Wind Chill vs Heat Index: Which Feels-Like Number Applies?

What does 91°F mean?

It means the heat index applies at 90°F: the air is above 80°F, so the Rothfusz regression runs, and 90°F at 40% humidity feels like 91°F on exposed skin. The same page at 20°F and 15 mph would say the wind chill applies instead and it feels like 6°F, and at 65°F it would say neither applies and it feels like 65°F. The answer is always one of three things — wind chill, heat index, or about the air temperature — and which one it is depends only on the air temperature: 50°F or below is wind chill, 80°F or above is heat index, and the 30 degrees in between are neither.

Three things to check before you trust the number

Check the temperature first, not the result. The first thing this page decides is which formula applies, and that decision comes from the air temperature alone. If you type 65°F and the page prints a wind chill or a heat index, something is wrong: between 50°F and 80°F neither index applies, and the feels-like temperature is about the air temperature. The wind speed and the humidity only matter once the temperature has picked a side.

Each side has its own range. The wind chill formula applies at 50°F or below with wind from 3 to 110 mph: below 3 mph the wind chill is the air temperature, and above 110 mph there is no wind chill to work out. The heat index formula applies at 80°F or above, with humidity from 0 to 100%: below 80°F there is no heat index, and at 80°F and 40% humidity the simple formula gives 79.580000°F, below the air temperature, so the heat index is about the air temperature there. A calculator that prints a number outside its side’s range is answering a question the formula does not ask.

Each side has its own warning. On the cold side the warning is frostbite: the National Weather Service lists frostbite within 30 minutes at −18°F wind chill, within 10 minutes at −32°F, and within 5 minutes at −48°F, and no frostbite time at all above −18°F. On the hot side the warning is the danger level: Caution from 80 to 90°F, Extreme Caution from 90 to 103°F, Danger from 103 to 125°F, and Extreme Danger at 125°F and above. The two warnings never appear together, because the two indices never apply together.

Worked example

On the defaults: 90°F, 15 mph, 40% humidity — and the cold-side equivalent, 20°F at 15 mph.

1. Which side? 90°F is above 80°F, so the heat index applies; the wind speed is not used.
2. The heat index: the Rothfusz regression gives 90.679691°F, 91°F in whole degrees.
3. In Celsius: (90.679691 − 32) × 5 ÷ 9 = 32.60°C.
4. Feels-like rise: 91 − 90 = 1°F in whole degrees (0.68°F unrounded).
5. Danger level: 91°F is in Extreme Caution (90 to 103°F).
6. The cold side: 20°F is at or below 50°F, so the wind chill applies; the humidity is not used.
7. The wind chill: 35.74 + 0.6215 × 20 − 35.75 × 15^0.16 + 0.4275 × 20 × 15^0.16 = 6.218885°F, 6°F in whole degrees.
8. In Celsius: (6.218885 − 32) × 5 ÷ 9 = −14.32°C.
9. Feels-like drop: 20 − 6 = 14°F.
10. Frostbite: at 6°F wind chill the NWS chart lists no frostbite time — the 30-minute band starts at −18°F.
11. The middle: 65°F is between 50°F and 80°F, so neither applies, and it feels like 65°F.

