1
Choose your mode — select Auto to let the calculator decide (wind chill below 50°F, heat index above 80°F), or manually force Wind Chill or Heat Index mode.
2
Pick your units — toggle between °F/°C for temperature and mph/km/h for wind speed. All conversions happen automatically.
3
Enter conditions — type in air temperature and either wind speed (wind chill) or relative humidity (heat index). Results update instantly.
4
Read the danger level — the colored card shows your NWS danger tier and health risk. Check the Condition Matrix tab for a full range view, or the Clothing & Safety Guide for practical advice.

NWS Wind Chill (2001)

WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16

T = air temp °F · V = wind speed mph · valid for T ≤ 50°F and V > 3 mph

Rothfusz Heat Index (NWS)

HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541TR − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH²

T = air temp °F · RH = relative humidity % · valid for T ≥ 80°F and RH ≥ 40%

Wind Chill The perceived decrease in air temperature felt on exposed skin due to wind. Moving air accelerates heat loss from your body, making it feel colder than the actual air temperature.

Heat Index The "feels like" temperature combining air temperature and relative humidity. High humidity reduces sweat evaporation, impairing the body's ability to cool itself and making it feel hotter than the actual temperature.

Relative Humidity The amount of water vapor in the air relative to the maximum it can hold at that temperature, expressed as a percentage. At 100% RH, air is saturated and evaporation is essentially impossible.

Dew Point The temperature at which air becomes saturated and dew forms. Dew points above 60°F feel humid; above 70°F feel oppressive. Dew point is a more absolute measure of moisture than relative humidity.

Apparent Temperature A general term for the subjective experience of temperature, taking into account humidity, wind, and solar radiation. Wind chill and heat index are both types of apparent temperature.

Frostbite Freezing of skin and underlying tissue, typically affecting extremities (fingers, toes, nose, ears). Risk increases sharply when wind chill drops below −18°F (−28°C). Cover all exposed skin in these conditions.

🥶

Cold Day with Wind

Air temp: 5°F · Wind speed: 20 mph

WC = 35.74 + 0.6215(5) − 35.75(20^0.16) + 0.4275(5)(20^0.16)
20^0.16 ≈ 1.7411
WC = 35.74 + 3.11 − 62.25 + 3.72 ≈ −19.7°F

Feels like −20°F — Frostbite in 30 minutes on exposed skin. Full face and extremity protection required.

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Hot Humid Day

Air temp: 95°F · Humidity: 80%

Rothfusz polynomial with T=95, RH=80:
HI ≈ 133°F (after adjustments)
NWS tier: Extreme Danger

Feels like 133°F — Extreme Danger. Heat stroke highly likely. Avoid all outdoor exertion. Seek air conditioning immediately.

The Body's Cooling System

The human body maintains its core temperature of 98.6°F through sweating. When sweat evaporates from skin, it carries heat away. In dry conditions, a 95°F day is uncomfortable but manageable — sweat evaporates quickly, cooling the skin effectively. In humid conditions, the air is already saturated with moisture, so sweat cannot evaporate. The cooling mechanism fails, and body temperature rises.

The 95°F / 80% Humidity Paradox

At 95°F with 80% relative humidity, the NWS heat index is approximately 133°F — 38 degrees hotter than the actual temperature. This is because at 80% humidity, your body must work 5–6 times harder to shed the same amount of heat as it would in dry air. The Rothfusz polynomial captures this non-linear relationship between temperature and humidity that makes muggy days genuinely life-threatening for elderly people, those with cardiovascular disease, and anyone doing physical labor outdoors.

Why the NWS Formula Uses a Polynomial

Simple temperature-humidity combinations were studied by R.G. Steadman in 1979, leading to the first heat index tables. The NWS adopted a regression equation developed by Rothfusz in 1990 that fits Steadman's original data with nine polynomial coefficients. Two adjustments were added: one for very low humidity (below 13% with temps 80–112°F), and one for high humidity (above 85% with temps 80–87°F). These correction terms address edge cases where the polynomial overestimates or underestimates perceived temperature.

