Heat index is the “feels like” temperature from air temperature plus humidity. Enter a dry-bulb temperature in °F, °C, or Kelvin and either relative humidity or dew point to see the National Weather Service estimate and a Caution through Extreme danger category.
Effects of the heat index
| Celsius | Fahrenheit | Notes |
|---|---|---|
| 27–32°C | 80–90°F | Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps. |
| 32–39°C | 90–103°F | Extreme caution: heat stroke, heat cramps, or heat exhaustion are possible with prolonged exposure or activity. |
| 39–52°C | 103–125°F | Danger: heat cramps or heat exhaustion are likely; heat stroke is possible with prolonged exposure or activity. |
| 52°C or higher | 125°F or higher | Extreme danger: heat stroke is highly likely. |
Examples
85°F and 70% humidity (default)
The default inputs match a typical summer afternoon. The Rothfusz value is about 92.7°F, which rounds to 93°F (34°C or 307K) and lands in Extreme caution — heat cramps and heat exhaustion are possible if you keep working or exercising.
83°F and 70% humidity
At 83°F with 70% relative humidity, the calculated heat index is 88°F (31°C or 304K), in the Caution band.
Dew point instead of humidity
The same 85°F air with a 70°F dew point is about 61% humidity, so the heat index is 90°F (32°C or 305K) — still Caution, because the unrounded value sits just under 90°F.
Formula
Air temperature is converted to Fahrenheit (conversion.js). Relative humidity is used as a percent. If you enter dew point instead, humidity is
RH% = 100 × exp( 17.27 Td / (Td + 237.7) − 17.27 T / (T + 237.7) )
with T and Td in Celsius. Then the NWS heat index (°F) is:
simple = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094)
initial = (simple + T) ÷ 2
if initial < 80°F:
HI = initial
else:
HI = −42.379 + 2.04901523 T + 10.14333127 RH
− 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH²
+ 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH²
Dry air (RH under 13% and T between 80 and 112°F) subtracts
((13 − RH) / 4) × √((17 − |T − 95|) / 17). Very humid air (RH above 85%
and T between 80 and 87°F) adds ((RH − 85) / 10) × ((87 − T) / 5).
Displayed °C and K are converted from the unrounded Fahrenheit heat index
(not from the already-rounded °F), then rounded to whole degrees.
This is an educational estimate, not a medical diagnosis. Full sun can add up to 15°F. Follow local alerts and seek emergency help for signs of heat stroke such as confusion, loss of consciousness, or very high body temperature.