How to use the heat index calculator with humidity or dew point
Choose relative humidity when your weather observation provides RH directly, or choose dew point when that is the available moisture reading. Enter the shaded air temperature, verify the unit, and calculate to see the apparent temperature and risk category.
In dew point mode, the tool first converts air temperature and dew point into relative humidity. That derived RH is shown with the result so you can review the intermediate value used by the NWS heat-index procedure.
- Use shaded air temperature rather than a sun-heated surface reading.
- Enter relative humidity from 0% to 100%, or a dew point no higher than air temperature.
- Read the risk message together with the temperature result.
- Account separately for direct sun, wind, clothing, activity, and personal vulnerability.
NWS heat index formula using air temperature and relative humidity
Heat index estimates an apparent temperature in shaded, light-wind conditions. The National Weather Service procedure begins with a simpler Steadman estimate and switches to the Rothfusz regression when the screened heat index reaches about 80°F.
The regression includes air temperature, relative humidity, their squares, and interaction terms. Low-humidity and high-humidity adjustments apply within defined temperature ranges. The calculator implements those branches before converting the Fahrenheit result to Celsius and Kelvin.
HI = -42.379 + 2.04901523T + 10.14333127RH − …How to calculate heat index from air temperature and dew point
When dew point is selected, the calculator first derives relative humidity from air temperature and dew point. It then passes that humidity into the same NWS heat-index procedure and displays the derived percentage for review.
This mode is useful when a weather station reports dew point but not relative humidity. Air temperature and dew point should describe the same location and observation time.
Heat index example for 90°F air temperature and 70% humidity
At 90°F (32.2°C) with 70% relative humidity, the Rothfusz calculation and applicable adjustments produce a heat index of about 106°F (41.1°C). That falls in the NWS Danger category, where heat cramps or heat exhaustion are likely with continued exposure and heat stroke is possible.
The example represents shaded, light-wind conditions. Full sun can make conditions feel hotter, while work intensity, clothing, acclimatization, hydration, age, and health can change individual risk without changing the calculated weather value.
NWS heat index danger levels for outdoor heat exposure
The result is grouped using commonly published NWS heat-index categories. These labels describe potential effects under assumed shaded conditions and do not measure an individual's body temperature or personal medical risk.
- 80–89°F: Caution — fatigue is possible with prolonged exposure.
- 90–104°F: Extreme caution — heat cramps or heat exhaustion are possible.
- 105–129°F: Danger — heat illness becomes increasingly likely.
- 130°F and above: Extreme danger — heat stroke is imminent.
- Full sunshine can make conditions feel up to about 14°F hotter than the shaded estimate.
When not to rely on a heat index result alone
Heat index is a weather screening value, not a measurement of core body temperature or a complete worksite heat assessment. It does not directly model radiant heat from the sun or machinery, heavy protective clothing, metabolic workload, or limited air movement.
Use official local warnings and an appropriate occupational or medical protocol when consequences are serious. Stop activity and seek qualified help for symptoms of heat illness rather than waiting for a calculator category to change.
Heat index calculator questions about humidity and sunlight
Why is heat index sometimes lower than air temperature?
At lower temperatures or very dry conditions, the simplified apparent-temperature estimate can fall slightly below air temperature. The Rothfusz regression is reserved for hotter screened conditions.
Does wind affect heat index?
The published equation assumes light wind. Strong airflow, clothing, activity, sunlight, and hydration can all change how conditions feel.
Can heat index be used indoors?
The arithmetic can be applied indoors, but the assumptions and danger categories were developed for environmental heat exposure and may not capture every indoor condition.