What Wind Chill Measures
Wind chill is not a change in air temperature. The thermometer reads the same whether the air is still or moving. What changes is the rate at which your body loses heat.
Still air trapped against your skin forms an insulating boundary layer warmed by your own body. Wind strips that layer away and replaces it with cold air, so heat leaves faster. Wind chill expresses that faster heat loss as the temperature that would produce the same loss in calm conditions.
The Formula
Twc = 35.74 + 0.6215T − 35.75V0.16 + 0.4275T × V0.16
where T is air temperature in Fahrenheit and V is wind speed in miles per hour. The metric form is:
Twc = 13.12 + 0.6215T − 11.37V0.16 + 0.3965T × V0.16
with T in Celsius and V in kilometres per hour. This is the joint standard adopted by the United States and Canada in 2001, replacing a 1945 formula based on how fast water froze in plastic cylinders. The current version models heat loss from a human face at 1.5 metres above ground, walking at 3 mph into the wind.
When It Applies
Wind chill is defined only for temperatures at or below 50°F (10°C) and wind speeds above 3 mph (5 km/h). Outside those bounds the calculation is not meaningful, which is why the calculator declines to produce a number.
Note the exponent of 0.16 on wind speed. Because it is well below 1, the effect saturates: going from calm to 10 mph makes a large difference, while going from 40 to 50 mph makes very little.
Wind Chill Chart
Feels-like temperature in °F:
| Wind (mph) | 40°F | 30°F | 20°F | 10°F | 0°F | −10°F | −20°F |
|---|---|---|---|---|---|---|---|
| 5 | 36 | 25 | 13 | 1 | −11 | −22 | −34 |
| 10 | 34 | 21 | 9 | −4 | −16 | −28 | −41 |
| 15 | 32 | 19 | 6 | −7 | −19 | −32 | −45 |
| 20 | 30 | 17 | 4 | −9 | −22 | −35 | −48 |
| 25 | 29 | 16 | 3 | −11 | −24 | −37 | −51 |
| 30 | 28 | 15 | 1 | −12 | −26 | −39 | −53 |
| 40 | 27 | 13 | −1 | −15 | −29 | −43 | −57 |
| 50 | 26 | 12 | −3 | −17 | −31 | −45 | −60 |
Frostbite Risk
| Wind chill | Time to frostbite on exposed skin |
|---|---|
| Above −15°C (5°F) | Low risk |
| −15 to −28°C | 30 minutes or more |
| −28 to −40°C | 10 to 30 minutes |
| −40 to −48°C | 5 to 10 minutes |
| −48 to −55°C | 2 to 5 minutes |
| Below −55°C | Under 2 minutes |
These are for dry, exposed, uncovered skin. Wet skin freezes considerably faster, and wind speeds are measured in the open — a gap between buildings can channel wind well above the forecast figure.
What Wind Chill Does Not Do
- It does not cool objects below air temperature. A parked car, a water pipe or a bottle of milk will cool faster in wind, but never below the actual air temperature. Wind chill applies only to bodies generating their own heat.
- It assumes bare skin. Clothing changes the picture entirely; a windproof outer layer removes most of the effect.
- It ignores sunshine. Direct sun can make conditions feel 5–10°F warmer than the wind chill suggests.
- It ignores humidity. Damp cold feels worse than dry cold at the same reading, and the formula does not capture that.
Staying Safe in the Cold
- Layer. A wicking base layer, an insulating middle layer and a windproof shell outperform one thick coat.
- Cover the extremities. Fingers, toes, ears, nose and cheeks freeze first because blood flow to them is cut back to protect the core.
- Stay dry. Wet clothing loses most of its insulating value and accelerates heat loss dramatically.
- Watch for warning signs. Numbness, waxy white or grey skin and clumsiness signal frostbite or hypothermia and need immediate warmth and medical attention.
- Avoid alcohol. It dilates surface blood vessels, which feels warm and increases heat loss.
Frequently Asked Questions
Why is there no wind chill above 50°F?
Because the formula was fitted to cold conditions and heat loss above that point is dominated by other factors. In warm weather the equivalent measure is the heat index, which adds humidity rather than wind.
Does wind chill affect my car or pipes?
Only the speed at which they reach air temperature, never the final temperature. A pipe cannot freeze from wind chill unless the actual air temperature is below freezing.
Why do old wind chill values look colder?
The pre-2001 formula overstated the effect substantially. A 1970s chart might show −40 where the current formula gives −22 for the same conditions.
Does wind chill change how fast I lose body heat indoors?
Only if there is moving air. A fan in a cold room produces a real wind chill effect on your skin.