What is the Heat Index?
The heat index is often referred to as humiture, and is similar to wind chill in its attempt to measure the perceived rather than the actual temperature. For example, an actual air temperature of 83°F with a high relative humidity of 70% would result in an estimated 88°F perceived temperature.
This difference in perceived and actual temperature is the result of a mixture of air temperature, relative humidity, and wind speed.
Why Does Humidity Make It Feel Hotter?
The perception of heat is subjective, but the physics behind it are universal. The human body cools itself through perspiration, where heat is removed from the body as a result of the evaporation of sweat. A higher relative humidity hinders normal body cooling by reducing the rate of evaporation of sweat into the atmosphere.
This lower rate of evaporation subsequently lowers the rate at which the body cools, increasing the perception of heat. This perception of heat is what the heat index seeks to measure. While it can technically be used indoors, it is most often used in reference to the outside temperature.
How is the Heat Index Calculated?
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 on how significantly these assumptions vary from the reality of an individual, heat index estimates may not accurately reflect the perceived temperature.
The core 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.
Potential Health Effects of High Heat Index
Under high air temperature and humidity conditions, perspiration is hindered. When this cooling mechanism is blocked, overheating and dehydration can occur with varying severity.
Note: Exposure to full, direct sunshine can increase heat index values by up to 14°F.
Below is a table indicating possible complications at varying levels of heat index values, as categorized by the National Weather Service.
| Celsius | Fahrenheit | Danger Level & Notes |
|---|---|---|
| 27 - 32°C | 80 - 90°F | Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps. |
| 32 - 41°C | 90 - 105°F | Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke. |
| 41 - 54°C | 105 - 130°F | Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity. |
| Over 54°C | Over 130°F | Extreme danger: heat stroke is imminent. |
Who is Most at Risk?
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. It is important to remain hydrated before, during, and after outdoor activities. In addition to children, people with certain conditions—including obesity, diabetes, heart disease, and cystic fibrosis—are at greater risk of overheating and dehydration.