Humidex, Heat Index and Wind Chill Calculators

Humidex is Environment Canada’s humidity index and the heat index is its US National Weather Service counterpart; wind chill uses the joint NWS / Environment Canada 2001 formula. Includes comfort ratings, frostbite risk, and metric/imperial support.

Humidex Calculator

Humidex describes how hot humid air feels to the human body. It is Environment Canada’s index; the US National Weather Service instead uses the heat index. Environment Canada reports humidex at 20°C (68°F) and above, but the equation stays valid at lower temperatures whenever humidity adds perceptible heat — so this calculator shows those values too.

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)
Defined whenever e > 10 hPa    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

Heat Index Calculator

The heat index is the US National Weather Service’s measure of how hot humid air feels — the American counterpart to Canada’s humidex, using a different equation (the Rothfusz regression). The NWS applies it at 80°F (27°C) and above; below that the apparent temperature is essentially the air temperature, shown here for reference.

Air Temperature
°C
Relative Humidity
%

Heat Index Equation (NWS — Rothfusz regression)

HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 TR − 0.00683783 T2
   − 0.05481717 R2 + 0.00122874 T2R + 0.00085282 TR2 − 0.00000199 T2R2
T = air temperature (°F)    R = relative humidity (%)
Applied when HI ≥ 80°F; below that a simpler Steadman formula is used
Low-humidity (RH < 13%) and high-humidity (RH > 85%) corrections applied per NOAA/WPC
Heat Index Caution Levels (NWS):
80–90°F (27–32°C) — Caution: fatigue possible with prolonged exposure    90–103°F (32–39°C) — Extreme caution: heat cramps and exhaustion possible
103–124°F (39–51°C) — Danger: heat exhaustion likely, heat stroke possible    125°F+ (52°C+) — Extreme danger: 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 (Environment Canada / NWS, 2001)

Environment and Climate Change Canada uses two formulas, chosen by wind speed; they meet at 5 km/h.
Formula 1 (V ≥ 5 km/h): WC = 13.12 + 0.6215 Tair − 11.37 V0.16 + 0.3965 Tair × V0.16
Formula 2 (0 < V < 5 km/h): WC = Tair + [(−1.59 + 0.1345 Tair) / 5] × V
Tair = air temperature (°C)    V = wind speed at 10 m (km/h)    WC in °C
Applied for Tair ≤ 10°C (50°F); Formula 2 is formally defined by ECCC for Tair ≤ 0°C. Imperial display converts to/from °F and mph.
Source: Environment and Climate Change Canada — hourly data technical documentation. The 1945 Siple-Passel formula is 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.

Heat index is the US counterpart to humidex, based on the work of R.G. Steadman and refined into the regression equation used by the National Weather Service. It combines air temperature and relative humidity into an apparent temperature expressed in the same units as the thermometer — a heat index of 105°F means the body experiences heat stress comparable to dry air at 105°F. Humidex and heat index answer the same question with different equations and scales, so their numbers are not directly interchangeable.

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. Environment and Climate Change Canada actually publishes two equations: the standard formula for winds of 5 km/h or more, and a separate light-wind formula for winds between 0 and 5 km/h — where the standard formula breaks down and would otherwise read warmer than the air itself. The two join smoothly at 5 km/h. Importantly, wind chill only affects how quickly an object reaches the ambient air temperature — it cannot cool anything below that temperature.

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