A thermometer gives the air's temperature, but your body cares how fast it loses heat. Wind carries heat away faster; humid air slows the cooling of sweat, which ties back to relative humidity. The National Weather Service turns these effects into two "feels like" numbers.
Key idea
Wind chill is the temperature that would cool exposed skin as fast in calm air as the real temperature does in the wind. The heat index is how hot it feels when humidity slows the evaporation of sweat. Both are read from NWS charts in °F and mph. Wind chill applies at 50 °F or below with winds over 3 mph; the heat index is figured for shade, and full sunshine can raise it by up to about 15 °F.
Why it works
Your body is warmer than cold air around it and loses heat into it. In still air, a thin layer of warmed air clings to your skin and insulates it; wind strips that layer away, so heat flows out faster: it feels colder. In heat, your body cools mainly by evaporating sweat, which takes latent heat from your skin. In humid air (a high dew point), sweat evaporates slowly, so you cool less: it feels hotter. Neither effect changes the air's real temperature.
| Air temperature | Wind 10 mph | Wind 20 mph | Wind 30 mph |
|---|---|---|---|
| 30 °F | 21 | 17 | 15 |
| 20 °F | 9 | 4 | 1 |
| 10 °F | −4 | −9 | −12 |
| 0 °F | −16 | −22 | −26 |
Worked example: Reading the wind chill chart
It is 10 °F with a 20 mph wind. What is the wind chill?
- Find the row for 10 °F and the column for 20 mph.
- The chart gives −9 °F: exposed skin cools as fast as it would in still air at −9 °F.
Answer: −9 °F
Worked example: Can the wind freeze the car?
The air is 38 °F with a 25 mph wind, and the wind chill is 27 °F. Can the wind make the water in a parked car's radiator freeze?
- Wind only speeds up cooling toward the air temperature.
- A parked car makes no heat of its own, so the wind brings it down to 38 °F and no lower.
- No: 38 °F is above freezing, so the wind can't freeze the water.
Answer: No
Common mistake
Thinking wind chill lowers the temperature of things. Wind makes warm objects lose heat faster, but it can only bring them down to the air temperature, never below it. A car, a water pipe or a thermometer in the wind settles at the air's temperature, not the wind chill. Check: wind chill matters for people and animals, which make their own heat.
| Air temperature | RH 40% | RH 50% | RH 60% | RH 70% |
|---|---|---|---|---|
| 90 °F | 91 | 95 | 100 | 106 |
| 96 °F | 101 | 108 | 116 | 126 |
| 100 °F | 109 | 118 | 129 | — |