Wind Chill and Heat Index Calculator (NWS)
NWS wind chill from air temperature and wind speed, and NWS heat index with its category from temperature and humidity, in °F or °C. How it works
Use the Wind Chill and Heat Index Calculator (NWS)
About the Wind Chill and Heat Index Calculator (NWS)
The NWS wind chill at 20 °F with a 15 mph wind is 6.2 °F, and the NWS heat index at 90 °F with 70% humidity is 105.9 °F, in the Danger category. Wind chill is defined only at 50 °F or below with wind above 3 mph, and the heat index is used from 80 °F up. Outside those ranges the page shows a not-applicable card instead of a number.
NWS wind chill (°F) by air temperature and wind speed
| Air temperature | 5 mph | 10 mph | 15 mph | 20 mph | 30 mph |
|---|---|---|---|---|---|
| 30 °F | 24.7 | 21.2 | 19.0 | 17.4 | 14.9 |
| 20 °F | 13.0 | 8.9 | 6.2 | 4.2 | 1.3 |
| 10 °F | 1.2 | -3.5 | -6.6 | -8.9 | -12.3 |
| 0 °F | -10.5 | -15.9 | -19.4 | -22.0 | -25.9 |
| -10 °F | -22.3 | -28.3 | -32.2 | -35.1 | -39.4 |
Computed with the NWS formula, shown to one decimal as the calculator shows it. Wind chill is defined only at 50 °F or below with wind above 3 mph.
NWS heat index (°F) by air temperature and relative humidity
| Air temperature | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|
| 80 °F | not applicable | 80.8 | 81.8 | 83.0 | 84.2 |
| 85 °F | 84.3 | 86.5 | 89.3 | 92.7 | 96.8 |
| 90 °F | 90.7 | 94.6 | 99.7 | 105.9 | 113.3 |
| 95 °F | 99.0 | 105.2 | 113.1 | 122.6 | 133.8 |
At 80 °F and 40% humidity the NWS check (the simple formula averaged with the air temperature) is 79.8 °F, below 80 °F, so no heat index is given.
NWS heat index categories and the effect text the calculator shows
| Category | Heat index | NWS effect |
|---|---|---|
| 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 under 125 °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. |
Ranges from the NWS heat index page. The page adds a seek-care line for Danger and Extreme Danger.
Assumptions
- NWS heat index values assume shade and light wind.
- The category names and ranges are the NWS ones; the page adds no other bands and shows no frostbite times.
How to use this tool
- Choose imperial (°F, mph) or metric (°C, km/h).
- Type the air temperature, the wind speed and the relative humidity. All three fields must hold a value in range before either card appears; if you only need the wind chill, leave the humidity at its starting value, and the same for the wind when you only need the heat index.
- Read the wind chill card and the heat index card. Each shows either a value or the reason it does not apply.
Frequently asked questions
- What is the wind chill at 0 °F with a 10 mph wind?
- The NWS formula gives -15.9 °F. With the same 10 mph wind it is 8.9 °F at 20 °F and -28.3 °F at -10 °F. In metric, -10 °C with a 20 km/h wind gives -17.8 °C.
- When does the wind chill not apply?
- The NWS defines wind chill only for temperatures at or below 50 °F and wind speeds above 3 mph. At 51 °F, or at 20 °F with a 3 mph wind, the page shows Wind Chill: not applicable, and the air temperature is the figure to go by.
- How is the heat index calculated?
- The NWS first computes a simple formula, 0.5 × (T + 61 + 1.2 × (T − 68) + 0.094 × RH), and averages it with the air temperature. If the average is 80 °F or higher it uses the Rothfusz regression, with a low-humidity adjustment below 13% and a high-humidity adjustment above 85%. The NWS says the regression approximates the heat index to within about 1.3 °F.
- Why does the heat index say not applicable at 75 °F and 80% humidity?
- The simple formula gives 76.0 °F there. Averaged with the 75 °F air temperature that is 75.5 °F, below 80 °F, so the NWS procedure does not use the regression and the page shows no heat index.
- Does the heat index include direct sun?
- No. NWS values assume shade and light wind, and direct sun can raise the heat index by up to 15 °F. At 100 °F and 55% humidity the page shows 123.6 °F, close to the 124 °F the NWS gives for that case.
- Does wind chill apply to my car, pipes or other objects?
- No. The NWS says wind chill applies only to people and animals, and that an inanimate object will not cool below the actual air temperature, so the air temperature is the figure to use for pipes and machinery.
Related Tools
View all toolsDew Point Comfort Zone
Dew point from air temperature and relative humidity by the Magnus formula, with the tool’s own comfort bands from Dry to Very oppressive.
Thermal Mass Calculator
Heat stored in a concrete, brick, stone, adobe, water, steel or custom mass from its area, thickness and temperature rise, in kWh or BTU, for passive solar.
Crosswind Component Calculator
Calculate the crosswind and headwind or tailwind component for a runway from the runway heading, wind direction and wind speed in knots, mph or km/h.
Fuel Endurance Calculator
Aircraft fuel endurance to your reserve and to dry tanks, plus range in still air and in wind, from fuel and fuel flow in gal, L, kg or lb and cruise speed.
Refrigerant Charge Calculator
R-410A line-set charge from nameplate charge, line length and liquid-line size: ounces to add or remove and the total. Other refrigerants need a typed rate.
Specific Heat Calculator
Heat to warm or cool a mass of 16 preset materials, from water to copper, or your own specific heat: Q = m × c × ΔT, in J, kJ, MJ or BTU.