GS 3: Environment -Environmental impact assessment

Why in News?

Severe heat wave conditions will continue in many parts over the next couple of days, especially in Rajasthan and several parts of north and central India.

What is Heat Wave?

  • A Heat Wave is a period of abnormally high temperatures, more than the normal maximum temperature that occurs during the summer season in the North-Western parts of India.
  • Heat Waves typically occur between March and June, and in some rare cases even extend till July. The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death.

The Indian Meteorological Department (IMD) criteria for Heat Waves:

  • Heat Wave need not be considered till maximum temperature of a station reaches atleast 40*C for Plains and atleast 30*C for Hilly regions
  • When normal maximum temperature of a station is less than or equal to 40*C :-Heat Wave Departure from normal is 5*C to 6*C, Severe Heat Wave Departure from normal is 7*C or more
  • When normal maximum temperature of a station is more than 40*C :-Heat Wave Departure from normal is 4*C to 5*C Severe Heat Wave Departure from normal is 6*C or more
  • When actual maximum temperature remains 45*C or more irrespective of normal maximum temperature, heat waves should be declared.
  • Higher daily peak temperatures and longer, more intense heat waves are becomingly increasingly frequent globally due to climate change.
  • India too is feeling the impact of climate change in terms of increased instances of heat waves which are more intense in nature with each passing year, and have a devastating impact on human health thereby increasing the number of heat wave casualties.


A heat wave is caused by a high pressure system that hovers over an area. It traps heat beneath it like an oven. High-pressure systems force air downward. Hot air on the ground cannot escape into higher levels. Without rising air, there are no rain or clouds. The sun just bakes the area until a new pressure system is strong enough to push the high-pressure system away.


  • Heat stress causes dehydration and loss of body salt.
  • It can also lead to failure in people with heart conditions.
  • When the core body temperature rises beyond 104 F, organs fail. The gut leaks toxins into the body, creating an inflammatory response called heat stroke.

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