What is the Heat Index?
The heat index, also known as the "apparent temperature," is a measure of how hot it really feels when relative humidity is combined with the air temperature. High humidity reduces the body's ability to cool itself through sweating, making it feel hotter than the actual temperature. This calculator uses the formula developed by the National Weather Service to determine the heat index.
The Heat Index Formula
The calculation is complex and involves multiple terms. The primary formula (the Steadman formula, adapted by the NWS) is:
HI = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - 6.83783e-3*T2 - 5.481717e-2*RH2 + 1.22874e-3*T2*RH + 8.5282e-4*T*RH2 - 1.99e-6*T2*RH2
Where:
- T is the air temperature in degrees Fahrenheit.
- RH is the relative humidity as a percentage.
This calculator applies this formula and necessary adjustments for different conditions to provide an accurate "feels like" temperature.
Health Risks: High heat index values can be dangerous, leading to heat exhaustion or heat stroke. The calculator will show the associated danger level based on the calculated heat index.