Heat Index Calculator

This calculator estimates the temperature felt by the body as a result of air temperature and relative humidity. Heat index is often referred to as humiture, and is similar to wind chill in its attempt to measure perceived, rather than actual temperature. For example, an air temperature of 83°F with a relative humidity of 70% would result in an estimated 88°F perceived temperature.

Air Temperature
Relative Humidity   OR   Dew Point Temperature

RelatedWind Chill Calculator | Dew Point Calculator

Heat Index Formula

Like the Wind Chill Temperature Index, the heat index used by the National Weather Service (NWS) in the United States is based on many assumptions such as body mass, height, clothing, individual physical activity, blood thickness, and wind speed. As such, depending how significantly these assumptions vary from the reality of an individual, heat index estimates may not accurately reflect perceived temperature. The equation used by the NWS to estimate heat index was developed by George Winterling in 1978, and is meant to be valid for temperatures of 80°F or higher, and relative humidity of 40% or more. The actual equation used by the NWS (as well as this calculator) is available through the NWS website. Below is a chart listed results of the equation.

Potential effects of heat index

The temperature equivalent perceived by humans as a result of air temperature, relative humidity, and wind speed, is sometimes referred to as apparent temperature. This apparent temperature can have potentially severe medical effects. Under high air temperature and humidity (high heat index) conditions, perspiration is hindered due to reduced evaporation as a result of high humidity. Sweat is the human body's physiological response to high temperatures, and is an attempt to lower body temperature through evaporation of sweat. When this is hindered, overheating and dehydration can occur, with varying severity. Below is a table indicating possible complications at varying levels of heat index values, obtained from Wikipedia.

Effects of the heat index1

27-32°C80-90°FCaution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32-41°C90-105°FExtreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41-54°C105-130°FDanger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
Over 54°COver 130°FExtreme danger: heat stroke is imminent.

Note that exposure to full sunshine can increase heat index values by up to 14°F. Heat index values are particularly important for children. Young children are generally in more danger due to factors including larger skin surface relative to their small bodies, higher heat production as a result of exercise, and typically sweating less than adults. Also, children are often less aware than adults of the need to rest and re-hydrate.

Thirst is a late sign of dehydration, and it is important to remain hydrated, particularly before, during, and after outdoor activities, especially those involving heavy physical exertion. In addition to children, people with certain conditions including obesity, diabetes, heart disease, cystic fibrosis, and mental retardation are at greater risk of overheating and dehydration.

  1. Wikipedia: The Free Encyclopedia. "Humiture." en.wikipedia.org/wiki/Humiture