products are very useful for Weather Forecasters, Media & general public.
Rainfall recorded by observatory/AWS (in mm)
Rainfall estimated using the newly developed model (in mm)
Rainfall Comparison for 5th July 2020 between the observed rainfall and the rainfall estimated using the newly developed model
Precipitation Accumulation (PAC) Product, dated 5 July 2020, generated by the Doppler Weather Radar, situated at IMD HQ, Lodhi Road, New Delhi
Myth #1 – Lightning never strikes twice in the same place.
Fact: Lightning often strikes the same place repeatedly, especially if it’s a tall, pointy, isolated object. The Empire State Building was once used as a lightning laboratory because it is hit nearly 25 times per year, and has been known to have been hit up to a dozen times during a single storm.
Myth #2 – Lightning only strikes the tallest objects.
Fact: Lightning is indiscriminate and it can find you anywhere. Lightning may hit the ground instead of a tree, cars instead of nearby telephone poles, and parking lots instead of buildings.
Myth #3 – If you’re stuck in a thunderstorm, being under a tree is better than no shelter at all.
Fact: Sheltering under a tree is just about the worst thing you can do. If lightning does hit the tree, there’s the chance that a “ground charge” will spread out from the tree in all directions. Being underneath a tree is the second leading cause of lightning casualties.
Myth #4 – If you don’t see rain or clouds, you’re safe.
Fact: Lightning often strikes more than three miles from the thunderstorm, far outside the rain or even the thunderstorm cloud. Though infrequent, “bolts from the blue” have been known to strike areas as distant as 10 miles from their thunderstorm origins, where the skies appear clear.
Myth #5 – A car’s rubber tires will protect you from lightning
Fact: True, being in a car will likely protect you. But most vehicles are actually safe because the metal roof and sides divert lightning around you—the rubber tires have little to do with keeping you safe. Convertibles, motorcycles, bikes, open shelled outdoor recreation vehicles and cars with plastic or fiberglass shells offer no lightning protection at all.
Myth #6 – If you’re outside in a storm, lie flat on the ground.
Fact: Lying flat on the ground makes you more vulnerable to electrocution, not less. Lightning generates potentially deadly electrical currents along the ground in all directions—by lying down, you’re providing more potential points on your body to hit.
Myth #7 – If you touch a lightning victim, you’ll be electrocuted.
Fact: The human body doesn’t store electricity. It is perfectly safe to touch a lightning victim to give them first aid.
Myth #8 – Wearing metal on your body attracts lightning.
Fact: The presence of metal makes very little difference in determining where lightning will strike. Height, pointy shape and isolation are the dominant factors in whether lightning will strike an object (including you). However, touching or being near metal objects, such as a fence, can be unsafe when thunderstorms are nearby. If lightning does happen to hit one area of the fence—even a long distance away—the metal can conduct the electricity and electrocute you.
Myth #9 – A house will always keep you safe from lightning.
Fact: While a house is the safest place you can be during a storm, just going inside isn’t enough. You must avoid any conducting path leading outside, such as electrical appliances, wires, TV cables, plumbing, metal doors or metal window frames. Don’t stand near a window to watch the lightning. An inside room is generally safe, but a home equipped with a professionally installed lightning protection system is the safest shelter available.
Myth #10 – Surge suppressors can protect a home against lightning.
Fact: Surge arresters and suppressors are important components of a complete lightning protection system, but can do nothing to protect a structure against a direct lightning strike. These items must be installed in conjunction with a lightning protection system to provide whole house protection.
A joint training program webinar was conducted on 14 July 2020 (10:30 AM to 01:00 PM) by National Institute of Disaster Management (NIDM) and India Meteorological Department (IMD) to sensitize participants regarding assessment, forecast, preparedness and mitigation of Hydro meteorological Disasters in the form of Thunderstorms and lightning. The program was inaugurated by Shri Nityanand Rai (Hon. Minister of State, Ministry of Home Affairs, Government of India) in the presence of Dr. N. Rajeevan (Secretary Ministry of Earth Sciences), Dr. M. Mohapatra (Director General of Meteorology, India Meteorological Department) and Maj.Gen. N.K.Bindal (ED NIDM). Dr Thiruppugazh (Addl. Secretary, NDMA), Prof Surya Prakash (NIDM) and Dr S.Sen Roy (NWFC, IMD) were the resource persons for the meet.
The webinar also aims to highlight scientific and engineering endeavours aimed at addressing critical gaps in knowledge about these phenomena and understanding adverse effects of thunderstorms & lightning as well as available resources for timely response and recovery. Experts also believe that the severity and frequency of thunderstorm/dust storms are expected to rise in years ahead due to rising global temperature. The increase in occurrence and severity is a wake-up call for all agencies to take appropriate action for prevention, preparedness and mitigation in order to save lives, livestock, property and infrastructure. It becomes a challenge for disaster managers to take preventive and mitigation measures through preparation of action plan.
