# 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.

Интерактивная версия: https://www.calcopenly.com/ru/earth/wind-chill-heat-index-dew-point
Тема: Калькуляторы погоды, Земли и досуга

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).

## Входные данные

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

## Результаты

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

## Формула

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

## Примеры с решением

### 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**
- Источник проверки: 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**
- Источник проверки: 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**
- Источник проверки: 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**
- Источник проверки: 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**
- Источник проверки: 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**
- Источник проверки: 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

## Вопросы

### 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.

### Насколько точен «Wind chill, heat index and dew point calculator»?

Точность зависит от введённых данных и допущений метода. Десятичная арифметика использует 50 значащих цифр, но оценки, численные методы и исходные данные могут быть менее точными; округление на экране не устраняет эти ограничения. Решённые примеры, проверенные по независимым источникам: 6. Например, «0 °F with 15 mph wind» проверяется по источнику NWS wind chill chart: 0 °F, 15 mph → −19 °F (formula −19.40).

### Откуда взята методика?

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.

## Источники

- [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)