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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.

Aggiornato Esempi verificati: 6

Measured at the standard 10 m height, as in weather reports.
%
Prova
Feels like
°C
Feels like: 1.1 °C
Cifre decimali: 1; Al più vicino; a parità verso la cifra pari
Feels like (°F)
34.0°F
Index used
Wind chill
Risk or comfort
Low risk of frostbite
Wind chill
1.1°C
Dew point
0.0°C / 32.0°F
Humidity feel
Dry and comfortable

At 5.0 °C (41.0 °F) with a 20 km/h (12.4 mph) wind and 70% humidity it feels like 1.1 °C (34.0 °F) by the wind chill. Low risk of frostbite. The dew point is 0.0 °C — dry and comfortable.

Feels-like temperature (°C) and time for exposed skin to freeze

< 2 min10–30 minModerate riskLow risk-65-55-48-40-28-1020Air1.1 °C

Dew point (°C)

Dry, comfortableStickyOppressive-2012.818.3300.0 °C
Come si calcola S

  1. Work in °F and mph (NWS units)

    T=41 °F,V=12.43 mph,RH=70%T = 41\ °\text{F},\quad V = 12.43\ \text{mph},\quad RH = 70\%

  2. Wind chill

    35.74+0.6215(41)−35.75(1.4966)+0.4275(41)(1.4966)=33.95 °F35.74 + 0.6215(41) - 35.75(1.4966) + 0.4275(41)(1.4966) = 33.95\ °\text{F}

    Valid for T ≤ 50 °F and V ≥ 3 mph.

  3. Dew point

    γ=ln⁡70100+17.625×5.00243.04+5.00=−0.00139,Td=243.04γ17.625−γ=−0.02 °C\gamma = \ln\frac{70}{100} + \frac{17.625 \times 5.00}{243.04 + 5.00} = -0.00139,\quad T_d = \frac{243.04\gamma}{17.625 - \gamma} = -0.02\ °\text{C}

  4. Back to °C

    (33.95−32)×59=1.08 °C(33.95 - 32) \times \tfrac{5}{9} = 1.08\ °\text{C}

Informazioni su Wind chill, heat index and dew point calculator

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).

Esempi svolti

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

Fonte di verifica: 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

Fonte di verifica: 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

Fonte di verifica: 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

Fonte di verifica: NWS heat index chart: 96 °F, 65% → 121 °F, in the Danger band (103–124 °F)

Domande

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.

Quanto è preciso «Wind chill, heat index and dew point calculator»?

La precisione dipende dai dati inseriti e dalle ipotesi del metodo. Il calcolo decimale usa 50 cifre significative, ma stime, metodi numerici e dati di origine possono essere meno precisi; l’arrotondamento visualizzato non elimina questi limiti. Esempi svolti verificati con fonti indipendenti: 6. Per esempio, «0 °F with 15 mph wind» viene verificato con NWS wind chill chart: 0 °F, 15 mph → −19 °F (formula −19.40).

Da dove proviene il metodo?

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.

Informazioni su questa calcolatrice

WC=35.74+0.6215T−35.75V0.16+0.4275TV0.16(°F, mph);Td=bγa−γ, γ=ln⁡RH100+aTb+TWC = 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}

Fonti

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

Verificato con le fonti

Questa calcolatrice include 6 esempi svolti con risposte da fonti indipendenti. Fanno parte della suite di test e puoi eseguirli anche qui.

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