IMD has precribed the following criteria for heatwaves :

• Heat wave will only be considered if maximum temperature of a station reaches 40 degree celsius for the plains and 30 degree celsius for hilly areas

• In case normal maximum temperature of a station is lower or equal to 40 degree celsius, heat wave departure from normal is 5 to 6 degree celsius and severe heat wave departure from normal is 7 degree celsius or more

• In case normal maximum temperature of station is more than 40 degree celsius, heat wave departure from normal is 4 to 5 degree celsius and severe heat wave departure from normal is 6 degree celsius or more

• When actual maximum temperature remains 45 degree celsius or more regardless of normal maximum temperature, heat waves should be declared

http://www.imd.gov.in/pages/heatwave.php

Criteria for Heat Wave:

Heat wave need not be considered till maximum temperature of a station reaches at least 40º C for Plains, 30º C for Hilly regions and 37º or more for coastal region.

a) When normal maximum temperature of a station is less than or equal to 40º C

Heat Wave Departure from normal is 5º C to 6º C & Severe Heat Wave Departure from normal is 7º C or more.

b) When normal maximum temperature of a station is more than 40º C

Heat Wave Departure from normal is 4º C to 5º C & Severe Heat Wave Departure from normal is 6º C or more

c) When actual maximum temperature remains 45ºC or more irrespective of normal maximum temperature, heat wave should be declared.

To declare Heat Wave, the above criteria should be met at least in two station in a meteorological sub-division for at least two consecutive days and it will be declared on the second days.

Source IMD

A heat wave over a region refers to a prolonged period of excessively hot weather (above certain threshold temperature value) over the region, which may be accompanied by high humidity. Various criteria have been used to identify heat waves such as on the basis of percentile thresholds of maximum temperature , Excess heat indices (Rohini et al., 2016) (Panda et al., 2017) etc. India Meteorological Department has given following criteria for declaring heat waves:

Heat wave is considered if maximum temperature of a station reaches at least 40°C or more for Plains, 37°C or more for coastal stations and at least 30°C or more for Hilly regions. Following criteria are used to declare heat wave:

(a) Based on Departure from Normal

i) Heat Wave: Departure from normal is 4.5°C to 6.4°C

ii) Severe Heat Wave: Departure from normal is >6.4°C

Based on Actual Maximum Temperature (for plains only)

i) Heat Wave: When actual maximum temperature ≥ 45°C

ii) Severe Heat Wave: When actual maximum temperature ≥47°C

To declare heat wave, the above criteria should be met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it will be declared on the second day (IMD, 2018).

Indian Institute of Tropical Meteorology has given following criteria for Heat Waves:

a) If maximum temperature is ≥ 39°C and minimum temperature ≥ 90th percentile of the observed minimum temperature for that day

b) If maximum temperature ≥ 95th percentile of the observed maximum temperature for that day, and actual maximum temperature is ≥ 39°C, and maximum temperature departure from normal is ≥ 3.5°C or maximum temperature is ≥ 44°C

c) If maximum temperature ≥ 99th percentile of the observed maximum temperature for that day, and actual maximum temperature is ≥ 39°C, and maximum temperature departure from normal is ≥ 5.5°C or maximum temperature is ≥ 46°C

Hot days(HOT): if satisfying A

Heat Wave (HW): if satisfying B

Severe Heat Waves(SHW): if satisfying C

https://www.tropmet.res.in/erpas/

https://www.nature.com/articles/srep26153

https://pdfs.semanticscholar.org/efda/f878f1e458094b480b775e5afe1f45ea95da.pdf

A heatwave refers to a prolonged period of hot weather, which may be accompanied by high humidity. The World Meteorological Organization guidance around the definition of a heatwave is “A marked unusual hot weather (Max, Min and daily average) over a region persisting at least two consecutive days during the hot period of the year based on local climatological conditions, with thermal conditions recorded above given thresholds.” They are common in the northern and southern hemisphere during summer, but classification and impacts vary globally.

For India,

As per India Meteorological Department (IMD):

Heat wave is considered if maximum temperature of a station reaches at least 40°C or more for Plains, 37°C or more for coastal stations and at least 30°C or more for Hilly regions.

Following criteria are used to declare heat wave:

a) 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

b) Based on Actual Maximum Temperature (for plains only)

· Heat Wave: When actual maximum temperature ≥ 45°C

· Severe Heat Wave: When actual maximum temperature ≥47°C

To declare heat wave, the above criteria should be met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it will be declared on the second day

As per Indian Institute of Tropical Meteorology (IITM):

For Observation Gridded data

A) IF MAXTmp more than 39°C and MINTmp more than 90th Percentile of the observed MINTmp for that day

B) IF MAXTmp more than 95th Percentile of the observed MAXTmp for that day and MAXTmp more than 39°C and MAXTmp departure is more than 3.5°C

Or MAXTmp more than 44°C

C) IF MAXTmp more than 99th Percentile of the observed MAXTmp for that day and

MAXTmp more than 39°C and MAXTmp departure is more than 5.5°C

Or MAXTmp more than 46°C

Hot Days (HOT): Satisfying A and B

Heat Wave(HW): Satisfying B

Severe Heat Wave(SHW): Satisfying C

Therefore SHW⊂HW⊂HOT, i.e. HOT conditions are inclusive of HW, and HW includes SHW. This is different from the IMD manual.

Source:

https://ndma.gov.in/images/guidelines/guidelines-heat-wave.pdf

https://www.preventionweb.net/files/50954_50954roadmapforurbanheatwavewarning.pdf

http://www1.tropmet.res.in/~lip/Publication/RR-pdf/RR-141.pdf

The Indian Meteorological Department (IMD) has given the following criteria for Heat Waves:

Heat Wave : Heat wave is considered if maximum temperature of a station reaches at least 40°C or more for Plains, 37°C or more for coastal stations and at least 30°C or more for Hilly regions.

Following criteria are used to declare heat wave:

a) Based on Departure from Normal

1.Heat Wave: Departure from normal is 4.5°C to 6.4°C

2.Severe Heat Wave: Departure from normal is >6.4°C b)

b) Based on Actual Maximum Temperature (for plains only)

1. Heat Wave: When actual maximum temperature ≥ 45°C

2. Severe Heat Wave: When actual maximum temperature ≥47°C

To declare heat wave, the above criteria should be met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it will be declared on the second day.

Source: India Meteorological Department

a. Heat Wave

Criteria for Heat Wave (as defined by IMD):

Heat wave need not to be considered till maximum temperature of a station reaches at least 40º C for plains and at least 30º C for hilly regions.

i. When normal maximum temperature of a station is less than or equal to 40º C

Heat Wave Departure from normal is 5º C to 6º C

Severe Heat Wave Departure from normal is 7º C or more

ii. When normal maximum temperature of a station is more than 40º C

Heat Wave Departure from normal is 4º C to 5º C

Severe Heat Wave Departure from normal is 6º C or more

iii. When actual maximum temperature remains 45ºC or more irrespective of normal maximum temperature

Heat Wave as an extreme weather event

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

Heat Waves typically occur between March and June,

and in some rare cases even extend till July.

The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, typically involving 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 39oC i.e.102oF.
• Heat Exhaustion: Fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.
• Heat Stoke: Body temperatures of 40oC i.e. 104oF or more along with delirium, seizures or coma. This is a potential fatal condition sometimes resulting in death.