To prepare for a thunderstorm, you should do the following:
· Do remember that vivid and frequent lightning indicates the probability of a strong thunderstorm. · To begin preparing, you should build an emergency kit and make a family communications plan.
· Remove dead or rotting trees and branches that could fall and cause injury or damage during a severe thunderstorm.
· Postpone outdoor activities.
· Remember the 30/30 Lightning Safety Rule: Go indoors if, after seeing lightning, you cannot count to 30 before hearing thunder. Stay indoors for 30 minutes after hearing the last clap of thunder.
· Secure outdoor objects that could blow away or cause damage.
· Get inside a home, building, or hard top automobile (not a convertible). Although you may be injured if lightning strikes your car, you are much safer inside a vehicle than outside.
· Remember, rubber-soled shoes and rubber tires provide NO protection from lightning. However, the steel frame of a hard-topped vehicle provides increased protection if you are not touching metal.
· Shutter windows and secure outside doors. If shutters are not available, close window blinds, shades or curtains.
· Unplug any electronic equipment well before the storm arrives.
During Thunderstorms and Lightning
If thunderstorm and lightning are occurring in your area, you should:
· Avoid contact with corded phones and devices including those plugged into electric for recharging. Cordless and wireless phones not connected to wall outlets are OK to use.
· Avoid contact with electrical equipment or cords. Unplug appliances and other electrical items such as computers and turn off air conditioners. Power surges from lightning can cause serious damage.
· Avoid contact with plumbing. Do not wash your hands, do not take a shower, do not wash dishes, and do not do laundry. Plumbing and bathroom fixtures can conduct electricity.
· Stay away from windows and doors, and stay off porches.
· Do not lie on concrete floors and do not lean against concrete walls.
· Avoid natural lightning rods such as a tall, isolated tree in an open area.
· Avoid hilltops, open fields, the beach or a boat on the water.
· Take shelter in a sturdy building. Avoid isolated sheds or other small structures in open areas.
· Avoid contact with anything metal-tractors, farm equipment, motorcycles, and bicycles.
· If you are driving, try to safely exit the roadway and park. Stay in the vehicle until the Strong rain ends. Avoid touching metal or other surfaces that conduct electricity in and outside the vehicle.
After a Thunderstorm or Lightning Strike
If lightning strikes you or someone you know, call for medical assistance as soon as possible. The following are things you should check when you attempt to give aid to a victim of lightning:
·Breathing – if breathing has stopped, begin mouth-to-mouth resuscitation.
·Heartbeat – if the heart has stopped, administer CPR.
·Pulse – if the victim has a pulse and is breathing, look for other possible injuries. Check for burns where the lightning entered and left the body. Also be alert for nervous system damage, broken bones and loss of hearing and eyesight.
Rainbow is one of the most spectacular light shows observed on the earth. The formation of rainbow is as much as an optical phenomenon, as it is a meteorological phenomenon in the atmosphere. Generally, the rainbow can be observed during morning or evening hours, when the rain is falling on the one part of the sky & the sun is shining on the another. It causes due to the reflection and refraction of sunlight by the rain droplets suspended in the air.
The sunlight consists different wavelengths (380-750 nm) in the visible light corresponds to their colours: red, orange, yellow green, blue indigo & violet. The violet light has least wavelength (380-450nm) while, red light has longer wavelength (520-750 nm).
As shown in figure 1, when sunlight enters from air to denser medium water, a part of it gets reflected & remaining gets refracted or split into its component colors. Generally, the rays of light refracts twice within the droplet. The violet light of the ray (shorter wavelength) refracts the most and red light (longer wavelength) the least. Although most of this light passes right on through the drop and then reflect off the back of the drops. The ray bend again while emerging out the drops. During this optical process, each color light bends differently from others due to slight difference in wavelength and each color ray emerges from the drop at a slightly different angle. For red light, the angle is 42° from the beam of sunlight and for violet light, it is 40° (figure 1). The light leaving the drop dispersed into a spectrum of colors from red to violet and appears as a rainbow.
Shape of rainbow:
The shape & brightness of rainbow depends upon the size of water droplets and the relative location of sun & observer. Brightest rainbow can be seen when the water droplets are large, usually right after a rain shower. If the sun is very low in the sky, either just before sunset or just after sunrise, it can be seen as half circle. The higher the sun is in the sky, lesser the rainbow will be seen.
Generally, the actual shape of rainbow is circular, however it can be seen as half circle or a part of it on the earth surface. The only way to see the full circle of a rainbow in the sky from an airplane and have the sun behind the observer.
2. Double rainbow:
Sometimes, a larger secondary rainbow with its colors reversed can be seen above the primary rainbow. Usually this secondary bow is much dimmer than the primary one. It is caused when sunlight enters the raindrops at an angle that allows the light to make two internal reflections in each drop. Each reflection weakens the light intensity and results the dimmer secondary bow. The reversal of the color order causes the way the light emerges from each drop after going through two internal reflections. Double rainbow spotted in Jaipur on 7th June, 2020 just after the rain shower in the evening (Figure 2).