About IMD

• The Indian Meteorological Department (IMD) was established in 1875. 
• It functions under the  Ministry of Earth Sciences (MoES). 
• It is the National Meteorological Service of the country and the principal government agency in all  matters relating to meteorology and allied subjects. 

Why in News?

• The Indian Meteorological Department (IMD) has issued a red alert for severe heatwave conditions over the plains of Northwest, East, and Central India for the next four days. 

About heat waves

• Qualitatively, heat wave is a condition of air temperature which becomes fatal to human body when exposed. Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal. 
• Heat Waves typically occur between March and June, and in some rare cases even extend till July. 

Criteria for heat waves

• • The Indian Meteorological Department (IMD) has given the following criteria for Heat Waves :
    • Heat wave is considered if the maximum temperature of a station reaches at least 40°C or more for Plains and at least 30°C or more for Hilly regions;

• Based on Departure from Normal

• • • • Heat Wave: Departure from normal is 4.5°C to 6.4°C
      • Severe Heat Wave: Departure from normal is >6.4°C

• Based on Actual Maximum Temperature

• • • Heat Wave: When actual maximum temperature ≥ 45°C
    • Severe Heat Wave: When actual maximum temperature ≥47°C
  • For coastal regions, when maximum temperature departure is 4.5°C or more from normal, Heat Wave may be described provided the actual maximum temperature is 37°C or more.

• Higher daily peak temperatures and longer, more intense heat waves are becoming 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.

Duration of a heatwave spell

• A heatwave spell generally lasts for a minimum of four days and on some occasions, it can extend up to seven or ten days. 

Impacts

• The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke. The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death.
• Heatwaves can also increase strain on water, energy and transportation resulting in power shortages or even blackouts. Food and livelihood security may also be strained if people lose their crops or livestock due to extreme heat.
• Multiple areas of the economic sector experience reduced worker productivity during heatwaves, especially agriculture and construction. 
• Heatwaves, without concomitant increases in precipitation, can lead to water shortages and increased stress for plants, particularly in arid regions. This has the effect of reducing plant growth, the basis of energy production and the food chain, with an overall drying-out of the landscape.

Vulnerable Regions

• Heat waves are common over the Core Heatwave Zone (CHZ) — Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana, as categorised by India Meteorological Department.
• The regions in the extreme north, northeast and southwestern India are less prone to heatwaves.

https://www.newsonair.gov.in/imd-issues-red-alert-for-severe-heatwave-conditions-over-plains-of-northwest-east-central-india/