Heat Index Calculator
Calculate apparent temperature from air temperature and humidity
Calculation Steps
How Heat Index Is Calculated
The calculator follows the NWS approach for combining temperature and humidity.
Humidity effect
High humidity slows evaporative cooling, so the same air temperature can feel much hotter.
NWS formula range
A simple apparent-temperature estimate is used first. When the result reaches about 80°F, the Rothfusz regression is applied.
Outdoor planning
Use heat index as one signal alongside sun exposure, activity level, clothing, wind, and local weather alerts.
Heat Index Examples
Compare how humidity changes the feels-like temperature.
Hot and humid
90°F with 70% humidity feels like about 106°F
Metric summer day
32°C with 70% humidity feels like about 41°C
Lower humidity
The same air temperature feels less intense when relative humidity falls.
Heat Index Calculator FAQ
Common questions about humidity, apparent temperature, sun exposure, and heat safety.
What does heat index mean?
Heat index is an apparent temperature estimate that combines air temperature and relative humidity to show how hot conditions may feel to the human body.
Why does humidity make the heat index higher?
High humidity slows sweat evaporation. When sweat evaporates less efficiently, the body has a harder time cooling itself and the heat feels more intense.
When is the heat index formula most useful?
It is most useful in hot, humid weather. At lower temperatures or low humidity, the calculator may use a simpler apparent temperature estimate.
Should I use shade or direct sun temperature?
Use the air temperature from a reliable weather reading, usually measured in shade. Direct sun can make conditions feel hotter than the reported heat index.
Is this heat index result medical advice?
No. It is an educational planning estimate. Follow local heat advisories and seek medical help if someone shows signs of heat illness.