Why in news:

  • The India Meteorological Department (IMD) has declared a heatwave in various regions of Maharashtra, including Mumbai, Palghar, Thane, Konkan, central Maharashtra and Vidarbha.
  • This is the fourth heatwave alert for the Konkan region and the first in May, the IMD said.

What is a heatwave?

  • Heat wave is a condition of air temperature which becomes fatal to human body when exposed.
  • It is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
  • In certain countries it is defined in term of the heat index based on temperature and humidity or based on extreme percentile of the temperatures.
  • In India Heatwaves occurs mainly during March to June and in some rare cases even in July.
  • The peak month of the heat wave over India is May.

What are the heat wave prone states over India?

  • Heat wave generally occurs over plains of northwest India, Central, East & north Peninsular India during March to June.
  • It covers Punjab, Haryana, Delhi, Uttar Pradesh, Bihar, Jharkhand, West Bengal, Odisha, Madhya Pradesh, Rajasthan, Gujarat, parts of Maharashtra & Karnataka, Andhra Pradesh and Telengana.
  • Sometimes it occurs over Tamilnadu & Kerala also.
  • Heat waves adversely affect human and animal lives.
  • However, maximum temperatures more than 45°C observed mainly over Rajasthan and Vidarbha region in month of May.

What is the temporal range of temperature forecast issued by IMD?

  • IMD issues temperature forecast & warning in following range:

a) Short to medium range (lead time/validity of 1 to 5 days)

b) Extended range (lead time/validity upto 4 weeks)

c) Seasonal range (lead time/validity upto 3 months)

What is criterion for declaring heat wave?

  • Heat wave is considered if maximum temperature of a station reaches at least 40 C or more for Plains and at least 30 C or more for Hilly regions.

          a) Based on Departure from Normal Heat Wave:

Departure from normal is 4.50C to 6.40C Severe Heat Wave: Departure from normal is >6.40C

         b) Based on Actual Maximum Temperature Heat Wave:

When actual maximum temperature ≥ 45 C Severe Heat Wave: When actual maximum temperature ≥47 C

If above criteria met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it declared on the second day.

What is a criterion for describing Heat Wave for coastal stations?

  • When maximum temperature departure is 4.50C or more from normal, Heat Wave may be described provided actual maximum temperature is 37 C or more.

What is warm night?

  • It is considered only when maximum temperature remains 40 C or more.
  • It is defined based on departures or actual minimum temperatures as follows:

Warm night: minimum temperature departure is 4.50C to 6.40C

Very warm night: minimum temperature departure is >6.40C

What is heat index?

  • The heat index is the combination of air temperature and relative humidity, it measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

What is impact based heat wave warning issue by India Meteorological Department (IMD)?

India Meteorological Department issues following colour code impact based heat warning jointly with National Disaster Management Authority.

Syllabus: Prelims; Geography