What is a heat wave? How to stay safe? As north India reels under intense sun, here's how to beat the heat

Parts of northern India are now reeling under a heat wave, particularly Rajasthan, Jammu and Kashmir, Punjab, West Rajasthan, West Madhya Pradesh. The Indian weather agency IMD on Wednesday (June 11) declared heat wave conditions in parts of Himachal Pradesh, Haryana, Chandigarh and the national capital Delhi and some places over Uttar Pradesh, East Rajasthan and East Madhya Pradesh. Warm night to severe warm night conditions were observed in isolated pockets of West Uttar Pradesh and West Rajasthan, and in isolated pockets of Punjab, East Uttar Pradesh and Haryana.

It is important to understand what a heat wave is, and the health risks associated with it. Here is what you should know about how heat waves are declared in India, and how to stay safe during such severe weather.

What is a heat wave?

A heat wave is defined as a long period of excessively hot weather, usually accompanied by humidity. Heat waves can pose health risks, and put a strain on infrastructure, particularly due to high electricity consumption for cooling.

Heat waves are declared when temperatures exceed the average for a specific region.

Geologically, heat waves are caused by a combination of intense sunlight, high-pressure systems that trap warm air and lead to hot winds, reduced cloud cover, and lack of cooling breezes.

Heat waves typically last for days, but can extend to weeks.

Heatstroke, dehydration are among health risks associated with heat waves, which are typically accompanied by power outages due to a rise in cooling demand.

How does IMD declare heat waves in India?

The Indian weather agency, India Meteorological Department (IMD), defines a heat wave based on specific temperature thresholds and deviations from the normal conditions for various regions of the country.

Usually for the plains, a heat wave is declared when the maximum temperature hits 40 degree Celsius and is between 4.5 to 6.4 degrees Celsius above the normal for that region.

A severe heat wave is declared if the temperature exceeds 6.4 Degrees Celsius above normal, or goes to 47 Degrees Celsius or higher.

For coastal India, which has high humidity, the heat wave threshold is 37 Degrees, while the deviation criteria remain the same as above.

For hilly areas, a heat wave is declared when temperatures reach 30 Degrees Celsius with an above-normal variation of 4.5 Degrees to 6.4 Degrees Celsius.

Heat waves and red or yellow alerts are declared by the IMD when the above-normal temperatures are recorded for two consecutive days at a specific location.

IMD also takes into account regional climate variations. This is because what constitutes extreme heat in coastal cities is different from a dry region like Rajasthan.

With these criteria, the IMD can issue warnings, alerting the public to take health measures and governments to mitigate the impact.

In India, heat waves usually run between the months of April and June. Some heat waves can be deadly, with several deaths reported every year, with the casualties are often found among low-income communities in urban and poor areas.

How to be safe during a heat wave

Here are some ways to minimise heat exposure and maintain hydration during heat waves:


Stay hydrated: Even if you are not thirsty, drink a lot of water throughout the day. Carry a water bottle with you. At the same time, avoid sugary drinks, caffeine, and alcohol, as they can cause more dehydration. If you are sweating too much, try drinking electrolyte-rich fluids like oral rehydration solutions.

Stay cool: Stay inside cool or air-conditioned spaces and shaded areas of homes, malls, public places. If air conditioning is not available near you, try to take cool showers, or wipe your body with damp cloths.

As much as possible, stay in indoor or shaded areas, particularly during the peak heat hours that are between 10 a.m and 4 p.m.

Dress for the summer : Go for heat-reflecting light colours like white. Use clothes made from material that's lightweight, like linen. When possible, wear loose-fitting clothes. Breathable fabrics like cotton are the way to dress in summer. If possible, wear hats, caps and sunglasses.