Heat wave is a condition of air temperature which becomes fatal to human body when exposed. Often times, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
Heat wave is considered if maximum temperature of a station reaches at least 400C or more for Plains and at least 300C 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 ≥ 450C
Severe Heat Wave: When actual maximum temperature ≥470C
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
It is occurring mainly during March to June and in some rare cases even in July. The peak month of the heat wave over India is May.
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.
a. Transportation / Prevalence of hot dry air over a region (There should be a region of warm dry air and appropriate flow pattern for transporting hot air over the region).
b. Absence of moisture in the upper atmosphere (As the presence of moisture restricts the temperature rise).
c. The sky should be practically cloudless (To allow maximum insulation over the region).
d. Large amplitude anti-cyclonic flow over the area.
Heat waves generally develop over Northwest India and spread gradually eastwards & southwards but not westwards (since the prevailing winds during the season are westerly to northwesterly).
The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke. The signs and symptoms are as follows:
1. Heat Cramps: Ederna (swelling) and Syncope (Fainting) generally accompanied by fever below 39*C i.e.102*F.
2. Heat Exhaustion: Fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.
3. Heat Stoke: Body temperatures of 40*C i.e. 104*F or more along with delirium, seizures or coma. This is a potential fatal condition.