It’s a common saying during summer: “It’s not the heat, it’s the humidity.” While partly true, the reality is, it’s actually both. The heat index, also known as the apparent temperature or “feels like” temperature, is a measure of what the air temperature feels like to the human body when relative humidity is factored in. This is crucial because our bodies react differently to heat depending on the moisture content in the air.
When our bodies overheat, we naturally begin to sweat. This perspiration is our body’s cooling mechanism. As sweat evaporates from our skin, it takes heat away, helping to regulate our internal temperature. However, this evaporation process is significantly affected by humidity. High relative humidity in the atmosphere means there’s already a lot of moisture in the air, reducing the rate at which our sweat can evaporate. Think of it like the air being already “saturated” with water, making it harder for more water (your sweat) to turn into vapor. Because your sweat isn’t evaporating as efficiently, your body can’t cool down as effectively, and you feel hotter than the actual air temperature. Conversely, in dry conditions with low relative humidity, sweat evaporates more readily, and you can feel cooler than the actual air temperature. This direct relationship means that as both air temperature and relative humidity rise, the heat index also increases, and vice versa.
To understand the heat index better, consider the heat index chart.
Heat Index Chart
Figure 1. Heat index chart showing the relationship between air temperature, relative humidity, and the resulting heat index.
Using this chart, you can determine the heat index by finding the intersection of the air temperature and relative humidity. For example, if the air temperature is 100°F and the relative humidity is 55%, the heat index reads a sweltering 124°F. Interestingly, when humidity is very low, the apparent temperature can be lower than the recorded air temperature. For instance, at 100°F with a mere 15% relative humidity, the heat index drops to a more bearable 96°F (Use this heat index calculator for precise calculations). In regions with hot summers but typically low humidity, like the Panhandles, dangerously high heat index values (103°F or greater) are less common due to the dryness of the air. A more detailed heat index chart covering a wider range of temperatures and humidity levels is available at NOAA.
It’s important to note that these heat index values are calculated for shady locations. Direct sunlight can increase the heat index significantly, by as much as 15°F. High heat index values are not just about discomfort; they pose real health risks. As detailed in the classification table below, prolonged exposure or physical activity when the heat index is high can lead to serious heat-related illnesses.
| Classification | Heat Index | Effect on the body |
|---|---|---|
| Caution | 80°F – 90°F | Fatigue possible with prolonged exposure and/or physical activity |
| Extreme Caution | 90°F – 103°F | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure/activity |
| Danger | 103°F – 124°F | Heat cramps or heat exhaustion likely, heat stroke possible with prolonged exposure/activity |
| Extreme Danger | 125°F or higher | Heat stroke highly likely |
For those who prefer a numerical approach, a complex equation accurately approximates the heat index. While it involves multiple variables and regression analysis, resulting in a small error margin of ±1.3°F, it provides a precise calculation:
Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 x 10-3T2 – 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 – 1.99 x 10-6T2R2
Where:
T = air temperature (in Fahrenheit)
R = relative humidity (in percentage)
Understanding the heat index is more than just knowing how hot it feels; it’s about being informed about potential health risks and taking necessary precautions during hot and humid weather. Always check the heat index in your area and take appropriate measures to stay safe and cool.
