Health & Body Tables
Wind Chill and Heat Index Table
Two different numbers answer the same question — what will this weather feel like on a body — and both are computed, not measured. Wind chill estimates heat loss from exposed skin as moving air strips away the thin layer of warmth around it; heat index estimates how much harder sweating becomes when the air is already full of water. Neither changes the actual temperature, and neither applies to a surface: your car's radiator freezes at 32 °F no matter what the wind is doing.
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The wind chill figures here use the 2001 North American formula, adopted by the US National Weather Service and Environment Canada after research volunteers had their faces chilled in a wind tunnel. It is defined only for air at or below 50 °F and wind above 3 mph, because in still air there is no wind chill — the concept needs moving air to exist. The United Kingdom and several other services use a different formulation, so a Met Office figure and an NWS figure for the same conditions can differ by a couple of degrees without either being wrong.
The heat index uses the Rothfusz regression, the same polynomial behind the NWS chart, including its two correction terms for very dry and very humid air. It is only published at or above 80 °F: below that the regression returns values slightly under the air temperature, which is a signal that humidity has stopped being the constraint. Both tables list the Celsius equivalent of the air temperature alongside, because the formulas themselves require Fahrenheit — converting the input, not the output, is what keeps the numbers honest.
Wind chill: what the air feels like at each temperature and wind speed
| Air temp (°F) | Air temp (°C) | 5 mph | 10 mph | 15 mph | 20 mph | 25 mph | 30 mph | 35 mph | 40 mph |
|---|---|---|---|---|---|---|---|---|---|
| 40 | 4.4 | 36 | 34 | 32 | 30 | 29 | 28 | 28 | 27 |
| 30 | -1.1 | 25 | 21 | 19 | 17 | 16 | 15 | 14 | 13 |
| 20 | -6.7 | 13 | 9 | 6 | 4 | 3 | 1 | 0 | -1 |
| 10 | -12.2 | 1 | -4 | -7 | -9 | -11 | -12 | -14 | -15 |
| 0 | -17.8 | -11 | -16 | -19 | -22 | -24 | -26 | -27 | -29 |
| -10 | -23.3 | -22 | -28 | -32 | -35 | -37 | -39 | -41 | -43 |
| -20 | -28.9 | -34 | -41 | -45 | -48 | -51 | -53 | -55 | -57 |
The formula is only defined for air at or below 50 °F with wind above 3 mph; outside that range the "feels like" number stops meaning anything, because in still air your body heat stays around you. A 0 °F reading with a 25 mph wind is −24 °F — frostbite territory in half an hour.
Heat index: what the air feels like at each temperature and humidity
| Air temp (°F) | Air temp (°C) | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH |
|---|---|---|---|---|---|---|---|
| 80 | 26.7 | 80 | 81 | 82 | 83 | 84 | 86 |
| 85 | 29.4 | 84 | 86 | 89 | 93 | 97 | 102 |
| 90 | 32.2 | 91 | 95 | 100 | 106 | 113 | 122 |
| 95 | 35.0 | 99 | 105 | 113 | 123 | 134 | 147 |
| 100 | 37.8 | 109 | 118 | 129 | 143 | 158 | 176 |
| 105 | 40.6 | 121 | 134 | 149 | 166 | 187 | 209 |
The heat index is only published above 80 °F. Below that, the regression returns numbers slightly under the air temperature, and the honest answer is that humidity is not yet the limiting factor. At 90 °F and 60% humidity the index is 100 °F: the air is as hard to cool in as still 100 °F air.
The bands the US National Weather Service uses for warnings
| Index | Range | Classification | What happens |
|---|---|---|---|
| Wind chill | −10 to −24 °F | Cold | Frostbite possible in 30 minutes of exposed skin |
| Wind chill | −25 to −34 °F | Very cold | Frostbite in 10 minutes |
| Wind chill | −35 to −59 °F | Dangerous | Frostbite in 5 minutes |
| Wind chill | −60 °F and below | Extreme | Frostbite in under a minute |
| Heat index | 80 to 90 °F | Caution | Fatigue with prolonged exposure |
| Heat index | 90 to 103 °F | Extreme caution | Heat cramps and exhaustion possible |
| Heat index | 103 to 124 °F | Danger | Cramps and exhaustion likely; heat stroke possible |
| Heat index | 125 °F and above | Extreme danger | Heat stroke highly likely with continued exposure |
These are the thresholds that trigger advisories, warnings and the closure of outdoor work. They are set on the index value, not on the air temperature — which is the whole point of computing one.
How to Use These Tables
- Read the wind chill table across a row for the wind effect and down a column for the temperature effect. Wind matters most when it is already cold: at 40 °F a 40 mph wind takes 6 degrees off, while at −20 °F the same wind takes 36.
- For heat, the index rises steeply with humidity once the air is above 90 °F: 95 °F at 40% humidity feels like 100 °F, and at 90% humidity it feels like 132 °F. That non-linearity is why humid heat kills more people than dry heat at the same thermometer reading.
- Use the warning bands rather than the raw number when deciding what to do. The NWS issues a heat advisory at an index of 100–104 °F and an excessive heat warning at 105 °F or above, with local adjustments for acclimatisation.
- Never use either index for equipment. Wind chill does not make pipes freeze faster and heat index does not make asphalt hotter — both describe human thermoregulation, and a thermometer is the right instrument for everything else.
Common Questions
Does wind chill mean the temperature actually drops?
No. A thermometer reads the same temperature with or without wind. Wind chill describes the rate at which exposed skin loses heat, expressed as the still-air temperature that would cause the same loss. That is why it is labelled "feels like" and why it only applies to living tissue and wet surfaces, not to pipes or engines.
Why do the US and UK wind chill figures differ?
They use different formulas. The 2001 North American model is based on a face-cooling experiment with 12 volunteers; the UK Met Office uses a model derived from different assumptions about skin temperature and heat transfer. For the same conditions the two can differ by 2–4 °F, and neither is more correct — they are calibrated against different experiments.
At what heat index is it dangerous to exercise?
Most sports medicine guidance puts the risk of heat illness rising sharply above an index of 90 °F, with cancellation or relocation recommended above 105 °F for youth and amateur events. The index assumes light wind and average clothing, so direct sun adds up to 15 °F and heavy protective kit makes it worse.
Why is the heat index only given above 80 °F?
Because below that, humidity is not the limiting factor for cooling and the regression produces values slightly lower than the air temperature, which reads as nonsense. The NWS withholds the number rather than publishing a figure that would suggest the air feels cooler than it is.
Can the heat index be lower than the air temperature?
Yes, in very dry air. At 100 °F and 10% humidity the index is around 95 °F, because sweat evaporates so efficiently that the air feels cooler than a thermometer suggests. This is why desert heat at 110 °F is survivable in shade while 95 °F at 80% humidity is not.
More Health & Body Tables
Sources
- Wind chill temperature index — the 2001 formula and its derivation — US National Weather Service
- The heat index equation (Rothfusz regression), Technical Attachment SR 90-23 — US National Weather Service, Southern Region
- Wind chill — cold weather safety guidance — Environment and Climate Change Canada
- Heat index and heat-related illness thresholds — US Centers for Disease Control and Prevention
Filed with Health calculations.