Heat Index

According to the National Weather Service, “Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.”  Heat index temperature is also referred to as the “real feel” or “apparent” temperature.  When relative humidity is high, the rate that perspiration evaporates is slowed by water vapor in the surrounding air.  Given that the body dissipates a great deal of heat through evaporation, understanding and monitoring heat index is critical.

To accurately determine the heat index, you can use a Wet Bulb Thermometer or a Heat Stress Meter.  If you don’t have a either of these, you can make one or use a tool like the Heat Index Chart shown below.

Heat Index Chart

Heat Index Chart

To use the heat index chart, you must know the ambient temperature and relative humidity. It is best to obtain these using a thermometer (temperature) and hygrometer (humidity) at the location of your activity. However, if you don’t have either of these you can use a weather report or weather app. Once you know the temperature and percentage of relative humidity, find the approximate temperature, along the left of the Heat Index Chart, that is closest to (but not less than) the ambient temperature. Next, find the approximate relative humidity, along the top of the Heat Index Chart, that is closest to (but not less than) the actual relative humidity. Follow the temperature row to the right, and the relative humidity column down.  Where these two intersect is the heat index temperature. For example, if the temperature is 96°F and the relative humidity is 65%, the heat index temperature is 121°F.  (See Example)

You can also use a heat index calculators like the one provided by NOAA.

Keep in mind that this method is general and not the most reliable way to determine the heat index…so err on the side of caution.

Heat Index Chart Example

Once you know the heat index temperature, you can use tools like the WBGT Guidelines (shown at bottom) to determine appropriate activity intensity and duration, rest periods, and fluid consumption.

Wet Bulb Globe Temperature

The Wet Bulb Globe Temperature (WBGT) is a means by which to measure the effect of humidity, infrared radiation, temperature, and wind.

Indoors and outside (when there is no direct sunlight), the equation is expressed as:

WBGT = 0.7NWB + 0.3GT

Outside with a solar load, the equation is expressed as:

WBGT = 0.7NWB + 0.2GT + 0.1DB


WBGT = Wet Bulb Globe Temperature Index
NWB = Natural Wet-Bulb Temperature (Lowest temperature that can be achieved through the evaporation of water given current ambient temperatures) *Indicates humidity in combination with the dry bulb temperature)
DB = Dry-Bulb Temperature (Air temperature not accounting for radiation and moisture)
GT = Globe Thermometer Temperature (Measures radiant heat transfer)


Equations aside, there are numerous affordable Wet Bulb Thermometers available for purchase that will do the math for you. Digital versions provide you with timely and accurate data, as well as alarms to warn you of hazardous conditions. Like the heat index chart, the heat index temperature obtained from a Wet Bulb Thermometer should be used to determine appropriate activity intensity and duration, rest periods, and fluid consumption (WBGT Guidelines). 

Next: Heat Acclimatization