How the Heat Index Calculator Works
The heat index, also called the feels-like temperature in hot weather, tells you how hot it feels to the body when the humidity is added to the air temperature. Your body cools itself by sweating, and sweat cools you only when it can dry. When the air is humid, sweat dries slowly, so you feel hotter than the thermometer shows. The Heat Index Calculator uses the method of the US National Weather Service (NWS).
What you enter
- The air temperature, in degrees Fahrenheit or Celsius. It should be the temperature in the shade.
- The relative humidity, from 0% to 100%.
The steps
1. The simple formula. This is a quick estimate, which works well in mild weather:
HI = 0.5 x (T + 61 + (T - 68) x 1.2 + RH x 0.094)
2. The regression. If the average of that result and the air temperature is 80°F or more, the heat index is worked out again with the Rothfusz regression, which has nine terms and was fitted to the results of a model of how the body loses heat:
HI = -42.379 + 2.04901523 T + 10.14333127 RH
- 0.22475541 T RH - 0.00683783 T x T - 0.05481717 RH x RH
+ 0.00122874 T x T RH + 0.00085282 T RH x RH
- 0.00000199 T x T RH x RH
3. The corrections. In two special cases, a small change is made to the result:
- Very dry air (humidity under 13%, air 80°F to 112°F): subtract ((13 - RH) / 4) x the square root of ((17 - |T - 95|) / 17).
- Very humid air (humidity over 85%, air 80°F to 87°F): add ((RH - 85) / 10) x ((87 - T) / 5).
T is the air temperature in °F and RH is the relative humidity in percent. A temperature in Celsius is changed to Fahrenheit first, and the answer is changed back.
Worked example: 90°F and 70% humidity
| Step | Result |
|---|---|
| Simple formula | 0.5 x (90 + 61 + 26.4 + 6.58) = 91.99 °F |
| Average with the air (91.99 + 90) / 2 | 91.0 (80 or more, so the regression is used) |
| Rothfusz regression | 105.9 °F |
| Difference from the air | +15.9 °F |
| Level | Danger (103 to 124 °F) |
The default of 85°F and 70% humidity gives 92.7 °F, which is extreme caution.
The danger levels of the NWS
| Heat index | Level | What can happen |
|---|---|---|
| Below 80°F (27°C) | Little risk | Heat is not a special risk for most people |
| 80°F to 89°F (27°C to 32°C) | Caution | Fatigue is possible with long exposure or hard activity |
| 90°F to 102°F (32°C to 39°C) | Extreme caution | Heat cramps and heat exhaustion are possible |
| 103°F to 124°F (39°C to 51°C) | Danger | Heat cramps and heat exhaustion are likely, and heat stroke is possible |
| 125°F (52°C) or more | Extreme danger | Heat stroke is highly likely |
- Heat index: Below 80°F (27°C)
Level: Little risk
What can happen: Heat is not a special risk for most people - Heat index: 80°F to 89°F (27°C to 32°C)
Level: Caution
What can happen: Fatigue is possible with long exposure or hard activity - Heat index: 90°F to 102°F (32°C to 39°C)
Level: Extreme caution
What can happen: Heat cramps and heat exhaustion are possible - Heat index: 103°F to 124°F (39°C to 51°C)
Level: Danger
What can happen: Heat cramps and heat exhaustion are likely, and heat stroke is possible - Heat index: 125°F (52°C) or more
Level: Extreme danger
What can happen: Heat stroke is highly likely
Chart: the danger scale
Each block is about 2°F, from 70°F to 140°F. The calculator draws this scale with a marker at your heat index.
70°F ███████████████████████████████████ 140°F gray under 80 | yellow 80-89 | orange 90-102 | red 103-124 | dark red 125 and more
Table: the heat index (°F) for air temperature and humidity
| Air | 40% | 50% | 60% | 70% |
|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 |
| 85°F | 84 | 86 | 89 | 93 |
| 90°F | 91 | 95 | 100 | 106 |
| 95°F | 99 | 105 | 113 | 123 |
| 100°F | 109 | 118 | 129 | 143 |
Chart: how much hotter it feels at 70% humidity
Each block is 2°F above 70°F. The colors are the danger levels.
80°F ██████ 83.0 °F 85°F ███████████ 92.7 °F 90°F ██████████████████ 105.9 °F 95°F ██████████████████████████ 122.6 °F
The difference between the air temperature and the heat index gets bigger as it gets hotter. At 95°F and 70% humidity, it feels about 28°F hotter than the thermometer says. The regression was made for the range of the NWS chart, so values far above it, such as 105°F with high humidity, are not reliable.
Staying safe in the heat
- Drink water often, before you feel thirsty.
- Rest in a cool place and take breaks from hard work. Do the hard work in the morning or the evening.
- Wear light, loose clothes, and use a hat in the sun.
- Never leave a child or a pet in a car. The inside of a car heats up very fast.
- Watch for warning signs: heavy sweating, dizziness, headache, nausea, or hot, dry skin with a high temperature. These need a doctor at once.
Limits of the heat index
- It is for shade and light wind. Direct sun can add up to 15°F (8°C) to how hot it feels.
- It is a model for a typical adult. Children, older people and people with health problems are at more risk, and so are people who are not used to the heat.
- It is not accurate in mild weather, and the regression is best for 80°F or more and 40% humidity or more. Below 80°F, the calculator uses the simple formula.
- It does not include wind, the sun or the clothes you wear, which the Wet Bulb Globe Temperature (WBGT) does.
This tool gives an estimate for general information. It is not medical advice. In a heat emergency, call your local emergency number.