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Heat Index Formula

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  • Last Updated : 03 Mar, 2022

In shaded locations, the heat index is a temperature that combines air temperature and relative humidity. Perspiration or sweating is a natural way for the human body to cool down. Sweat evaporation is responsible for removing heat from the body. High relative humidity, on the other hand, slows evaporation. When the temperature is 32 degrees Celsius and the relative humidity is 70%, the heat index number equals the actual air temperature.

Heat Index

The heat index formula is based upon the temperature and relative humidity to indicate the air temperature perceived by the body. There is a difference between the actual temperature of the atmosphere and the temperature that we perceive or feel. 


If the temperature is 32°C and 70% relative humidity, the heat index is 41°C. This heat index has an implied humidity of 20%. It’s the value of the relative humidity for which the heat index formula shows 41°C. A heat index temperature of 32°C holds implied relative humidity of 38%.

Perspiration, or sweating, is how the human body generally cools itself. Sweat evaporates, which removes heat from the body. High relative humidity, on the other hand, slows evaporation. As a result, the pace at which heat is removed from the body slows down, giving the sense of being overheated. This effect is subjective, with different people perceiving heat differently for various reasons (such as differences in body shape, metabolic differences, hydration differences, pregnancy, menopause, drug effects and/or drug withdrawal); its measurement is based on subjective descriptions of how hot people feel for a given temperature and humidity. As a result, a heat index is calculated, which compares one temperature and humidity combination to another.


The heat index formula is expressed as,

HI = c1 + c2T + c3R + c4TR + c5T2 + c6R2 + c7T2R + c8TR2 + c9T2R2


  • HI denotes the heat index in degrees Fahrenheit
  • R denotes the relative humidity
  • T refers to the temperature in °F
  • c1 = −42.379
  • c2 = −2.04901523
  • c3 = −10.14333127
  • c4 = −0.22475541
  • c5 = −6.83783 × 10−3
  • c6 = −5.481717 × 10−2
  • c7 = −1.22874 × 10−3
  • c8 = 8.5282 × 10−4
  • c9 = −1.99 × 10−6

Function of Heat Index

It’s utilized to simulate the human body’s feeling of humidity and heat in order to produce healthy or public programs that protect people and animals. The heat index of the parts in this environment that are exposed to sunlight could be greater or lower. Furthermore, people engaged in various activities may sense heat differently and may not feel the heat index measured for that day or location.

Sample Problems

Question 1. Calculate the heat index for a temperature of 185° F and 86% relative humidity.


Given: T = 185° F and R = 86%.

Since, HI = c1 + c2T + c3R + c4TR + c5T2 + c6R2 + c7T2R + c8TR2 + c9T2R2

Substituting the given values in the above formula, we have:

HI = -42.379 + -2.04901523 × 185 + -10.14333127 × 86 + -0.22475541 × 185 × 86 + -6.83783 × 10−3 × (185)2+-5.481717 × 10−2 × (86)2+ -1.22874 × 10−3 × (185)2 × 86 + 8.5282 × 10−4 × 185 × (86)2 + -1.99 × 10−6 × (185)2 × (86)2

HI = 1274° F

Question 2. Calculate the heat index for a temperature of 90° F and 60% relative humidity.


Given: T = 90° F and R = 60%.

Since, HI= c1 + c2T + c3R + c4TR + c5T2 + c6R2 + c7T2R + c8TR2 + c9T2R2

Substituting the given values in the above formula, we have:

HI = -42.379 + -2.04901523 x 90 + -10.14333127 x 60 + -0.22475541 x 90 x 60 + -6.83783 x 10−3 x (90)2+-5.481717 x 10−2 x (60)2+ -1.22874 x 10−3 x (90)2× 60 + 8.5282 x 10−4 x 90 x (60)2 + -1.99 x 10−6 x (90)2 x (60)2

HI = 100° F

Question 3. Why is the heat index higher than the actual temperature?


The heat index is a measurement of how hot it really feels when the relative humidity is incorporated with the actual temperature. Heat indices were designed for use in the shade with light wind conditions. If in direct sunlight, the heat index can increase as much as 15 degrees. With very hot and dry air, strong winds can also be extremely dangerous. When a human being perspires, the water in his or her sweat evaporates. … When humidity is high, the rate of evaporation and cooling is reduced, resulting in it feeling hotter than it actually is.

Question 4. Can the heat index be lower than the actual temperature?


It most certainly is, and it occurs when high temperatures are combined with low dew points. Despite the fact that the desert is incredibly hot, the heat index is lower than the temperature due to dry conditions. In 2017, the temperature in Phoenix hit 119 degrees, while the dew point was just 37. This resulted in a relative humidity of 6% and a heat index of 111, which was still deadly but eight degrees lower than the temperature. Chicago’s highest heat index occurred during the city’s catastrophic 1995 heat wave, when a 106-degree temperature at Midway Airport coupled with a suffocating 81-degree dew point to produce a heat index of 125. Dhahran, Saudi Arabia, had the world’s highest heat index of 178, with a temperature of 108 and a dew point of 95.

Question 5. Why is wind speed not considered while measuring heat index?


All weather agencies record ‘ambient temperature’— how warm the air is in the shade and sheltered from the wind. This is done by placing weather recording instruments in a Stevenson Screen. The height above ground that Stevenson Screens are placed is between 1.25 and 2 m (4 ft. 1 in and 6 ft. 7 in). By using this approach weather readings from around the world can be regarded being consistent and relatable.

When weather forecasters produce a temperature forecast they try to produce a number that will be similar to what would be recorded by instruments in a Stevenson Screen.

Knowing the effect humidity and wind has on how humans feel temperature most weather agencies also try to forecast the apparent temperature. In colder parts of the world, agencies issue the forecast temperature and the wind chill factor. This isn’t generally done for warmer regions, partly because predicting wind speeds and wind consistency can be difficult.

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