What is a heatwave, how is it declared, and what are the many types of alerts in India?

Source:WION Web Team

 | Updated: May 01, 2022, 05:18 PM IST

May has arrived, after April scorched large parts of India, setting a 122-year record. The country is experiencing a heatwave, with numerous states issuing orange alerts.

But what exactly is a heatwave, how is it declared, and what are the many types of alerts?

Let's find out:

What is heatwave

Heat wave is a state of high air temperature that is lethal to the human body when exposed. It is defined based on temperature thresholds over a region, either in terms of actual temperature or deviation from normal. In certain nations, it is measured by the heat index, which is based on temperature and humidity, or by the extreme percentile of temperatures.


What criteria is used to declare a heat wave

As per the Indian Meteorological Department, when the highest temperature of a station reaches at least 40.0°C for Plains and at least 30.0°C for Hilly regions, it is considered a heatwave.

Based on temperature deviation: If the temperature varies from normal by 4.50°C to 6.40°C.

Alternatively, when the maximum temperature exceeds 45.0°C, it is called a heatwave, and when the maximum temperature exceeds 470°C it is categorised as a severe heatwave.


heatwave notice

In collaboration with the National Disaster Management Authority, the India Meteorological Department provides a colour-coded impact-based heat warning.



It means that no action is necessary. The maximum temperatures are in the typical range and there is no need to take any precautionary measures.


Yellow Alert

This signal is issued when a heat wave lingers for two days in isolated pockets but the temperature remains moderate. 

Heat is bearable for most people, but it poses a moderate health risk for the most vulnerable, such as infants, the elderly, and persons with chronic conditions.

In such a situation, the IMD recommends that you (a) avoid heat exposure. (b) Dress in light-colored, loose-fitting cotton clothing. (c) Use a cap, a cloth, or an umbrella to protect yourself.


Orange alert

Issued when heat waves that are severe last for two days; heat waves that are not severe last for four days or more. 

People who are either exposed to the sun for a long time or doing heavy work are more likely to get heat illness symptoms when the temperature is high.  Infants, the elderly, and persons with chronic conditions are among the most vulnerable.

In such a case, the IMD advises you to stay cool and avoid sun exposure. Dehydration should be avoided at all costs.

(a) Drink plenty of water, even if you're not thirsty. (b) Stay hydrated by drinking ORS or homemade drinks like lassi, torani (rice water), lemon water, buttermilk, and so on.


Red alert

When a severe heat wave lasts more than two days or the total number of heat/severe heat wave days exceeds six days, an Extreme Heat Alert is issued.In such conditions, heat illness and heat stroke are quite common in people of all ages.