Heatwave alerts

• India Meteorological Department (IMD) has issued an orange alert over high temperatures in 3 states (West Bengal, Bihar and Andhra Pradesh) and claimed that heatwave conditions are to prevail in 4 other states (Sikkim, Jharkhand, Odisha and Uttar Pradesh).
• Heatwave is a condition of air temperature which becomes fatal to human body when exposed.
• It is defined based on temperature thresholds over a region in terms of actual temperature or its departure from normal.
• Reasons for Heatwaves: Prevalence of hot dry air over a region, Absence of moisture in upper atmosphere, Cloudless Sky, Anti-cyclonic conditions.

Criteria of declaring Heatwaves

• IMD has given following criteria for Heatwave –
• 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 temperature,

• Heat Wave -Departure from normal is 4.50°C to 6.40°C.
• Severe Heat Wave: Departure from normal is >6.40 degree C.

Based on Actual Maximum Temperature:

• Heat Wave: When actual maximum temperature ≥ 45 degree C.
• Severe Heat Wave: When actual maximum temperature ≥47 degree C
• If the above criteria is met at least in 2 stations in a Meteorological subdivision for at least two consecutive days and is declared on the second day.