It’s a common saying during summer: “It’s not the heat, it’s the humidity.” While partly true, the reality is, it’s both. The heat index, also known as the apparent temperature, tells us what the temperature actually feels like to our bodies when we factor in relative humidity alongside the air temperature. This is crucial because it directly impacts our comfort and health.
When our bodies overheat, we naturally start to sweat to cool down. This cooling process works through evaporation. As sweat evaporates from our skin, it takes heat with it, effectively lowering our body temperature. However, when the air is humid, meaning it already holds a lot of moisture, sweat doesn’t evaporate as easily. This reduced evaporation rate hinders our body’s ability to cool itself, making us feel hotter. Conversely, in dry conditions with low relative humidity, sweat evaporates more quickly, making us feel cooler than the actual air temperature. Therefore, the heat index rises and falls in direct relation to both air temperature and relative humidity.
Heat Index Chart
Figure 1. A heat index chart illustrating how air temperature and relative humidity combine to determine the apparent temperature.
To understand the heat index, you can use a heat index chart like the one above. You’ll need to know the air temperature and the relative humidity. For instance, if the air temperature is a sweltering 100°F and the relative humidity is 55%, the heat index soars to 124°F. Interestingly, when humidity is low, the apparent temperature can be lower than the recorded air temperature. For example, even with an air temperature of 100°F, if the relative humidity drops to 15%, the heat index is a more manageable 96°F. You can easily calculate this using online heat index calculators available from sources like the National Weather Service (weather calculator). For a more comprehensive heat index chart covering a wider range of temperatures and humidity levels, resources are available online (at this link).
Many are surprised to learn that 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 pose real health risks. As shown in the table below, heat index values of 103°F or higher can lead to dangerous heat-related illnesses, especially with prolonged exposure or physical activity in the heat.
| 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 quick calculations, utilize a weather calculator if you prefer manual input over chart reading. For those mathematically inclined, a complex equation closely approximates the heat index. This equation, derived from multiple regression analysis, has a margin of error of ±1.3°F.
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)
