'Feels Like' Temperature: Wind Chill vs Heat Index Explained
Wind chill measures how cold it feels with wind; heat index how hot with humidity. Learn when each applies, danger thresholds, and why thermometers miss it.
Your phone's weather app shows one temperature and a second one in parentheses: "feels like". That second number is a wind chill in winter and a heat index in summer — two different formulas answering the same question your skin already knows the answer to: how hard is it for your body to hold 37°C right now?
What each measures
- Wind chill — how quickly cold air removes heat from exposed skin when the wind is blowing. Applies at cold temperatures with wind. Below ~10°C (50°F).
- Heat index — how hot the air actually feels when humidity blocks sweat evaporation. Applies at hot temperatures with humidity. Above ~27°C (80°F).
They never overlap. Above about 10°C with no wind, or hot without humidity, "feels like" ≈ actual temperature.
Why a thermometer misses it
A thermometer measures the air. Your body cares about the *rate at which heat leaves you* (in cold) or the *rate at which sweat can cool you* (in heat). Wind accelerates heat loss; humidity blocks evaporative cooling. Neither shows up on the mercury.
Wind chill — cold + wind
The NWS wind chill formula (in °F, wind mph) is:
Wind chill = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16
You don't need to memorize it — the Wind Chill Calculator does the arithmetic. What matters is the frostbite window:
| Air temp | Wind | Feels like | Frostbite risk (exposed skin) |
|---|---|---|---|
| 20°F | 10 mph | 9°F | Low |
| 0°F | 15 mph | −19°F | 30 minutes |
| −10°F | 25 mph | −37°F | 10 minutes |
| −20°F | 30 mph | −53°F | 5 minutes |
Frostbite time is more important than the number itself. Below −20°F feels-like, cover every square inch.
Heat index — hot + humid
The heat index (Rothfusz regression) combines air temperature and relative humidity into an apparent temperature. Rough thresholds from the NWS:
| Heat index | Category | Risk |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure |
| 90–103°F | Extreme Caution | Heat cramps and exhaustion possible |
| 103–124°F | Danger | Heat stroke likely with continued activity |
| 125°F+ | Extreme Danger | Heat stroke imminent |
Humidity is the amplifier. At 95°F actual air temperature, the heat index jumps from ~95°F at 40% humidity to about 131°F at 90% humidity — the difference between "hot day" and "medical emergency in an hour." Run scenarios through the Heat Index Calculator.
The other cousin: dew point
If humidity feels abstract, dew point is the cleaner human-comfort number: the temperature at which air becomes saturated. Above 65°F (18°C) dew point feels muggy; above 70°F (21°C) feels oppressive; 75°F+ (24°C) is tropical. Meteorologists watch dew point more than relative humidity because it tracks how it *feels* without depending on the current air temperature. The Dew Point Calculator converts between the three.
Practical rules
- Cold work outdoors: dress for feels-like, not air temperature. A 20°F day with a 25-mph wind is a −5°F day for your face.
- Summer exercise: if heat index passes 100°F, cut intensity or move indoors. Above 105°F, postpone.
- Dry heat vs humid heat: 100°F in Phoenix (dew point 40°F) is genuinely more manageable than 92°F in Miami (dew point 76°F). Sweat evaporates in the first case; it doesn't in the second.
FAQ
What is the difference between wind chill and heat index? Wind chill applies in cold conditions with wind and measures how fast you lose heat. Heat index applies in hot conditions with humidity and measures how hard it is to cool through sweat. They never overlap.
Why does 90°F feel worse when it's humid? Because your body cools primarily by evaporating sweat, and high humidity slows or stops that evaporation. The same 90°F at 30% humidity is comfortable; at 90% humidity it can push the heat index over 120°F.
At what wind chill does frostbite happen? At a wind chill of around −18°F (−28°C), frostbite on exposed skin can occur within 30 minutes. At −40°F feels-like, it's about 10 minutes; at −55°F, about 5 minutes.
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*Check current conditions fast with the free Wind Chill and Heat Index calculators — or use Dew Point for a single humidity-comfort number.*