A heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature; “how hot it feels.” The result is also known as the “felt air temperature” or “apparent temperature.” For example, when the temperature is 90°F with very high humidity, the heat index can be about 106°F.

The human body normally cools itself by perspiration, or sweating. Heat is removed from the body by evaporation of that sweat. However, relative humidity reduces the evaporation rate because the higher vapor content of the surrounding air does not allow the maximum amount of evaporation from the body to occur.

Higher humidity results in a lower rate of heat removal from the body, hence the sensation of being overheated. This effect is subjective; however, studies have been performed to normalize heat index representation (ksi.uconn.edu).

The table above is a listing of the calculated heat index values which match the U.S. National Oceanic and Atmospheric Administration’s table. The colors correspond to WIAA’s categorization (NOTICE, CAUTION, WARNING, and DANGER).

Looking at the table, it is important to note that any temperature above 86°F will “feel” at least 86°F. Virtually every heat index will be at least the current temperature, if not hotter.

The WIAA website uses a custom script to calculate a given heat index and forecasted heat index for a given location (i.e. city name or ZIP code). This is accomplished via a weather API service through AccuWeather.

This resource allows us to plug in a specific location and get back a data packet with pertinent information to determine a heat index. Specifically, we extract the returned current temperature, relative humidity, and forecasted high temperature. Our custom calculator then takes those variables to get a current heat index and a forecasted “high heat index”. The calculator will then show WIAA recommendations in accordance with the WIAA practice model.

Two variables, temperature (T) and relative humidity (R), are necessary to calculate a heat index. Given a current temperature and a relative humidity, a heat index can be calculated easily using the following polynomial equation. The formula approximates the heat index in degrees Fahrenheit and conforms to the NOAA National Weather Service’s Heat Index Lookup Table.

This equation is the result of a multivariate fit (temperature ≥ 80°F and relative humidity ≥ 40%) to a model of the human body. This equation does not work with temperatures under 80°F nor a relative humidity under 40% (values outside of that range are assumed to be the given temperature; i.e. 96°F with relative humidity of 25% would yield a heat index of 96°F).

There are other website services out there that can report a heat index. Those sites (like NOAA’s heat index maps) provide a heat index forecast for a broad area and are updated less frequently that the weather service WIAA implements. Most weather providers rely on National Weather Service (NWS) observation stations that are primarily located at airports and military bases. Each state only has a handful of these stations. The entire state of New York has less than fifty such sites, for example. Their reports are only accurate and relevant in the immediate vicinity of the observation station and generally updated only once every hour.

The AccuWeather API service generates forecasts with Superior Accuracy™ harnessing the expert skills of the world’s leading meteorologists in tandem with the most robust database of forecast models, most advanced global forecast engine, proprietary patents, and comprehensive validation results pinpointed for every location on Earth. This allows the WIAA Heat Index Calculator to use data for geographically specific results and up-to-date information.
It is recommended that WIAA schools check their practice location’s heat index through the WIAA Heat Index Calculator, no other site. The heat index is categorized and suggested practice modifications are provided. This helps us ensure consistency and lends an easier approach for school administrators and coaches.