Wind Chill vs "Feels Like": How the US Measures It
Your app's "feels like" number is really one of two different NWS formulas — wind chill when it's cold, heat index when it's hot.
"Feels like" is an umbrella term for apparent temperature — how hot or cold the air feels to your body. In the United States the National Weather Service calculates it two ways: wind chill in cold weather (air temperature plus wind, at or below 50°F) and heat index in hot weather (temperature plus humidity, at or above 80°F). "Feels like" simply reports whichever formula applies to current conditions.
What is "feels like" temperature?
A thermometer measures the air, but your comfort depends on how fast your body loses or gains heat. Wind strips warmth away from exposed skin in the cold; humidity slows the sweat evaporation that cools you in the heat. Apparent temperature translates those effects into a single number on the same Fahrenheit scale, so "feels like 5°" or "feels like 105°" is an estimate of the thermal stress you'd actually experience.
How does the NWS calculate wind chill?
The current Wind Chill Temperature index has been in use since November 2001. It's based on a model of heat loss from the human face and uses wind speed measured at about 5 feet — average face height — rather than the standard 33-foot anemometer height. It applies only when the air temperature is at or below 50°F and winds are above 3 mph.
The official formula is:
Wind Chill (°F) = 35.74 + 0.6215T − 35.75(V0.16) + 0.4275T(V0.16)
where T is air temperature in °F and V is wind speed in mph. One important limit: wind chill describes how quickly you lose heat, not the actual air temperature. Wind cannot cool an object below the true air temperature — it just gets there faster. The NWS wind chill chart also flags how quickly exposed skin can suffer frostbite at dangerous combinations of cold and wind.
How does the NWS calculate heat index?
The heat index combines air temperature and relative humidity using the Rothfusz regression, and applies when the temperature is at or above 80°F and relative humidity is at or above 40%. Your body cools itself by sweating; when humidity is high, sweat evaporates more slowly, so you feel hotter than the thermometer reads. That's why 90°F in a humid climate can feel like 105°F or worse.
Wind chill vs heat index at a glance
| Wind chill | Heat index | |
|---|---|---|
| When it applies | Temp ≤ 50°F, wind > 3 mph | Temp ≥ 80°F, RH ≥ 40% |
| Inputs | Temperature + wind speed | Temperature + relative humidity |
| Effect on the body | Speeds heat loss from skin | Slows sweat evaporation (cooling) |
| Main danger | Frostbite, hypothermia | Heat exhaustion, heat stroke |
| Direction | Feels colder than air temp | Feels hotter than air temp |
What do the formulas leave out?
Both indices are deliberately simplified. The heat index is calculated for shady, light-wind conditions — direct sunshine can add as much as 15°F to the value the formula produces, which is why standing in full sun feels dramatically worse than the posted number. The wind chill index, meanwhile, assumes a clear night and doesn't account for solar warming either, so a sunny, windy winter day can feel a touch less brutal than the figure suggests. Neither formula knows how you're dressed, how active you are, or how well-hydrated you are — all of which change how the same conditions actually affect your body.
This is why the apparent temperature is a guide, not a verdict. Use it to gauge relative risk and to decide how to dress, but pay attention to sun exposure and your own exertion on top of the number.
Why does the "feels like" number matter for safety?
Apparent temperature is the figure the NWS ties its cold- and heat-safety guidance to, because it tracks the actual stress on your body better than the raw air temperature does. Dangerous wind chill values drive the risk of frostbite and hypothermia; dangerous heat index values drive heat exhaustion and heat stroke. When the NWS issues cold-weather or extreme-heat alerts, they are keyed to these apparent-temperature thresholds — so the "feels like" reading, not the thermometer, is usually the number that should shape your plans on an extreme day.
Why do apps disagree on "feels like"?
Different apps pull temperature, wind, and humidity from different sources and different underlying forecast models, and some blend in extra factors like sunshine or their own proprietary adjustments. So a 15°F morning can show "feels like 3°" in one app and "feels like 6°" in another. The NWS wind chill and heat index formulas are fixed and public — the disagreement comes from the input data feeding them. Comparing forecast models directly, as we do at checkweather.io, shows you when those inputs are in broad agreement versus when they're a genuine toss-up.
When is there no "feels like" number?
Between roughly 50°F and 80°F — or on a cold day with calm wind — neither formula meaningfully applies, so the apparent temperature is essentially just the air temperature. In that mild range, the thermometer reading already tells the whole story.
It's also why you'll sometimes see a forecast list an air temperature with no separate "feels like" value at all: the conditions simply don't meet the threshold for either index. That's not an error or missing data — it's the forecast telling you the air temperature is the number that matters. When a "feels like" figure does appear, it's a signal that wind or humidity is significant enough to change how the day will actually feel on your skin, and worth factoring into how you dress and how long you stay outside.