Wind Chill: The Other Extreme

Wind chill was reformulated by the NWS in 2001 using a model based on a human face at walking speed (3 mph). The old formula from the 1940s (Siple-Passel) was based on water freezing on a cylinder and dramatically overstated the cooling effect at low wind speeds. The modern NWS formula adds a correction for solar radiation and better matches physiological heat loss. Key insight: wind does not lower the actual air temperature — it accelerates heat loss from your body. At calm wind, wind chill equals air temperature. Speeds above 40 mph add diminishing returns; going from 40 to 60 mph only changes wind chill by a few degrees.

What's the difference between wind chill and actual temperature?

Wind chill is the perceived temperature on exposed skin — not the actual air temperature. Wind chill can be well below 0°F when the thermometer reads 20°F, but a parked car's engine, metal surfaces, and pipes will only cool to the actual air temperature, not the wind chill temperature. Wind chill only affects heat loss from warm-blooded surfaces.

When does the NWS use heat index vs. wet-bulb temperature?

The NWS issues public heat advisories using heat index because it is intuitive (a "feels like" temperature in °F). Wet-bulb temperature is used by meteorologists and physiologists to assess the absolute human survivability limit — around 95°F wet-bulb (about 35°C), at which point even a healthy person in the shade cannot survive more than a few hours. Heat index values above 130°F approach or exceed this survivability threshold.

Why does the calculator show "out of range" sometimes?

The NWS wind chill formula is only valid when the air temperature is 50°F or below and wind speed exceeds 3 mph. The heat index formula is only valid when temperature is 80°F or above and relative humidity is at least 40%. Outside these ranges, the formulas produce inaccurate results. The calculator shows actual temperature instead of a meaningless calculated value.

Does wind chill affect how quickly water or pipes freeze?

No. Wind chill only describes how fast exposed human (or animal) skin loses heat to the environment. Inanimate objects will cool to the actual air temperature — no colder. A pipe will freeze when the air temperature drops to 32°F or below, regardless of wind speed. However, pipes exposed to wind may reach that freezing point faster, since wind accelerates heat loss from warm surfaces.

What humidity level is dangerous with heat?

At 90°F, a humidity of 90% produces a heat index of about 122°F — in the Danger zone. Even moderate temperatures become dangerous: 85°F at 100% humidity yields a heat index of around 108°F (Danger). As a general rule, dew points above 70°F (which corresponds to roughly 80%+ humidity at 85–90°F) are hazardous for outdoor physical activity, especially for the elderly and those with heart conditions.

How accurate is the NWS heat index formula?

The Rothfusz polynomial is accurate within ±1.3°F for temperatures between 80°F and 110°F and humidity between 40% and 100%. The two Steadman adjustment terms improve accuracy at the edges of that range. At extremes (110°F+, very high humidity), the formula's accuracy decreases, but the values serve as valid worst-case indicators for issuing safety alerts.

Wind Chill & Heat Index Calculator

Clothing Recommendation

Activity & Exercise

Pet Safety

Car & Vehicle Tips

How to Use This Calculator

Official NWS Formulas

Key Terms

Worked Examples

Cold Day with Wind

Hot Humid Day

Why Humidity Makes Heat More Dangerous Than Dry Heat

The Body's Cooling System

The 95°F / 80% Humidity Paradox

Why the NWS Formula Uses a Polynomial

Wind Chill: The Other Extreme

Frequently Asked Questions

Wind Chill & Heat Index Calculator

Clothing Recommendation

Activity & Exercise

Pet Safety

Car & Vehicle Tips

How to Use This Calculator

Official NWS Formulas

Key Terms

Worked Examples

Cold Day with Wind

Hot Humid Day

Why Humidity Makes Heat More Dangerous Than Dry Heat

The Body's Cooling System

The 95°F / 80% Humidity Paradox

Why the NWS Formula Uses a Polynomial

Wind Chill: The Other Extreme

Frequently Asked Questions

Related Calculators