Climate & Environment

Heat Index Calculator

What this does

Turn temperature and relative humidity into the NWS heat index; the “feels like” temperature; with health risk banding.

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Calculator inputs

How to use it

Using the heat index calculator

  1. 01

    Enter the air temperature

    Use the shade temperature in °F of at least 80°F. The heat index is not defined below that.

  2. 02

    Add relative humidity

    Weather apps and station observations report RH directly. Anything outside 0–100% is rejected.

  3. 03

    Act on the risk band

    Caution means pace yourself; extreme caution calls for shade and water; danger and above call for moving activity indoors or cancelling.

Good to know

Why humidity makes heat dangerous

Humans cool themselves by evaporating sweat. When the air is already moisture-laden, evaporation slows and body heat accumulates; the same thermometer reading turns lethal as humidity climbs. The heat index quantifies that combination.

Heat index versus “real feel” marketing

Some weather services blend sun angle, wind and other effects into proprietary feels-like numbers. This calculator implements the official NWS heat index: shade, light wind, humidity only; comparable across sources because it is standardized.

Using the risk bands

Bands map to NWS guidance: caution (80–90°F) warrants pacing; extreme caution (90–103°F) demands hydration and shade; danger (103–124°F) brings likely cramps and exhaustion; above 124°F heat stroke is highly likely without intervention.

How it's calculated

The math behind this calculator

HI = −42.379 + 2.049T + 10.143R − 0.2248TR − 0.00684T² − 0.05482R² + … plus NWS adjustments for very dry (RH<13%) or very humid (RH>85%) air

The heat index comes from the Rothfusz multiple regression of Steadman’s apparent-temperature table, using the standard coefficients the National Weather Service publishes for temperatures of 80°F and above.

Two standard corrections follow: very dry air (below 13% RH) evaporates sweat faster, lowering the index; extremely humid mild heat (above 85% RH between 80–87°F) raises it slightly.

When even the simple screening formula falls below 80°F, humidification of the air cannot meaningfully impair cooling and the calculator returns the plain temperature with a note instead of an inflated figure.

Assumptions & limitations

  • Valid for air temperatures ≥ 80°F; below that the concept is not defined.
  • Shade conditions and light winds assumed; full sun can add up to 15°F.
  • Relative humidity must lie between 0% and 100%.
  • Individual heat tolerance varies with acclimatisation, hydration, age and health.

Worked example

A steamy 90°F afternoon at 70% relative humidity feels closer to 106°F; firmly inside the “danger” band where heat cramps and exhaustion become likely with exertion.

FAQ

Frequently asked questions

Why won’t it compute below 80°F?
The NWS regression was fitted for hot conditions; below 80°F humidity barely impairs the body’s cooling and the index loses meaning. The calculator says so rather than returning a misleading number.
Does sun exposure change the result?
Yes; the official index assumes shade. In direct sunlight, effective heat stress can run up to 15°F higher than the calculated value.
What if humidity reads over 100%?
That is physically impossible for relative humidity, so inputs outside 0–100% are rejected. Supersaturated foggy air still reports at or near 100%.
Is the heat index valid at very low humidity?
Only with the NWS dry-air adjustment, which this calculator applies automatically below 13% RH for temperatures between 80°F and 112°F.

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