Humidex and Wind Chill Calculators

Current official formulas from Environment Canada and the US National Weather Service. Includes comfort ratings, frostbite risk, and metric/imperial support.

Humidex Calculator

Humidex describes how hot humid air feels to the human body. Valid when temperature ≥ 20°C (68°F).

Air Temperature
°C
Relative Humidity
%

Humidex Equations (Environment Canada)

Humidex = T + 0.5555 × (e − 10)
e = 6.11 × exp[ 5417.753 × (1/273.16 − 1/(273.16 + Td)) ]
Td = 237.3 × γ / (17.27 − γ)    (dew point from RH)
γ = 17.27 T / (237.3 + T) + ln(RH/100)
Valid for T ≥ 20°C    e = vapour pressure (hPa)    Td = dew point (°C)
Humidex Comfort Scale (Environment Canada):
20–29 — Little or no discomfort    30–39 — Some discomfort    40–45 — Great discomfort; avoid exertion
45–54 — Dangerous    55+ — Heat stroke imminent

Wind Chill Calculator

Uses the 2001 NWS/Environment Canada formula — the current international standard, based on human clinical trials. Replaces the old 1945 Siple-Passel formula. Valid when T ≤ 10°C (50°F) and wind speed ≥ 4.8 km/h (3 mph).

Air Temperature
°C
Wind Speed
km/h

Wind Chill Equations (NWS / Environment Canada, 2001)

Metric: WC (°C) = 13.12 + 0.6215T − 11.37V0.16 + 0.3965T × V0.16
Imperial: WC (°F) = 35.74 + 0.6215T − 35.75V0.16 + 0.4275T × V0.16
T = air temperature (°C or °F)    V = wind speed (km/h or mph)
Valid: T ≤ 10°C (50°F), V ≥ 4.8 km/h (3 mph)
Old 1945 Siple-Passel formula: WC = 0.045×(5.2735√V + 10.45 − 0.2778V)×(T−33)+33 — now obsolete
Frostbite Risk (NWS):
Above −27°C (−17°F) — Low risk    −27 to −39°C — Increasing risk (10–30 min on exposed skin)
−40 to −47°C — High risk (5–10 min)    −48 to −54°C — Very high risk (2–5 min)
Below −55°C (−67°F) — Extreme danger (under 2 min)

About Humidex & Wind Chill

Both humidex and wind chill are apparent temperature indices — they describe how the human body perceives temperature rather than what a thermometer measures. They exist because the thermometer alone is a poor guide to physiological comfort and safety: heat combined with humidity prevents sweat from evaporating efficiently, trapping body heat; cold combined with wind accelerates heat loss from exposed skin far beyond what still air would cause.

Humidex was developed by Canadian meteorologists J.M. Masterton and F.A. Richardson in 1979 and remains the standard used by Environment Canada. It is based on the vapour pressure of water in the air, which determines how effectively the body can cool itself through perspiration. A humidex of 40 means the body is working as hard to stay cool as it would in dry air at 40°C — even if the actual temperature is only 30°C.

Wind chill quantifies the rate of heat loss from exposed human skin caused by wind. The current formula — adopted jointly by the US National Weather Service and Environment Canada in November 2001 — replaced the 1945 Siple-Passel formula, which was derived from experiments cooling water in Antarctica and significantly overestimated the effect of wind. The 2001 formula was developed using heat transfer theory and validated in clinical trials with human volunteers in wind tunnels, wearing standardized clothing and walking at 1.34 m/s (3 mph) to simulate normal pedestrian movement. Importantly, wind chill only affects how quickly an object reaches the ambient air temperature — it cannot cool anything below that temperature.

Related Reference Pages