Western disturbances are the weather systems which are seen as extra tropical upper air trough (extended low pressure area) or/and cyclonic circulations (CCs) (winds that circulates anti-clockwise around the low pressure area) in mid- latitude westerlies that move from west to east across Himalayan region. Under the influence of these systems, sometimes CCs develop south of the system at lower levels called induced CC. Sometimes the western disturbances can appear as a low pressure area under favourable condition. A low can sometimes (though rare) intensify into depression, which is called Western Depression.
These systems originate mostly over Mediterranean Sea & Caspian Sea. However some of the systems come from North Atlantic Ocean also. Thereafter, it moves eastward while causing precipitation over Iraq, Iran, Afghanistan and Pakistan. Subsequently, it causes weather in the form of rain/snow over Western Himalayan region (Jammu & Kashmir, Himachal Pradesh and Uttrakhand) and adjoining northern plains. If this system gets moisture from Arabian Sea, then it causes heavy precipitation over the region, otherwise it causes only light to moderate precipitation.
Sometimes, these systems do not completely weaken over Indian region and move east-northeastwards after getting moisture feed from Arabian Sea and cause precipitation over Nepal, Sikkim and northeastern states & adjoining East India. Sometimes, the WDs can cause rainfall even over Central & adjoining south Peninsular India. The typical example of WD, which causes intense precipitation over Western Himalayan region is given below in Figure 1.
In the above Figure 1, WD is seen as a cyclonic circulation over north Pakistan & neighbourhood along with a trough running from the centre of the circulation to Arabian Sea at mid-tropospheric levels. Sometimes under the influence of WD, a low or CC develops to the southeast sector of the main system at higher levels. Then, it is called as the ‘Induced low’ or ‘induced CC’. Most of the times, induced CC develop over West Rajasthan & neighbourhood at lower tropospheric levels and move mostly northeastward. In some cases, there is moisture feeding from the Arabian Sea also at lower tropospheric levels over northwest India due to induced CC, moderate to heavy rainfall also occurs over plains of northwest India as shown in following Figure 2.
In the figure 2, there is induced CC over central Pakistan adjoining areas of Punjab & West Rajasthan. Winds are from Arabian Sea indicating moisture feed from the sea. This is an ideal condition for intense rainfall over northwest India. Rainfall along with thunderstorm, lightning and hailstorm over central parts of country due to mixing of two air masses over Central India, one from Arabian Sea and other from Bay of Bengal due to anti-CC over Bay of Bengal.
In addition to above factors, the strength of Jet Stream (upper tropospheric narrow currents having width of a few hundred kms, length thousands of kms and wind speed>60 knot) at 200 hPa level plays very vital role in the intensity of the system. If there is strong Jet Stream wind of order 150 knots or more, it cause high divergence at higher altitudes, as a result system becomes more intense (Figure 3).
Typical example of high divergence due to strong jet stream wind is shown below in Figure 4.
Generally, high convergence at lower levels & high divergence at higher levels over Western Himalayan Region & adjoining plains are seen before commencement of widespread precipitation along with heavy falls.
Following meteorological features are seen before the approach of WD over a station, in general:
Fall of pressure (can be noticed by P24 P24 values).
Rise in minimum temperatures and the dew point.
Approaching cloud sequence: First High clouds, then medium and then low clouds and followed by rain/snow
Following meteorological features are observed after the passage of WD over a station:
Rise in pressure.
Fall in minimum temperature, sometimes leading to cold wave condition and fall in dew point indicates dry weather.
India Meteorological Department (IMD) is the National Meteorological Service of the country & the principal Government agency in all matters related to meteorological & allied disciplines. The Department provides weather & climate services for the Public safety and socio-economic benefits.
The vision of IMD is to provide efficient Weather and Climate Services for safety of life and property and to contribute towards the National development.
· To carry out meteorological observations for current weather information and to issue timely weather forecasts / meteorological information for optimum operation of weather sensitive sectors like agriculture, irrigation, shipping, aviation, offshore oil explorations, tourism, surface transport, power generation and distribution, etc.
· To warn against severe weather phenomena like tropical cyclones, thunderstorms, dust storms, wind squall, heavy rains, snow, cold and heat waves, etc., which cause destruction of life, property & livelihood.
· To issue Quantitative precipitation forecast for flood management to Central Water Commission and other agencies.
· To provide Climatological information for planning agriculture, water resource management, industries, oil exploration and other nation building activities.
· To conduct and promote research in Meteorology and allied disciplines.
Today, the 15th January 2020, when we enter into our 145th year of serving the Nation with weather & Climate information, thought it would be apt to look back a bit and reminisce on the evolution of IMD.
Long ago, in the year 1864, the city of Kolkata (then Calcutta) was devastated by a Cyclone, claiming innumerable lives. The cyclone had crossed this part of the east coast of India to the south of Hooghly River, around 10 AM of 5th October 1864. Huge Tidal waves washed away nearly everything in the City including the harbor (Gastrell & Blanford 1866).
The immediate 7 years that followed also witnessed two severe droughts & famines due to acute shortfall in the monsoon rainfall over India (Hunter, 1886).
These natural calamities led the Government to establish a central agency to organize the monitoring & forecasting of such meteorological & hydrological disasters for the benefit of the people.
India Meteorological Department was established in the year 1875 with its headquarters in Kolkata.
The headquarters was shifted to Shimla in 1905, to Pune in 1928, then to Delhi in 1944 and since then it is in Delhi.
