What is Heat Wave and its types?

Presently, North India is reeling under very hot weather. A common person hears the news of heat wave very often during the months from March to June in India. In this article we will discuss the exact definition of heat waves and the reason of heat waves.

What is Heat Wave?

We, often, think that heat wave has arrived when we start to feel hot and sweaty. However, it has proper definition. Indian Meteorological Department (IMD) has many criterions for declaring heat wave.

  • Firstly, the maximum temperature for the weather station should be 40°C or more and 30°C or more for the plains and hilly regions, respectively. Heat waves are divided into two types i.e. heat waves and severe heat waves. 
  • Secondly, the temperature should depart from the normal temperature by a certain limit. Please note that normal temperature is average of monthly temperature for the period of thirty years i.e. 1981-2010.
    • Heat Waves: When the Departure from normal temperature is 4.5°C to 6.4°C.
    • Severe Heat Waves: When the departure from the normal is more than 6.4°C.
  • Thirdly, the maximum temperature should be equal or greater than 45°C for heat waves and equal or greater than 47°C for severe heat waves.
  • Lastly, the coastal region has slightly different definition of heat waves. It is declared in coastal regions when the maximum temperature is equal or more than 37°C and the departure from normal is 4.5°C.

If the above criterions are met at least in 2 stations in a Meteorological sub-division for at least two consecutive days, then the IMD declares the heat wave on the second day.

Cause of Heat Wave

After 22nd December, the Sun starts to move towards Tropic of Cancer. As a result, the rays of Sun start to fall more vertically on the tropical and sub-tropical regions of north hemisphere. On 21st June, it is directly overhead on the Tropic of Cancer. Due to this movement of Sun, we experience summer in north hemisphere of Earth. The continuous influx of sunrays during the summer heats the land areas extremely which leads to accumulation of heat. As a result, we experience Heat Waves.

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