About heat waves:

• A Heat Wave is a period of abnormally high temperatures, more than the normal maximum temperature that usually occurs during the summer season in the North-Western parts of India.

When does it occur?

• 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 need not be considered till maximum temperature of a station reaches at least 40*C for Plains and at least 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.

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 potential fatal condition.

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.
• The longest recorded heat wave spell, in recent years, was between 18 – 31 May 2015. This spell had severely affected parts of West Bengal along with Odisha, Andhra Pradesh, and Telangana.

Why in the news?

The current heatwave spell commenced on May 22 and is likely to continue till May 29.

Does all of India experience heatwave conditions?

• Heatwaves 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.
• Several recent studies indicate that CHZ experience more than six heatwave days per year during these four months.
• Many places in the northwest and cities along the southeastern coast report eight heatwave days per season. 
• However, the regions in the extreme north, northeast and southwestern India are less prone to heatwaves.

Reason for unusual heat waves in India:

• Western disturbances

• Continuous inflow of Western Disturbances influenced the weather in the north till as late as April.
• When Western Disturbances interact with weather systems heading from the two southern seas, that is, warm winds blowing in from the Bay of Bengal or the Arabian Sea, they cause snowfall or rainfall over the north. 
• A significant influence of Western Disturbances is experienced during December to February. However, this year, its influence persisted till early May.
• The recent Western Disturbances got support from easterly winds blowing over from the Bay of Bengal. It resulted in rainfall and thunderstorm activities over parts of Rajasthan, Punjab, Uttar Pradesh, north Madhya Pradesh and Delhi until mid-May, keeping atmospheric conditions cooler than normal for summer standards.