Heat Index and Wind Chill Calculator
At 90 °F and 70% relative humidity the heat index is 106 °F (41.1 °C), in the NWS “Danger” category, and the dew point is 78.9 °F (26.1 °C). At −5 °F with a 25 mph wind the wind chill is −31 °F (−35.0 °C) (frostbite on exposed skin in about 30 minutes). A 80 °F day with a 65 °F dew point has a relative humidity of 60.3 %. The heat index uses the National Weather Service equation (Rothfusz regression with the NWS adjustments), the wind chill uses the 2001 NWS formula, and the dew point uses the Magnus formula. Results are informational, not medical advice.
Updated Runs in your browser: nothing you enter is uploadedHow this tool works
Heat index and dew point
Air temperature and relative humidity.
Heat index 106 °F (41.1 °C)
Danger: Heat cramps and heat exhaustion are likely, and heat stroke is possible with prolonged exposure and physical activity.
- Heat index
- 106 °F (41.1 °C)
- NWS category
- Danger
- Formula used
- Rothfusz regression
- Dew point
- 78.9 °F (26.1 °C)
Wind chill
Air temperature and wind speed. Defined at 50 °F (10 °C) or colder and wind of 3 mph (4.8 km/h) or more.
Wind chill −31 °F (−35.0 °C)
Frostbite possible in about 30 minutes on exposed skin.
- Wind chill
- −31 °F (−35.0 °C)
- Frostbite time on exposed skin
- 30 minutes
- Wind speed used
- 25.0 mph (40.2 km/h)
Relative humidity from dew point
Air temperature and dew point.
Relative humidity 60.3 %
- Relative humidity
- 60.3 %
- Gap between air temperature and dew point
- 15.0 °F
Heat index table
Heat index in °F by air temperature (rows) and relative humidity (columns), worked out with the same formula as above. Each cell is shaded and also carries a text code, so the category does not depend on colour.
- C = Caution (80 to 89 °F (27 to 32 °C))
- EC = Extreme Caution (90 to 102 °F (32 to 39 °C))
- D = Danger (103 to 124 °F (39 to 51 °C))
- XD = Extreme Danger (125 °F (52 °C) or higher)
| Air temp. \ RH | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 80 °F (26.7 °C) | 80 Caution | 81 Caution | 82 Caution | 83 Caution | 84 Caution | 86 Caution | 89 Caution |
| 85 °F (29.4 °C) | 84 Caution | 86 Caution | 89 Caution | 93 Extreme Caution | 97 Extreme Caution | 102 Extreme Caution | 108 Danger |
| 90 °F (32.2 °C) | 91 Extreme Caution | 95 Extreme Caution | 100 Extreme Caution | 106 Danger | 113 Danger | 122 Danger | 132 Extreme Danger |
| 95 °F (35 °C) | 99 Extreme Caution | 105 Danger | 113 Danger | 123 Danger | 134 Extreme Danger | 147 Extreme Danger | 161 Extreme Danger |
| 100 °F (37.8 °C) | 109 Danger | 118 Danger | 129 Extreme Danger | 143 Extreme Danger | 158 Extreme Danger | 176 Extreme Danger | 195 Extreme Danger |
| 105 °F (40.6 °C) | 121 Danger | 134 Extreme Danger | 149 Extreme Danger | 166 Extreme Danger | 187 Extreme Danger | 209 Extreme Danger | 234 Extreme Danger |
| 110 °F (43.3 °C) | 136 Extreme Danger | 152 Extreme Danger | 171 Extreme Danger | 194 Extreme Danger | 219 Extreme Danger | 247 Extreme Danger | 278 Extreme Danger |
NWS heat index categories
| Category | Heat index | What the NWS says |
|---|---|---|
| Caution | 80 to 89 °F (27 to 32 °C) | Fatigue is possible with prolonged exposure and physical activity. |
| Extreme Caution | 90 to 102 °F (32 to 39 °C) | Heat cramps and heat exhaustion are possible, and heat stroke with prolonged exposure and physical activity. |
| Danger | 103 to 124 °F (39 to 51 °C) | Heat cramps and heat exhaustion are likely, and heat stroke is possible with prolonged exposure and physical activity. |
| Extreme Danger | 125 °F (52 °C) or higher | Heat stroke is highly likely. |
Wind chill frostbite times
| Wind chill (°F) | Frostbite on exposed skin |
|---|---|
| Above −19 °F | Not shaded on the NWS chart |
| −19 to −37 °F | Possible in about 30 minutes |