The beginnings of meteorology in India can be traced to ancient times. Early philosophical writings of the 3000 B.C. era, such as the Upanishadas, contain serious discussion about the processes of cloud formation and rain and the seasonal cycles caused by the movement of earth round the sun. Kautilya’s (321 -296 BC) Arthashastra contains records of scientific measurements of rainfall and its application to the country’s revenue and relief work. Varahamihira’s classical work, the Brihat samhita, written around 500 A.D., provides clear evidence that a deep knowledge of atmospheric processes existed even in those times. It was understood that rains come from the sun (Adityat Jayate Vrishti – embedded in IMD’s emblem) and that good rainfall in the rainy season was the key to bountiful agriculture and food for the people. Kalidasa in his epic, ‘Meghdoot’, written around the seventh century, even mentions the date of onset of the monsoon over central India and traces the path of the monsoon clouds.
Meteorology, as we perceive it now, may be said to have had its firm scientific foundation in the 17th century after the invention of the thermometer and the barometer and the formulation of laws governing the behaviour of atmospheric gases. It was in 1636 that Halley, a British scientist, published his treatise on the Indian summer monsoon, which he attributed to a seasonal reversal of winds due to the differential heating of the Asian land mass and the Indian Ocean.
In India, we have some of the oldest meteorological observatories of the world. The British East India Company established several such stations, for example, those at Calcutta in 1785 and Madras (now Chennai) in 1796 for studying the weather and climate of India. The Asiatic Society of Bengal founded in 1784 in Calcutta, and in 1804 in Bombay (now Mumbai), promoted scientific studies in Meteorology in India. Captain Henry Piddington in Calcutta published 40 papers during 1835-1855 in the Journal of the Asiatic Society dealing with tropical storms and coined the word “cyclone”, meaning the coil of a snake. In 1842 he published his monumental work “The Sailors’ horn-book for the Law of Storms”. In the first half of the 19th century, several observatories began functioning in India under the provincial governments.
The above depicted ancient wisdom laid a strong foundation for the initial activities of IMD since its inception in 1875.
Evolution of different aspects of the Meteorological & Hydrological services in the early years in its finest details can be read at Hundred Years of Weather Service (1875-1975) view pdf
It may be noted that though the beginning was a humble one, it never fumbled in its duties.
India has some of the oldest Meteorological Observatories of the world. First Astronomical and Meteorological Unit was started at Madras in 1793.
In 1875 all Meteorological work in the country was brought under a central authority with the establishment of India Meteorological Department.
Advent of telegraphy enabled centralised data reception and publication of the All India Daily Weather Report since 1878. The first weather charts were printed in the Weather Reports in 1887.
Seismological activity started in India with the establishment of the first observatory at Alipore, Calcutta in 1888.
First Meteorological observatory in the country was established in Chennai in the year 1792.
Upper air measurements of winds started in 1905 by the method of tracking balloons with theodolites.
A separate division was created in 1932 for research activities in the field of Agricultural Meteorology.
RADARs were pressed into aviation weather service as early as 1954.
The positional Astronomy Centre- then known as Nautical Almanac Unit was established in 1955.
Environmental Meteorology took shape in India with the first Ozone measurement at Kodaikanal in 1957.
IMD started receiving satellite images from US Satellites in 1964.
Meteorological training facilities created in 1942 and upgraded to a Directorate in 1969 is now the Regional Meteorological Training Centre of the WMO.
Directorate of Telecommunication was set up in 1970 to rapidly exchange information amongst various centres.
The Telecom age ushered in the prospects of global data assimilation and numerical weather forecasting in 1973.
The National Data Centre at Pune was created in 1977 for scrutinising and archiving all Meteorological data in computerized form.
INSAT provided a Geostationary platform for remote sensing of the atmosphere and automatic data collection in 1982.
First Meteorological station at Antarctica was established in 1983 by IMD.
First Global Seismological Network (GSN) standard broad band seismological observatory was set up at Pune in 1996.
The Internet has opened avenues to provide better services. IMD created its official website in 2000.
125 Years of Service to the Nation (1875-2000) view pdf
In 2002 Doppler Weather Radars (DWR) were inducted in the cyclone detection network which enable precise estimate of intensity of cyclone.
Meteorological Data and INSAT Imagery were launched through World Space Digital Data Broadcast System in 2003.
From a modest beginning in 1875, IMD has progressively expanded its infrastructure for meteorological observations, communications, forecasting and weather services and it has achieved a parallel scientific growth. IMD has always used contemporary technology. In the telegraph age, it made extensive use of weather telegrams for collecting observational data and sending warnings. Later IMD became the first organisation in India to have a message switching computer for supporting its global data exchange. One of the first few electronic computers introduced in the country was provided to IMD for scientific applications in meteorology. India was the first developing country in the world to have its own geostationary satellite, INSAT, for continuous weather monitoring of this part of the globe and particularly for cyclone warning.
IMD has continuously ventured into new areas of application and service, and steadily built upon its infra-structure in its history of 145 years. It has simultaneously nurtured the growth of meteorology and atmospheric science in India. Today, meteorology in India is poised at the threshold of an exciting future.
Gastrell J E., Blanford, H.F, ‘Report on the Calcutta Cyclone of the 5th October 1864’,O.T. Cutter Military Orphan Press, 150 pp.
Sir William Wilson Hunter, ‘The Imperial Gazetteer of India’ 1886, vol.8, page 40.
Under the influence of the WD and its interaction with lower level easterlies, fairly widespread to widespread rain has been observed J&K, HP, Uttarakhand, Punjab, Haryana,Chandigarh & Delhi, Uttar Pradesh and north Rajasthan during last 24 hours. The WD has moved further eastwards. No rain is likely over Punjab, Haryana, Delhi and West Uttar Pradesh during next 3 days.
Now only isolated to scattered rain with thunderstorm & lightning is likely over Bihar, Jharkhand, Odisha, GWB, Assam & Meghalaya and Arunachal Pradesh during next 24 hours.