Feels Like Temperature

Calculating the “Feels Like” Temperature

The air temperature alone doesn’t tell you how weather actually feels on your skin — wind strips heat away faster in the cold, and humidity slows your body’s ability to cool itself in the heat. This calculator uses the two official U.S. National Weather Service formulas for “feels like” temperature: Wind Chill for cold, windy conditions, and Heat Index for hot, humid ones.

Both formulas are only meaningful at the temperature range they were built for — wind chill applies at 50°F and below, heat index applies at 80°F and above. Outside those ranges, wind and humidity have a negligible effect on perceived temperature, so the “feels like” value is just the actual air temperature.

The Formula

• Wind Chill (NWS 2001 formula): $35.74 + 0.6215\vA{T} - 35.75(\vB{V}^{0.16}) + 0.4275\vA{T}(\vB{V}^{0.16})$, where $\vA{T}$ is air temperature in °F and $\vB{V}$ is wind speed in mph. Only applies at 3 mph or faster — below that, there’s not enough moving air to meaningfully affect the perceived temperature.
• Heat Index (the Rothfusz regression): a more complex polynomial in temperature ($\vA{T}$) and relative humidity ($\vC{R}$), both in °F/percent — the official NWS approximation used by virtually every weather service and app.

Worked Example

Wind Chill: at -5°F with a 15 mph wind, the formula gives a wind chill of approximately -25.8°F — meaning exposed skin loses heat about as fast as it would in still air at -25.8°F.

Heat Index: at 90°F with 70% relative humidity, the formula gives a heat index of approximately 105.6°F — a widely-cited example from the NWS’s own heat index chart.

Key Factors to Consider

• Both figures describe risk, not just comfort — extreme values carry real health warnings. The National Weather Service issues formal advisories and warnings at specific wind chill and heat index thresholds (for frostbite risk on the cold side, and heat exhaustion/heat stroke risk on the hot side) — these aren’t just “feels uncomfortable” numbers, they’re used operationally for public safety alerts.
• Direct sunlight can make actual conditions feel meaningfully hotter than heat index alone suggests. Heat index describes shaded conditions — full sun exposure can add several more degrees to how hot conditions actually feel, which is why some weather services separately note a sun-exposure adjustment beyond the base heat index.
• Wind chill only describes air temperature effects on exposed skin, not overall body heat loss. Wet clothing, body size, and physical exertion all affect how quickly a person’s body actually loses heat in cold, windy conditions beyond what the wind chill formula alone captures.
• These formulas are empirically derived approximations, not exact physical laws. Both the NWS wind chill and heat index formulas were built by fitting equations to experimental and observational data within their respective valid ranges — they’re the official standard used by weather services, but like any model, they’re an approximation of a genuinely complex physiological response.

Common Mistakes

• Using wind chill or heat index outside their valid temperature range. As covered above, wind chill only applies at 50°F and below, and heat index only applies at 80°F and above — plugging in a mild, in-between temperature to either formula produces a number with no real meaning, since neither formula was calibrated for that range.
• Applying wind chill below 3 mph of wind. The wind chill formula assumes at least light moving air — below roughly 3 mph, there isn’t enough airflow to meaningfully strip heat away faster than still air, so a “wind chill” calculated at near-zero wind speed doesn’t reflect a real physical effect.
• Treating the heat index as accurate for full sun exposure. As noted above, heat index is calculated for shaded conditions — direct sunlight can add several more degrees to how hot it actually feels, so a heat index reading understates real heat risk for anyone working or exercising outdoors in direct sun.
• Ignoring wind chill/heat index advisories as just a comfort number. As covered above, the National Weather Service issues real safety warnings at specific thresholds on both ends of this scale — treating an extreme reading as merely “uncomfortable” rather than a genuine frostbite or heat-illness risk misses the point of why these formulas exist.

Useful to Know

• Both formulas were fitted to experimental and observational data rather than derived from pure physics, so a result a degree or two off from another source’s calculator is normal — differences usually come from small variations in implementation, not a wrong formula.
• Since wind chill and heat index apply at opposite ends of the temperature scale, a given set of conditions will only ever fall into one formula’s valid range or the other — there’s no in-between “feels like” calculation for a temperate, calm day, because wind and humidity barely affect perception there anyway.
• This calculator pairs naturally with the Dew Point Calculator (a more direct measure of how much moisture is in the air) and the BTU Calculator (for sizing heating or cooling equipment against real, not just nominal, temperature extremes).