What is Heat Index?

By : Neha Dhyani

Updated : May 27, 2022, 6:24

The Heat Index measures the humidity present in the atmosphere. We can also say that the Heat Index is the temperature that combines relative humidity and air temperature in shaded areas. 

Other terms used for measuring Heat Index are Real Feel, Felt Air temperature, and Apparent Temperature. George Winterling introduced the concept of Heat Index in 1978, and it was adopted by the US National Weather Service in 1979.

Heat Index Assumptions

The Heat Index follows the given below assumptions -

  •  The mass and height of humans.
  • Clothing of human beings
  • Amount of physical activity performed by humans
  • Individual heat tolerance by an individual
  • Wind speed
  • Exposure of people to sunlight and radiation

Uses of Heat Index

  1. It helps us know the accurate amount of heat and hotness present outside.
  2. It helps us to know the temperature by combining the air and relative humidity.
  3. It also helps us to know the rate of absorption of the humidity.
  4. Higher or lower Heat Index also depends on the areas which are more or less exposed to sunlight. For example, the areas which are exposed to more sunlight will have more Heat Index and vice versa.
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How to calculate Heat Index? 

The Heat Index is calculated based on actual temperature and relative humidity levels and the production of the amount of heat. There is both a chart and a formula to calculate the Heat Index. The Heat Index is based on the result of a Heat Index graph when we compare relative humidity and actual temperature.

The formula for Heat Index -

Heat Index (HI) = c1+c2T+c3R+c4TR+c5T2+c6R2+c7T2R+c8TR2+c9T2R2


  • HI stands for the Heat Index which is measured in Fahrenheit 
  • T stands for temperature R stands for relative humidity 

c1-c9 stands for constants

  • value of c1 is -42.379
  • c2 is -2.04901523
  • c3 is -10.1433
  • c4 is -0.22475541
  • c5 is -6.83783 x 10−3
  • c6 is -5.481717 x 10−2
  • c7 is -1.22874 x 10−3
  • c8 is 8.5282 x 10−4.

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Heat Index in India

The Indian Meteorological Department introduced the Heat Index in India in 2016. This Heat Index provides people with accurate and updated information regarding the heat conditions.

IMD (Indian Meteorological Department) 2017 found and brought out a map charting of the Heat Index in different states. The map is highlighted with different colours, from blue to orange, showing the stressed places to climatic conditions.

The Heat Index, also known as apparent temperature, is what the temperature feels like to humans. It is the temperature or the hotness which we feel outside in summers. The Heat Index is also determined by the combination between relative humidity and air present in our environment.

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FAQs on Heat Index

Q.1 What are the four alerts of the Heat Index?

The four alerts of the Heat Index are -

  • Caution 
  • High alert
  • Danger
  • Extreme Danger

Q.2 Name the SI unit of temperature for the Heat Index.

Kelvin is the SI unit of temperature. It is used to calculate the Heat Index.

Q.3 What are some common sources of heat that the Heat Index Considers?

Some common sources of heat are:

  • Electrical
  • Nuclear
  • Sun
  • Chemical

These are all factored into the Heat Index.

Q.4 What is the temperature range of caution alert on the Heat Index?

80 degrees F – 90 degrees F is the temperature range of caution alert on the Heat Index.