Heat Wave conditions over India

During hot weather period (March to July) surface temperatures over many parts of India abnormally shoot up, particularly over North and central India. Annual cycle of All-India daily maximum and minimum temperatures is as given in the below figure.

Figure 1: Annual cycle of All-India daily maximum and minimum temperatures.

As per the above figure, maximum temperatures are highest in pre-monsoon days and started decreasing in monsoon days.

As per India Meteorological Department’s criteria, Heat Wave conditions are defined as:

 Heat Wave:

It should be based on the actual maximum temperature of a station. Heat Wave is considered when maximum temperature of a station is 400C or more for plains and 30°C or less for Hilly regions. 

Based on Departure

Heat Wave:  Positive Departure from normal is 4.5°C to 6.4°C

Severe Heat Wave:  Positive Departure from normal is more than 6.4°C

Based on Actual Minimum Temperature    (For plain stations only)

Heat Wave:                                 When maximum temperature is ³45°C

Severe Heat Wave:                     When minimum temperature is ³47°C

Heat Wave conditions for coastal stations

When minimum temperature departure is +4.5°C or more over a station, “Heat Wave” may be described if the maximum temperature is 370C or more.

To declare heat wave, the following criteria should be met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it will be declared on the second day.  Forecasts of heat wave over a sub-division will be issued only if at least two stations in the sub-division are expected to experience such conditions.

  • Heat wave occurs mostly over an interior plain area when dry and warmer air is transported in a region with clear skies and hence maximum isolation during the summer season.
  • Bay islands, Lakshadweep, Tamil Nadu, Kerala, Coastal and South Interior Karnataka are not affected by heat waves due to the occurrence of maritime air over these regions.
  • Heat waves generally develop over Northwest India and spread gradually eastwards & southwards but not westwards (since the prevailing winds during the season are westerly to northwesterly). But on some occasions, heat wave may also develop over any region in situ under the favorable conditions.

Heat wave impacts rural and urban areas, natural habitats like forests, water resources, poultry and a range of sectors like agriculture, health, power etc, Though there is no universally acceptable or uniform definition for a heatwave, they are understood to be periods of abnormally high temperatures, more than the normal maximum temperature that occurs during the pre-monsoon (April to June) summer season. In India heat -waves typically occur between March to June, and in some rare cases even extend till July. Heat waves are more frequent over the Indo-Gangetic plains of India.

Health Impacts of Heat Waves

The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke. The signs and symptoms are as follows:

  • Heat Cramps: Ederna (swelling) and Syncope (fainting) generally accompanied by fever below 39°C i.e.102°F.
  • Heat Exhaustion: fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.
  • Heat Stroke: body temperatures of 40°C i.e. 104°F or more along with delirium, seizures or coma. This is a potentially fatal condition.

What causes a heat wave?

  • A heat wave is formed when static high pressure generated in the upper atmosphere over a region for several days. This static high pressure generates a hot mass of air, which is stagnant for many days, which resulted the trapping of more heat that also reduce the convection currents. The high pressure acts as a barrier and forces the mass of air to sink to the surface of the land that prevents heat to rise.
  • This hot mass of air accumulates only heat and humidity without any trace of precipitation that causes abnormally high temperatures. It is very often during the summer season, from March to July in the Indian Region.
  • Seasonal climatology map of number of HW days and SHW days over India during the hot weather season (March – July) for the period 1961-2010 is as in Figure 2.
Figure 2: Seasonal climatology map of number of HW days and SHW days over India during the hot weather season (March – July) for the period 1961-2010.

Monitoring and forecasting process:

IMD utilizes all its resources to monitor heat wave round the clock. It issues forecast& warning with a lead period of 05 days against the heat wave to the general public, disaster managers, media and other stake holders using following Impact Based Forecast table as per requirement of Disaster Management.

Figure 3: Impact Based Warning Forecast table followed by India Meteorological Department.

Compiled By: Surendra Pratap Singh, Naresh Kumar.

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