Weather science

Wind chill vs heat index

"Feels like 15°F" or "Feels like 108°F" — both are calculated numbers, not measurements. Here is what they mean, how they're calculated, and where they mislead.

The core idea

Neither wind chill nor heat index measures actual air temperature. Both estimate how the human body loses (or fails to lose) heat under specific conditions — and translate that into an equivalent still-air temperature.

Wind Chill

Wind chill is the perceived temperature effect of wind moving over exposed skin. Faster wind = faster heat loss = colder perceived temperature.

Only applies when actual temp is at or below 50°F.
Requires wind speed above 3 mph.
Assumes bare skin — clothed skin sees less effect.
The 2001 NWS formula replaced an older 1945 wartime formula that overestimated the effect.

Formula: WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16 (T in °F, V in mph).

Heat Index

Heat index estimates how hot it feels when humidity limits evaporative cooling. Sweat cools you by evaporating; humid air slows evaporation.

Only applies when actual temp is at or above 80°F.
Only applies when humidity is above 40%.
Assumes shade — direct sun adds 10-15°F.
Assumes light wind (5 mph).
The formula is a polynomial fit to biometric research from the 1970s.

Simplified: HI = -42.379 + 2.04901523T + 10.14333127R − 0.22475541T·R − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282T·R² − 0.00000199T²R² (T in °F, R in %RH).

Where they mislead

Wind chill assumes exposed skin. In clothing, the effect is much reduced.
Wind chill applies to living tissue — inanimate objects don't "feel" wind chill.
Heat index assumes shade. Direct sun adds a lot.
Heat index assumes fit, healthy adults.
Neither number is measured with a sensor — both are formulas.
Two people in identical conditions can perceive very different temperatures.

The alternatives

WBGT (Wet Bulb Globe Temperature)

Used by the military, athletics, industry. Accounts for humidity, wind, sun. Measured by three-thermometer instrument.

UTCI (Universal Thermal Climate Index)

European standard. Complex biometeorological model.

Apparent Temperature

Australian version. Broader validity range than heat index.

Wet Bulb

Simple humidity-adjusted temperature. Used in survival research.

When to use each

Winter: wind chill for frostbite planning.
Summer: heat index for heat exhaustion planning.
Athletic events: WBGT is the gold standard.
Ordinary cold: check the actual temperature and wear appropriate layers.
Ordinary heat: check the actual temperature, humidity, and sun exposure.

Wet bulb temperature and survivability

The 35°C (95°F) wet bulb threshold is often cited as the survivability limit for humans — above it, we cannot cool ourselves even with unlimited water and shade. As of 2026, small regions of the Persian Gulf and Indus Valley have briefly exceeded this. Climate models predict expansion.