Heat Index Calculator
Calculate what the air temperature 'feels like' based on the humidity level. High humidity makes hot weather feel significantly more intense.
What is the Heat Index?
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This has important considerations for the human body's comfort and safety from heat-related illnesses.
How does humidity affect how hot it feels?
When the body gets too hot, it begins to sweat to cool itself off. If the sweat is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. High humidity levels slow down evaporation, which makes the body feel much hotter than the actual air temperature.
What are the safety levels for Heat Index?
The National Weather Service uses these ranges: 80-90°F (Caution), 90-103°F (Extreme Caution), 103-124°F (Danger), and above 125°F (Extreme Danger). In the Danger and Extreme Danger ranges, heatstroke is highly likely with continued exposure.
Can I calculate Heat Index for temperatures below 80°F?
Technically, no. The heat index formula is specifically designed for warm conditions. At temperatures below 80°F (27°C), humidity has a negligible effect on apparent temperature, and the actual air temperature is used.