Every branch, checked

• 20°F at 15 mph, 40% humidity: 20°F at 15 mph, 40% humidity: Wind chill applies: 20°F at 15 mph feels like 6°F. 20°F at 15 mph — the wind chill applies: it feels like 6°F, 6.218885°F unrounded, −14.32°C, 14°F colder than the air, with no frostbite time listed on the NWS chart.
• 90°F at 15 mph, 40% humidity: 90°F at 15 mph, 40% humidity: Heat index applies: 90°F at 40% humidity feels like 91°F. 90°F at 15 mph, 40% humidity — the heat index applies: it feels like 91°F, 90.679691°F unrounded, 32.60°C, in Extreme Caution.
• 0°F at 20 mph, 40% humidity: 0°F at 20 mph, 40% humidity: Wind chill applies: 0°F at 20 mph feels like −22°F. 0°F at 20 mph — the wind chill applies: it feels like −22°F, −21.995223°F unrounded, −30.00°C, in the 30-minute frostbite band.
• 95°F at 15 mph, 60% humidity: 95°F at 15 mph, 60% humidity: Heat index applies: 95°F at 60% humidity feels like 113°F. 95°F at 15 mph, 60% humidity — the heat index applies: it feels like 113°F, 113.090308°F unrounded, 45.05°C, in the Danger level.
• 65°F at 10 mph, 50% humidity: 65°F at 10 mph, 50% humidity: Neither applies: 65°F feels like 65°F. 65°F at 10 mph, 50% humidity — neither applies: between 50°F and 80°F the feels-like temperature is about the air temperature, so it feels like 65°F.
• 32°F at 10 mph, 50% humidity: 32°F at 10 mph, 50% humidity: Wind chill applies: 32°F at 10 mph feels like 24°F. 32°F at 10 mph — the wind chill applies: it feels like 24°F, 23.727144°F unrounded, −4.60°C; freezing air with a light wind feels eight degrees colder than the thermometer.
• 100°F at 10 mph, 50% humidity: 100°F at 10 mph, 50% humidity: Heat index applies: 100°F at 50% humidity feels like 118°F. 100°F at 10 mph, 50% humidity — the heat index applies: it feels like 118°F, 118.315812°F unrounded, 47.95°C, in the Danger level.
• 55°F at 10 mph, 50% humidity: 55°F at 10 mph, 50% humidity: Neither applies: 55°F feels like 55°F. 55°F at 10 mph, 50% humidity — neither applies: 55°F is inside the 50–80°F band, so it feels like 55°F.
• 85°F at 20 mph, 70% humidity: 85°F at 20 mph, 70% humidity: Heat index applies: 85°F at 70% humidity feels like 93°F. 85°F at 20 mph, 70% humidity — the heat index applies: it feels like 93°F, 92.702149°F unrounded, 33.72°C, in Extreme Caution.
• −10°F at 20 mph, 40% humidity: −10°F at 20 mph, 40% humidity: Wind chill applies: −10°F at 20 mph feels like −35°F. −10°F at 20 mph — the wind chill applies: it feels like −35°F, −35.114225°F unrounded, −37.29°C, in the 10-minute frostbite band.

The reverses, worked backwards

• Smallest wind speed for 20°F to feel like 10°F or colder: 9 mph — at 9 mph the figures are Wind chill applies: 20°F at 9 mph feels like 10°F (9.511259°F unrounded); at the lower neighbor the figures are Wind chill applies: 20°F at 8 mph feels like 10°F (10.232972°F unrounded); at the upper neighbor the figures are Wind chill applies: 20°F at 10 mph feels like 9°F (8.854038°F unrounded) — verdict: STEP.
• Warmest air at 15 mph for a wind chill of 0°F or colder: 15°F — at 15°F the figures are Wind chill applies: 15°F at 15 mph feels like 0°F (−0.185325°F unrounded); at the upper neighbor the figures are Wind chill applies: 16°F at 15 mph feels like 1°F (1.095517°F unrounded); at the lower neighbor the figures are Wind chill applies: 14°F at 15 mph feels like −1°F (−1.466167°F unrounded) — verdict: STEP.
• Smallest humidity for 90°F to feel like 100°F or hotter: 61% — at 61% the figures are Heat index applies: 90°F at 61% humidity feels like 100°F (100.249790°F unrounded); at the lower neighbor the figures are Heat index applies: 90°F at 60% humidity feels like 100°F (99.677718°F unrounded); at the upper neighbor the figures are Heat index applies: 90°F at 62% humidity feels like 101°F (100.833496°F unrounded) — verdict: STEP.
• Coldest air at 60% humidity for a heat index of 110°F or hotter: 94°F — at 94°F the figures are Heat index applies: 94°F at 60% humidity feels like 110°F (110.168900°F unrounded); at the lower neighbor the figures are Heat index applies: 93°F at 60% humidity feels like 107°F (107.366937°F unrounded); at the upper neighbor the figures are Heat index applies: 95°F at 60% humidity feels like 113°F (113.090308°F unrounded) — verdict: STEP.
• Warmest air at 15 mph for the wind chill to still apply: 50°F — at 50°F the figures are Wind chill applies: 50°F at 15 mph feels like 45°F (44.644144°F unrounded); at the upper neighbor the figures are Neither applies: 51°F feels like 51°F (between 50°F and 80°F neither index applies); at the lower neighbor the figures are Wind chill applies: 49°F at 15 mph feels like 43°F (43.363302°F unrounded) — verdict: STEP.
• Coldest air at 40% humidity for the heat index to still apply: 80°F — at 80°F the figures are Heat index applies: 80°F at 40% humidity feels like 80°F (heat index ≈ air temperature) (simple formula 79.580000°F unrounded); at the lower neighbor the figures are Neither applies: 79°F feels like 79°F (between 50°F and 80°F neither index applies); at the upper neighbor the figures are Heat index applies: 81°F at 40% humidity feels like 81°F (heat index ≈ air temperature) (simple formula 80.652255°F unrounded) — verdict: STEP.

