What is a heatwave?

About Heat wave:

• The definition of a heatwave depends on the physiography of regions.
• 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.
• The IMD will declare a heatwave:
  • if the maximum temperature recorded at a station is 40 degrees Celsius or more in the plains, 37 degrees Celsius or more in the coast, and 30 degrees Celsius or more in the hills.
  • a heatwave’s severity is determined by its departure from normal temperature.
  • there is a ‘normal heatwave’ when the departure is by 4.5-6.4 degrees Celsius and a ‘severe heatwave’ if the departure is greater.
• Heat wave declaration could also be based on actual maximum temperature:
  • A ‘heatwave’ is when this figure is greater than 45 degrees Celsius and a ‘severe heatwave’ when greater than 47 degrees Celsius.
  • The IMD takes these two ‘routes’ only when at least two stations in a meteorological subdivision report such a high maximum or when at least one station has recorded a corresponding departure from the normal for at least two consecutive days.
• Governments at various levels — State, district, and city — have prepared Heat Action Plans (HAPs).
• HAPs aim to increase preparedness and lower the adverse impacts of extreme heat by outlining strategies and measures to prepare for, address and recover from heatwaves. 
• The National Disaster Management Authority and IMD are reported to be working with 23 States to develop HAPs.
• There is no centralised database on HAPs, but at least 23 HAPs exist at the State and city level, with a few States, such as Odisha and Maharashtra, laying out district-level HAPs.