| −38 to −67 °F | Possible in about 10 minutes |
| −68 °F or lower | Possible in about 5 minutes |
Worked heat index examples
| Air temperature | Relative humidity | Heat index | NWS category | Dew point |
|---|---|---|---|---|
| 80 °F | 40% | 80 °F | Caution | 53.5 °F |
| 90 °F | 70% | 106 °F | Danger | 78.9 °F |
| 95 °F | 55% | 109 °F | Danger | 76.3 °F |
| 100 °F | 40% | 109 °F | Danger | 71.4 °F |
| 105 °F | 60% | 149 °F | Extreme Danger | 88.3 °F |
| 110 °F | 40% | 136 °F | Extreme Danger | 80.2 °F |
Wind chill examples
| Air temperature | 5 mph | 15 mph | 25 mph | 40 mph |
|---|---|---|---|---|
| 30 °F | 25 °F | 19 °F | 16 °F | 13 °F |
| 20 °F | 13 °F | 6 °F | 3 °F | −1 °F |
| 10 °F | 1 °F | −7 °F | −11 °F | −15 °F |
| 0 °F | −11 °F | −19 °F | −24 °F | −29 °F |
| −10 °F | −22 °F | −32 °F | −37 °F | −43 °F |
| −20 °F | −34 °F | −45 °F | −51 °F | −57 °F |
Rothfusz regression coefficients
| Term | Coefficient |
|---|---|
| c1 | −42.379 |
| c2 (T) | 2.04901523 |
| c3 (RH) | 10.14333127 |
| c4 (T × RH) | −0.22475541 |
| c5 (T²) | −0.00683783 |
| c6 (RH²) | −0.05481717 |
| c7 (T² × RH) | 0.00122874 |
| c8 (T × RH²) | 0.00085282 |
| c9 (T² × RH²) | −0.00000199 |
How to find the heat index, wind chill and dew point
- Pick your units. Choose °F or °C for temperature and mph or km/h for wind. Typed values convert when you switch, and results show both temperature units.
- Find the heat index. Enter the air temperature and relative humidity in the first box. At 90 °F and 70% the heat index is 106 °F (41.1 °C), which is Danger.
- Read the dew point. The same box shows the dew point, the temperature at which the air would be saturated: 78.9 °F (26.1 °C) for the example.
- Find the wind chill. In the wind chill box enter the air temperature (50 °F or colder) and a wind speed of 3 mph or more. −5 °F with 25 mph gives −31 °F (−35.0 °C).
- Convert a dew point to humidity. In the last box enter the air temperature and the dew point to get relative humidity: 80 °F with a 65 °F dew point is 60.3 %.
- Check the table. The heat index table below the tool shows the heat index for common temperature and humidity pairs, shaded by category and labelled with a code (C, EC, D, XD).
Frequently asked questions
- What is the heat index?
- The heat index, or “feels like” temperature in the heat, combines air temperature and relative humidity into the temperature the body seems to feel, because humid air slows sweat evaporation. It is defined for shade and light wind. At 90 °F and 70% humidity it is 106 °F (41.1 °C).
- What formula does the heat index use?
- The NWS equation: a simple Steadman formula first, 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094). If that value averaged with the air temperature is 80 °F or more, the Rothfusz multiple regression with nine coefficients is used instead (listed in the table above). For 90 °F and 70% the result is 106 °F.
- What are the low- and high-humidity adjustments?
- When RH is below 13% and the air temperature is 80 to 112 °F, ((13 − RH) ÷ 4) × √((17 − |T − 95|) ÷ 17) is subtracted. At 100 °F and 10% that gives 94 °F (34.4 °C). When RH is above 85% and the temperature is 80 to 87 °F, ((RH − 85) ÷ 10) × ((87 − T) ÷ 5) is added. At 85 °F and 95% that gives 105 °F (40.6 °C).
- What are the NWS heat index categories?
- Caution is 80 to 89 °F, Extreme Caution 90 to 102 °F, Danger 103 to 124 °F and Extreme Danger 125 °F or higher. The example heat index of 106 °F (41.1 °C) falls in Danger. These are general guides, not medical advice.
- Why does the heat index show no category below 80 °F?
- The NWS chart starts at 80 °F. At 70 °F and 50% humidity the calculator returns 69 °F (20.6 °C) using the simple formula, which is close to the air temperature, and reports no category.
- How is wind chill calculated?
