What Is Heat Index?
The heat index, or "feels-like" temperature, combines air temperature and relative humidity to indicate the human-perceived equivalent temperature. High humidity prevents sweat from evaporating efficiently, making the body feel hotter than the actual air temperature. The National Weather Service (NWS) uses the heat index to issue heat warnings.
At 95°F with 50% humidity, the heat index is approximately 107°F. This significantly higher apparent temperature poses real health risks including heat exhaustion, heat cramps, and potentially fatal heat stroke. Understanding heat index helps people take appropriate precautions during hot weather.
NWS Heat Index Equation
Where T is temperature in Fahrenheit and R is relative humidity in percent. Valid for T >= 80°F.
Heat Index Danger Levels
| Heat Index | Level | Health Risk |
|---|---|---|
| 80-90°F | Caution | Fatigue possible |
| 90-105°F | Extreme Caution | Heat cramps, exhaustion |
| 105-130°F | Danger | Heat stroke likely |
| >130°F | Extreme Danger | Heat stroke imminent |
Frequently Asked Questions
Why does humidity make heat feel worse?
Your body cools itself primarily through sweat evaporation. High humidity means the air already holds a lot of moisture, slowing evaporation. With less cooling from sweat, your body temperature rises. At 100% humidity, sweat cannot evaporate at all, making even moderate temperatures dangerous.
Is heat index accurate in direct sunlight?
The heat index is calculated for shady conditions. Direct sunlight can add up to 15°F to the heat index value. Wind can lower the effective heat index by aiding evaporation. For outdoor activities in sun, add 15°F to the calculated heat index for a more realistic assessment.