Many people have heard, “It’s not the heat, it’s the humidity,” which is a common phrase used to express how uncomfortable it feels outside during the summer. In fact, it is not just the humidity that tells us this, but heat as well. The heat index is what the temperature feels like to the human body when relative humidity (moisture content in the atmosphere) is combined with the air temperature. When the body gets too hot, it begins to sweat to cool itself off.

The sweat needs to evaporate off the body for it to effectively reduce the body’s temperature. The amount of moisture in the atmosphere plays a key role in determining how much sweat will evaporate off the body. The more humid it is outside, the lower the rate of evaporation from the body. In other words, the body feels warmer in humid conditions and cooler in arid conditions. For example, say the air temperature is 100 degrees Fahrenheit and the relative humidity is 55%. The heat index in this case is 124 degrees. If the relative humidity is 15%, then the heat index would be 96 degrees, only 4 degrees cooler than what it actually is.

Surprisingly enough, the heat index values from the chart used by the National Weather Service are actually for shady locations. If exposed to direct sunlight, the heat index value can be increased by up to 15 degrees. The effect that the heat index has on the body depends greatly on how high the heat index value is. Effects can range from fatigue to heat exhaustion, heat cramps, or heat strokes.

Figure 1: Equation used to calculate the approximate value of the heat index. Photo courtesy of the National Weather Service.

To determine the value of the heat index in your area, you can use the weather calculator (Figure 1). The “T” refers to the air temperature and the “R” refers to the relative humidity. Both values can easily be found on weather apps. The equation may look lengthy, but it is not exact. In fact, the actual heat index value from this equation has an error of 1.3 degrees. No matter what your method is to finding the heat index, always remember to check the forecast ahead of time for any potentially dangerous summer-like conditions.