Climate Change

The rising temperature and how it actually feels

Heat index calculates the felt air temperature by capturing the changes in humidity levels, air temperature and wind speed

By Meenakshi Sushma
Published: Friday 31 May 2019
Representative Photo: Getty Images

On May 31, India Meteorological Department (IMD) released an all-India warning report stating that  heat waves will hit parts of Madhya Pradesh, Uttar Pradesh, Rajasthan, Telangana, Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Punjab, Haryana, Chandigarh & Delhi, Bihar, Jharkhand, Gujarat, Maharashtra, Karnataka, Tamil Nadu and Puducherry.

Chandrapur in Maharashtra’s Vidarbha region has been hit worst, facing heatwaves since May 23, with the maximum temperature touching 48 degree Celsius, according to the IMD report.

Heat wave conditions are declared when the maximum temperature of a station (local meteorological stations) reaches at least 40°C or more for plains, 37°C or more for coastal and 30°C or more for hilly regions.

However, while calculating the temperature the thermometer just measures hot air and not the heat from direct sunlight. At the global scale, a tool called Wet-Bulb globe temperature is used, which calculates the heat stress in direct sunlight, and also takes into account temperature, humidity, wind speed, sun angle and solar radiation.

There is, however, a difference between the actual temperature and the way temperature “feels like” to us also called felt air temperature. This is measured by combining air temperature and relative humidity, and is called the heat index.

Heat index calculates the felt air temperature by capturing the changes in humidity levels, air temperature and wind speed — if the humidity is high, it feels hotter than the actual temperature.

This scenario is mostly seen in coastal areas, for example, in Chennai the actual temperature is 36°C, but when people step out they will feel like it’s 40°C because of the levels of humidity and wind speed.

Following table shows the difference between the actual temperature and what it actually feels like. The inputs were taken from The Weather Channel



Feels like

Wind speed





WNW 11 km/h





S 21 km/h


Nagpur -vidharbha



SW 8 km/h





SSE 10 km/h





W 16 km/h


In 2017, IMD had released a map which shows the heat index in each region. “But, it is not being calculated currently,” says Brahm Prakash Yadav, Deputy Director General, IMD.

It is very important at this stage to calculate the heat index as it can help avoid deaths due to heat strokes. Excessive exposure to heat waves results in:

  1. Heat syncope: giddiness, vertigo headache and sudden onset drowsiness/ unconsciousness generally accompanied by fever below 102° fahrenheit
  2. Heat Cramps: Edema (swelling in legs feet or hands because of fluid retention in the body)
  3. Heat Exhaustion: Fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating
  4. Heat Stoke: Body temperatures of 104°F or more along with delirium, seizures or coma. This is a potential fatal condition, according to a report published by the National Institute of Disaster Management.

There have been 22,562 deaths due to heat wave in India between 1992 and 2015, states the report.

The same scenario is going till continue June 1, 2019, as IMD predicted. It is high time that we start calculating both the heat index and wet bulb globe temperature to avoid the deaths and health impacts caused due to heat waves; and timely action can be taken.

Subscribe to Daily Newsletter :
Related Stories

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.