What the defaults assume

The defaults are 90°F, 15 mph and 40% humidity — a hot summer afternoon with a light breeze — and the heat index applies: it feels like 91°F, 90.679691°F unrounded. They are a starting point, not a standard: your air may be colder or warmer, your wind calmer or stronger, your humidity drier or damper, and the right answer for you is the one your numbers describe. Change the temperature, the wind and the humidity to your own and the calculator recomputes instantly, and tells you which side applies before it tells you the number.

These notes repeat in every explainer section because this page is built to be read start to finish: the temperature picks the side before anything else is calculated; each side has its own range, and outside it there is no number; and each side has its own warning, and the two warnings never appear together.

Output · formula · meaning

Who uses this

• A parent reading the morning forecast — 20°F at 15 mph feels like 6°F on the wind chill side, 90°F at 40% humidity feels like 91°F on the heat index side, and 65°F is neither, so one page answers the winter morning, the summer afternoon and the mild day in between.
• A coach deciding whether practice goes ahead — 95°F at 60% humidity feels like 113°F, Danger, so practice moves indoors, while 85°F at 70% humidity feels like 93°F, Extreme Caution, so water breaks are scheduled.
• A construction foreman planning a winter shift — −10°F at 20 mph feels like −35°F, in the 10-minute frostbite band, so exposed skin needs covering and breaks need warming.
• A teacher explaining the weather — the same page shows why a 50°F day has a wind chill and a 51°F day does not, and why an 80°F day has a heat index and a 79°F day does not.

Accuracy & limitations

The calculator uses the National Weather Service formulas exactly as published — the wind chill formula on the cold side and the Rothfusz regression, with both NWS adjustments and the simple formula, on the hot side — and every figure on this page is checked against an independent Decimal model: the wind chill is printed to 6dp (6.218885°F for the cold headline) and the heat index to 6dp (90.679691°F for the hot headline). What it cannot do is describe your own tolerance: both formulas assume a typical person, and clothing, activity, sunshine, humidity and health all move the real risk. Bright sunshine can raise the feels-like temperature by 10 to 18°F in winter and add up to 15°F in summer. The frostbite bands and the danger levels are headline thresholds, not a guarantee for any one person, and at the extremes both formulas are extrapolated past the printed charts. It is not a medical device: if skin goes numb, white or waxy, or someone shows signs of heat exhaustion or heatstroke, get warm or cool and get help, whatever the calculator says.

Cite this calculator

20°F at 15 mph feels like 6°F; 90°F at 40% humidity feels like 91°F. Smallest wind speed for 20°F to feel like 10°F or colder: 9 mph. Warmest air at 15 mph for a wind chill of 0°F or colder: 15°F. Smallest humidity for 90°F to feel like 100°F or hotter: 61%. Coldest air at 60% humidity for a heat index of 110°F or hotter: 94°F. Warmest air at 15 mph for the wind chill to still apply: 50°F. Coldest air at 40% humidity for the heat index to still apply: 80°F. Source: Bright Side Kit, brightoolbox.com.

Frequently asked questions