Heat Index Calculator
Input the actual air temperature and relative humidity percentage to determine how hot it truly "feels like" to the human body.
Heat Index Estimator
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Understanding Heat Index and Humidity
In the summer, an 85°F day in the dry desert of Arizona feels completely different than an 85°F day in the humid swamps of Florida. The reason is the Heat Index.
How Humidity Traps Heat
Your body's primary mechanism for cooling down is sweating. As sweat evaporates off your skin, it absorbs thermal energy, lowering your body temperature. However, evaporation depends entirely on the air's capacity to absorb moisture.
When Relative Humidity (RH) is very high, the air is already saturated with water vapor. Therefore, your sweat cannot evaporate easily. It stays on your skin, and your body fails to cool itself. This causes your internal temperature to rise, making the ambient air "feel" much hotter than the thermometer reads.
The Heat Index Formula
The Heat Index is calculated using the Rothfusz regression equation, adopted by the National Weather Service (NWS). It is an incredibly complex formula that factors in both air temperature and relative humidity to produce a single "Feels Like" temperature.
Important note: Heat Index values are formulated for shady, light-wind conditions. If you are exposed directly to the blazing sun, the perceived heat index can increase by up to 15°F (8°C).
Health Risks of High Heat Index
Knowing the Heat Index is crucial for outdoor workers and athletes:
- 90°F - 103°F (Extreme Caution): Heat cramps, heat exhaustion, and dehydration become possible with prolonged exposure.
- 103°F - 124°F (Danger): Heat cramps and heat exhaustion are highly likely. Heatstroke is a severe risk if physically active.
- 125°F+ (Extreme Danger): Heatstroke is highly likely. The body's cooling mechanisms are entirely overwhelmed. Outdoor activity should be strictly avoided.