Humidex: Canada’s index for perceived heat and humidity

Definition and meaning

Humidex uses the air temperature together with the dew point—a direct measure of moisture in the air—to produce a single value that corresponds to the perceived temperature. The index is intended to convey discomfort from combined heat and humidity: high humidex values indicate conditions in which sweat evaporates more slowly and the body’s evaporative cooling is impaired. The humidex incorporates the physics of water vapor pressure, sometimes summarized as the effect of heat and humidity on human comfort.

Formula and calculation

The modern humidex formulation was published in 1979 by J. M. Masterton and F. A. Richardson. In its usual engineering form it is written as

• Humidex = T + 0.5555 × (e − 10),
• where e = 6.11 × exp(5417.7530 × (1/273.16 − 1/(dewpoint + 273.16))).

Here T and the dew point are expressed in degrees Celsius, e is the vapor pressure in hectopascals (hPa), and exp() denotes the exponential function. The calculation emphasizes dew point rather than relative humidity, which is the distinguishing factor compared with the U.S. heat index. For more technical background consult a discussion of the humidex formula.

Interpretation and examples

Humidex values are reported on roughly the same numeric scale as Celsius temperatures so they are easy to interpret. For example, if the air temperature is 30 °C and humid conditions produce a humidex of 40, the conditions will feel similar to a dry temperature of about 40 °C. Canadian advisories use approximate comfort thresholds: a humidex near 30 indicates some discomfort, around 40 signals great discomfort, values above 45 are considered dangerous, and readings approaching the mid‑50s are associated with very high risk of heat illness and heat stroke. These categories are commonly cited in summer public messaging and occupational guidance (summer safety notices).

History, records and notable facts

The index originated within Canada's Meteorological Service (now Environment Canada) to give the public a practical, easy‑to‑understand sense of summer discomfort. It has been adopted in forecasts and warnings across the country. Canada’s highest recorded humidex values include a peak of 53.0 on July 25, 2007 in Carman, Manitoba, which surpassed an earlier calculation of 52.1 from 1953 in Windsor. Those historical notes are frequently referenced when discussing extremes and public response to heat; see the record entry for the exact number. The index postdates the mid‑20th century so earlier heat events were not described with humidex at the time (humidex invention).

Uses, differences and limitations

Humidex is used for public health advisories, outdoor work planning, and personal decision‑making about hydration and activity. It differs from the U.S. heat index in its reliance on dew point rather than relative humidity and in some implementation details; compare the two when reading international reports (United States, heat index). Important limitations: the humidex does not account for wind speed, direct solar radiation, clothing, physical exertion, or population acclimatization. Because of these omissions, two situations with identical humidex values can produce different physiological stress. Practical guidance therefore combines humidex with local context and other indices where appropriate; for examples of guidance see public health materials or occupational safety references (example).

Quick reference

• Purpose: express perceived heat by combining temperature and moisture (index).
• Primary input: dew point rather than relative humidity (dew point, relative humidity).
• Typical thresholds: 30 (some discomfort), 40 (great discomfort), 45+ (dangerous).
• Use: weather reporting, public advisories, planning for heat exposure (summer).