What the heat index tells you
The heat index is the temperature it feels like when humidity is added to the heat. At 90 °F and 60% relative humidity, for example, it feels like about 100 °F, which the NWS puts in the "extreme caution" level. Your body cools itself by sweating, and the sweat cools you only when it evaporates. In humid air, less of it evaporates, so you feel hotter. In very dry air, the heat index can even come out below the air temperature. NWS Amarillo gives the example of 100 °F at 15% humidity, which feels like 96 °F (NWS Amarillo, What is the heat index?).
How the heat index is calculated
This calculator uses the method published by the NWS Weather Prediction Center. All values are in °F and percent; for °C, the temperature is converted first and the result is converted back.
- First the simple formula: HI = 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094). If the result is below 80 °F, that is the heat index.
- Otherwise, the regression by Rothfusz (NWS Technical Attachment SR 90-23, 1990) is used.
- If the relative humidity is below 13% and the temperature is between 80 and 112 °F, an adjustment is subtracted. If the humidity is above 85% and the temperature is between 80 and 87 °F, an adjustment is added.
With a dew point instead of humidity, the calculator first finds the relative humidity with the formula from the NWS vapor pressure notes: the vapor pressure e = 6.11 × 10^(7.5 × Td ÷ (237.3 + Td)) is computed for the dew point and for the air temperature (both in °C), and their ratio times 100 is the relative humidity.
Heat index levels
The NWS sorts heat index values into four levels by the likelihood of heat disorders with prolonged exposure or strenuous activity. The descriptions are quoted from the table of NWS Amarillo.
| Level | Heat index | Possible effects |
|---|---|---|
| Caution | 80 to 90 °F | Fatigue possible with prolonged exposure and/or physical activity |
| Extreme caution | 90 to 103 °F | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity |
| Danger | 103 to 124 °F | Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity |
| Extreme danger | 125 °F or higher | Heat stroke highly likely |
The published ranges share their end points. The calculator rounds the heat index to a whole degree and keeps 90 °F in caution and 103 °F in extreme caution, the way the NWS chart colors most of those cells.
Limits of the heat index
- It assumes shade. The NWS says the values were devised for shady, light wind conditions, and full sunshine can add up to 15 °F (NWS, Heat Index).
- Extremes are outside its range. The WPC notes that the Rothfusz regression is not valid for extreme temperature and humidity beyond the range of the data Steadman used. The NWS chart stops at 137 °F.
- It is an approximation. NWS Amarillo gives the regression an error of ±1.3 °F, so a reading of 102 and one of 104 are not far apart.
How to use the heat index calculator
- 1Pick °F or °C and enter the air temperature, measured in the shade. You can check a conversion with the temperature converter.
- 2Enter the relative humidity, or switch to "Dew point" and enter that.
- 3Read the heat index, its NWS level and the nearest cell on the chart. Copy the result or a link. In cold, windy weather, the wind chill calculator does the same for wind.
Frequently asked questions
What is the heat index?
The heat index is how hot it feels when the relative humidity is combined with the air temperature. The National Weather Service (NWS) also calls it the apparent temperature. Humid air slows the evaporation of sweat, so your body cools less well and the air feels hotter than the thermometer reads.
How is the heat index calculated?
The NWS uses a multiple regression by Lans P. Rothfusz (1990) with temperature in °F and relative humidity in percent, plus two small adjustments for very dry and very humid air. When conditions would give a heat index below about 80 °F, a simpler formula is used instead. This calculator follows the equation page of the NWS Weather Prediction Center step by step.
What heat index is dangerous?
The NWS lists four levels: caution from 80 to 90 °F, extreme caution from 90 to 103 °F, danger from 103 to 124 °F and extreme danger from 125 °F. NWS Amarillo writes that heat indices meeting or exceeding 103 °F can lead to dangerous heat disorders with prolonged exposure or physical activity in the heat.
Does sunshine raise the heat index?
Yes. The NWS says heat index values were devised for shady, light wind conditions, and exposure to full sunshine can increase them by up to 15 °F. It also warns that strong winds, particularly with very hot, dry air, can be extremely hazardous.
Why does my result differ from the chart by a degree?
The calculator uses the NWS equation, the chart prints values rounded to whole degrees, and the two do not always agree. NWS Amarillo notes that the regression equation has an error of ±1.3 °F. Both are close approximations of the same heat index.
Can I use the dew point instead of humidity?
Yes. Choose "Dew point" and the calculator first works out the relative humidity from the air temperature and dew point with the NWS vapor pressure formula, then the heat index. The dew point cannot be higher than the air temperature.
Is my data sent anywhere?
No. The calculation runs in your browser, nothing is stored or sent.