# Wind chill, heat index and dew point calculator

> Wind chill and heat index calculator: the feels-like temperature from the NWS formulas, plus dew point, frostbite risk and heat-stress level.

Interactive version: https://www.calcopenly.com/earth/wind-chill-heat-index-dew-point
Subject: Weather, earth and fun calculators

Wind chill uses the 2001 North American formula, in which the cooling grows with the 0.16 power of wind speed; it applies at 10 °C (50 °F) or colder with wind of at least 4.8 km/h (3 mph). Heat index uses the US National Weather Service (NWS) Rothfusz regression from 26.7 °C (80 °F) up. Dew point comes from the Magnus formula with Alduchov–Eskridge constants.

Runners, hikers and outdoor workers check it before going out. The default, 5 °C in a 20 km/h wind, feels like 1.1 °C; the NWS chart puts 0 °F in a 15 mph wind at −19 °F. Between 10 °C and 26.7 °C neither index applies, so the air temperature is shown as the feels-like value.

Enter the wind speed from a weather report, which is measured 10 m above the ground. Heat index values are for shade: the NWS says direct sunlight can add up to 15 °F (8.3 °C).

## Inputs

- **Air temperature**
- **Wind speed**: Measured at the standard 10 m height, as in weather reports.
- **Relative humidity**

## Results

- Feels like (°C) — main result
- Feels like (°F) (°F)
- Index used
- Risk or comfort
- Wind chill (°C)
- Heat index (°C)
- Dew point (°C)
- Dew point (°F)
- Humidity feel

## Formula

$$
WC = 35.74 + 0.6215T - 35.75V^{0.16} + 0.4275TV^{0.16}\quad(°F,\ \text{mph});\qquad T_d = \frac{b\gamma}{a-\gamma},\ \gamma = \ln\frac{RH}{100} + \frac{aT}{b+T}
$$

## Worked examples

### 0 °F with 15 mph wind

- Air temperature: 0 °F
- Wind speed: 15 mph
- Relative humidity: 50%
- **Feels like (°F): -19.0 °F**
- **Index used: Wind chill**
- Checked against: NWS wind chill chart: 0 °F, 15 mph → −19 °F (formula −19.40)

### 30 °F with 20 mph wind

- Air temperature: 30 °F
- Wind speed: 20 mph
- Relative humidity: 50%
- **Feels like (°F): 17.0 °F**
- Checked against: NWS wind chill chart: 30 °F, 20 mph → 17 °F (formula 17.36)

### 90 °F at 60% humidity

- Air temperature: 90 °F
- Wind speed: 5 mph
- Relative humidity: 60%
- **Feels like (°F): 100.0 °F**
- **Index used: Heat index**
- Checked against: NWS heat index chart: 90 °F, 60% → 100 °F (Rothfusz regression 99.68 in Python)

### 96 °F at 65% humidity

- Air temperature: 96 °F
- Wind speed: 5 mph
- Relative humidity: 65%
- **Feels like (°F): 121.0 °F**
- **Risk or comfort: Danger: heat exhaustion likely, heat stroke possible**
- Checked against: NWS heat index chart: 96 °F, 65% → 121 °F, in the Danger band (103–124 °F)

### Dew point at 25 °C and 60%

- Air temperature: 25 °C
- Wind speed: 10 km/h
- Relative humidity: 60%
- **Dew point: 16.7 °C**
- **Index used: Air temperature**
- Checked against: Psychrometric tables: 25 °C, 60% RH → dew point 16.7 °C; Python evaluation of the Alduchov–Eskridge Magnus formula gives 16.70

### Calm air at 20 °F: wind chill not defined

- Air temperature: 20 °F
- Wind speed: 2 mph
- Relative humidity: 70%
- **Feels like: -6.7 °C**
- **Feels like (°F): 20.0 °F**
- **Index used: Air temperature**
- Checked against: NWS: wind chill is defined only for winds of 3 mph or more, so the air temperature stands; (20 − 32) × 5/9 = −6.666667 °C

## Questions

### At what temperature does wind chill apply?

The NWS formula applies at 50 °F (10 °C) or colder with wind of at least 3 mph (4.8 km/h); outside those limits this calculator shows the air temperature. Environment and Climate Change Canada reports wind chill only when the temperature is 0 °C or below and the wind is 5 km/h or more. At 5 °C in a 20 km/h wind the index is 1.1 °C.

### What wind chill causes frostbite?

Environment and Climate Change Canada gives these times for exposed skin: 10 to 30 minutes from −28 °C, 5 to 10 minutes from −40 °C, 2 to 5 minutes from −48 °C, and under 2 minutes at −55 °C or colder. Air at −20 °C with a 30 km/h wind has a wind chill of −32.6 °C, inside the 10 to 30 minute band.

### What heat index is dangerous?

The NWS classifies a heat index of 80–90 °F (26.7–32.2 °C) as caution, 90–103 °F as extreme caution, 103–124 °F as danger and 125 °F (51.7 °C) or higher as extreme danger. Air at 96 °F (35.6 °C) with 65% humidity has a heat index of 121 °F (49.4 °C), in the danger band where heat stroke is possible.

### What dew point feels uncomfortable?

The NWS La Crosse office describes a dew point of 55 °F (12.8 °C) or lower as dry and comfortable, 55–65 °F as sticky with muggy evenings, and 65 °F (18.3 °C) or higher as oppressive. Air at 25 °C and 60% relative humidity has a dew point of 16.7 °C (62 °F), in the sticky range.

### Is the heat index the same as the humidex?

No. The humidex is Environment and Climate Change Canada's index, a different formula that also combines temperature and humidity, so the two give different numbers for the same weather. ECCC rates a humidex of 30–39 as some discomfort, 40–45 as great discomfort, and 46 or more as dangerous, with heat stroke possible.

### How accurate is the wind chill, heat index and dew point calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “0 °F with 15 mph wind” is checked against NWS wind chill chart: 0 °F, 15 mph → −19 °F (formula −19.40).

### Where does the method come from?

NWS — Wind chill chart and formula (2001); NWS Weather Prediction Center — The heat index equation (Rothfusz 1990 with adjustments); Environment and Climate Change Canada — Wind chill index and frostbite risk; O. A. Alduchov and R. E. Eskridge, “Improved Magnus form approximation of saturation vapor pressure”, J. Appl. Meteor. 35 (1996) 601–609; NWS La Crosse — dew point comfort guide.

## Sources

- [NWS — Wind chill chart and formula (2001)](https://www.weather.gov/safety/cold-wind-chill-chart)
- [NWS Weather Prediction Center — The heat index equation (Rothfusz 1990 with adjustments)](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)
- [Environment and Climate Change Canada — Wind chill index and frostbite risk](https://www.canada.ca/en/environment-climate-change/services/weather-health/wind-chill-cold-weather.html)
- O. A. Alduchov and R. E. Eskridge, “Improved Magnus form approximation of saturation vapor pressure”, J. Appl. Meteor. 35 (1996) 601–609
- [NWS La Crosse — dew point comfort guide](https://www.weather.gov/arx/why_dewpoint_vs_humidity)