Wind Chill and Heat Index: How 'Feels Like' Temperatures Are Calculated
October 29, 2026
The air temperature alone does not tell you how hot or cold it feels. In cold weather, wind strips heat from exposed skin faster, so it feels colder. In hot weather, humidity slows the evaporation of sweat, so it feels hotter. Meteorologists express these effects as the wind chill and the heat index, equivalent temperatures that describe the stress on the body.
The US National Weather Service wind chill formula (valid for temperatures at or below 50°F and winds above 3 mph) is: WC = 35.74 + 0.6215 T − 35.75 V^0.16 + 0.4275 T V^0.16, with T in °F and V in mph. For 20°F and a 15 mph wind, WC = 6.2°F.
Skip the math and get your answer instantly:
Open the Temperature Converter →Wind Chill Table
The values are in °F. Wind chill does not change the actual temperature of objects (a car will not freeze below the air temperature), but it affects the rate of heat loss from exposed skin. At a wind chill of −18°F or below, frostbite can occur on exposed skin in 30 minutes; at −35°F, in 10 minutes; and at −55°F, in 5 minutes or less, according to NWS guidance.
| Air temperature (°F) | 5 mph wind | 15 mph wind | 25 mph wind |
|---|---|---|---|
| 30 | 25 | 19 | 16 |
| 20 | 13 | 6 | 3 |
| 10 | 1 | -6 | -10 |
| 0 | -11 | -19 | -24 |
| -10 | -22 | -32 | -37 |
Heat Index Formula and Table
The heat index combines temperature T (in °F, at least about 80°F) and relative humidity R (in percent) using the Rothfusz regression: HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 T R − 0.00683783 T² − 0.05481717 R² + 0.00122874 T² R + 0.00085282 T R² − 0.00000199 T² R². At 90°F and 60% humidity, HI ≈ 100°F. Heat index values assume shade and light wind; direct sunlight can add up to 15°F. The NWS categorizes 80°F to 90°F as caution (fatigue possible), 90°F to 103°F as extreme caution, 103°F to 124°F as danger (heat cramps and exhaustion likely), and above 125°F as extreme danger (heat stroke highly likely).
| Air temperature (°F) | 40% humidity | 60% humidity | 80% humidity |
|---|---|---|---|
| 85 | 84 | 89 | 97 |
| 90 | 91 | 100 | 113 |
| 95 | 99 | 113 | 134 |
| 100 | 109 | 129 | 158 |
Staying Safe
In the cold, cover exposed skin, dress in layers, keep dry, and limit time outdoors when the wind chill is dangerous. Watch for signs of frostbite (numbness, pale or waxy skin) and hypothermia (shivering, confusion, drowsiness). In the heat, drink water, rest in the shade or air conditioning, avoid strenuous activity during the hottest hours, and never leave children or pets in vehicles. Check on elderly neighbors and those without cooling. Signs of heat exhaustion include heavy sweating, weakness, nausea, and a rapid pulse; heat stroke, with confusion and hot, dry skin, is an emergency.
Limits of the Formulas
The formulas are built from models of an average adult walking outdoors and from experiments, and they are not accurate in every situation. They do not account for clothing, activity level, acclimatization, sun, or individual susceptibility. The heat index regression is not valid below 80°F or in very dry or very humid extremes without adjustments. Other indices, such as the wet bulb globe temperature (WBGT), are used for athletes and workers because they include radiant heat and wind. Use the numbers as guides, and rely on how you feel.
Frequently Asked Questions
What is wind chill?
An index of how cold it feels on exposed skin when wind speeds up heat loss.
What is the heat index?
A measure of how hot it feels when humidity is combined with air temperature.
At what wind chill does frostbite occur?
Around −18°F or colder, frostbite can occur on exposed skin within 30 minutes.
Does wind chill apply to objects?
No. It applies to heat loss from living skin, not to the temperature of inanimate objects.
Does direct sunlight change the heat index?
Yes, it can add up to about 15°F to the feels-like temperature.