- With the 2001 NWS and Meteorological Service of Canada formula: 35.74 + 0.6215 × T − 35.75 × V^0.16 + 0.4275 × T × V^0.16, with T in °F and V in mph. It applies at 50 °F or colder and wind of 3 mph or more. At −5 °F and 25 mph it gives −31 °F (−35.0 °C). In metric units, 18.6 mph (30.0 km/h) at −10 °C gives −19.4 °C (−3 °F).
- How long does it take to get frostbite?
- On the NWS chart, frostbite on exposed skin is shaded as possible in about 30, 10 or 5 minutes at progressively lower wind chills. The calculator uses typical cut-offs of −19 °F, −38 °F and −68 °F. The example wind chill of −31 °F (−35.0 °C) is in the 30 minutes band. Real risk depends on skin exposure, wetness, activity and health, so treat this as informational, not medical advice.
- Why is wind chill not defined above 50 °F or in light wind?
- The formula was fitted to cold-weather heat loss from the face. Above 50 °F, or when the wind is under 3 mph, it does not describe the body’s heat loss, so the calculator says so instead of returning a number.
- How is the dew point calculated?
- With the Magnus formula using the Alduchov and Eskridge (1996) constants b = 17.625 and c = 243.04 °C: γ = ln(RH ÷ 100) + b·T ÷ (c + T), dew point = c·γ ÷ (b − γ). At 90 °F and 70% it gives 78.9 °F (26.1 °C). At 35 °C and 60% in metric units it gives 26.1 °C (78.9 °F). It is valid from −40 to 50 °C.
- How do I find relative humidity from the dew point?
- RH = 100 × exp(b·Td ÷ (c + Td) − b·T ÷ (c + T)) with the same constants, T and Td in °C. For 80 °F and a 65 °F dew point it gives 60.3 %. The dew point can never be higher than the air temperature.
- Is this medical advice?
- No. The categories and frostbite times are informational and describe typical conditions for healthy adults, in shade for heat and on exposed skin for cold. Direct sun, clothing, age, medication, health and activity all change the real risk. Follow local weather warnings and seek medical help for heat or cold illness.
- Does anything I type leave my device?
- No. All calculations run in your browser, and only a count of tool uses is recorded, never the numbers.
How it works
Everything is computed in °F and mph, the units the NWS formulas are published in, and converted for display. Heat index: the NWS Weather Prediction Center equation. A simple Steadman formula, HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}, is computed first and averaged with T. If that average is below 80 °F the simple value is the heat index. Otherwise the Rothfusz regression (nine coefficients, listed in the table) is used, and if RH is below 13% with T from 80 to 112 °F the amount [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17} is subtracted, or if RH is above 85% with T from 80 to 87 °F the amount [(RH − 85) ÷ 10] × [(87 − T) ÷ 5] is added. The NWS notes that the regression is not valid for extreme combinations beyond the data it was fitted to. The result is rounded to a whole °F, as the NWS does, before it is matched to a category (Caution 80 to 89, Extreme Caution 90 to 102, Danger 103 to 124, Extreme Danger 125 and above). The heat index is calculated for 40 to 140 °F. Wind chill: the 2001 NWS and Meteorological Service of Canada formula, WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16, T in °F and V in mph, defined for T up to 50 °F and V of 3 mph or more. Kilometres per hour are divided by 1.609344 to get mph. Frostbite bands use typical wind chill cut-offs of −19, −38 and −68 °F for about 30, 10 and 5 minutes on exposed skin; the NWS chart shades by wind speed as well, so these are estimates. Dew point: the Magnus formula with the Alduchov and Eskridge (1996) constants b = 17.625 and c = 243.04 °C, valid from −40 to 50 °C and relative humidity above 0. The humidity from a dew point is the inverse of the same formula. The table is the heat index at the listed temperatures and humidities, computed by the same function. Nothing you enter leaves your browser.
Sources
- NWS Weather Prediction Center, The Heat Index Equation
- NWS, Heat Index chart and categories
- NWS, Wind Chill Chart and formula
- Rothfusz, L. P. (1990), The Heat Index Equation, NWS Southern Region Technical Attachment SR 90-23
- Steadman, R. G. (1979), The assessment of sultriness, Journal of Applied Meteorology 18, 861-873
- Osczevski, R. and Bluestein, M. (2005), The new wind chill equivalent temperature chart, Bulletin of the American Meteorological Society 86, 1453-1458
- Alduchov, O. A. and Eskridge, R. E. (1996), Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology 35, 601-609
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