Humidex

Development and Purpose

The term 'humidex' was first coined in 1965 as Canada sought a way to communicate the combined effect of heat and humidity on human comfort and health. The original calculation was based on the Fahrenheit scale, which was in common use in Canada at the time. In 1979, meteorologists J. M. Masterton and F. A. Richardson of Canada's Atmospheric Environment Service, now part of Environment and Climate Change Canada, reformulated the index to adapt to the country's metrication. This update introduced a conversion factor to base the humidex on Celsius readings, making it the standard used in Canadian forecasts today. The core purpose of the humidex is to provide a more accurate representation of perceived temperature than the air temperature alone, helping the public understand when conditions become hazardous. The index is described as a nominally dimensionless quantity, though it is publicly understood and reported as an equivalent to degrees Celsius.

It is specifically designed to reflect how the average person experiences weather, accounting for the body's reduced ability to cool itself through sweat evaporation when humidity is high. This physiological focus makes it a vital public health tool, directly informing the heat warnings and advisories issued by Environment Canada. The development of the humidex placed Canada at the forefront of public weather communication regarding thermal comfort, creating a model that is distinct from similar indices used in other countries. The index serves as a practical guide for daily activities and official safety protocols during the summer season.

Calculation and Scale

The humidex is calculated using a formula that incorporates the air temperature and the vapour pressure derived from the dew point. The standard formula is Humidex = T_air + 0.5555(e - 10.0), where T_air is the air temperature in degrees Celsius and e is the vapour pressure in millibars. The vapour pressure is itself calculated from the dew point temperature using an exponential function involving constants related to the physical properties of water. The 0.5555 factor in the formula is a conversion factor from the original Fahrenheit-based calculation, equivalent to 5/9. This adjustment ensures the final humidex value aligns with a Celsius scale for public interpretation.

The result is a number that typically exceeds the actual air temperature when humidity is present. For example, when the air temperature is 30°C with a dew point of 15°C, the humidex calculates to approximately 34. If the dew point rises to 25°C while the temperature remains at 30°C, the humidex jumps significantly to about 42. The index is interpreted through a standardized comfort scale. A humidex between 20 and 29 indicates little to no discomfort for most people. Values from 30 to 39 signal some discomfort, while a humidex of 40 to 45 denotes great discomfort where avoidance of exertion is recommended. Any humidex reading above 45 is classified as dangerous, with heat stroke considered quite possible.

Comparison with the U.S. Heat Index

While similar in purpose to the heat index used in the United States, the Canadian humidex is derived from a different meteorological variable. The humidex is calculated using the dew point temperature, which is a direct measure of atmospheric moisture content. In contrast, the American heat index formula uses a combination of air temperature and relative humidity. Both dew point and relative humidity are related to atmospheric moisture, but dew point is often considered a more consistent indicator of how humid the air feels because it does not fluctuate with temperature changes throughout the day like relative humidity does. This foundational difference means that for the same air temperature and relative humidity, the calculated humidex value will generally be higher than the corresponding heat index value.

The humidex is derived from the dew point rather than relative humidity. This makes it a reliable metric that can be applied across different times of day and varying temperature conditions. The humidex differs from the heat index used in the United States in this key methodological approach. Both indexes serve the critical function of warning the public about the dangers of apparent temperature. The use of dew point provides a stable baseline for assessing humid heat.

Historical and Record Humidex Events

Canada has experienced numerous extreme humidex events, particularly during summer heat waves in southern Ontario, Quebec, and the Prairies. For decades, the highest recorded humidex in the country was 52.1, measured in Windsor, Ontario, on June 20, 1953. This record stood as a benchmark for extreme humid heat for over half a century. The record was finally surpassed on July 25, 2007, when the community of Carman, Manitoba, recorded a humidex of 52.6, establishing the new national extreme. In recent years, humidex values pushing into the mid-40s have become a more frequent feature of summer weather forecasts, prompting widespread heat warnings.

For instance, during a major heat event, the humidex was forecast to reach the mid 40s in Toronto. Environment Canada said the humidex could reach 45 in Windsor, Ontario. In Montreal, a high of 32 C with a humidex up to 43 or 44 was forecast, coinciding with the traditional moving day. These events demonstrate the threat of humid heat waves across the country, a trend also observed in the Prairies where multi day heat events are forecast.

• June 20, 1953: Windsor, Ontario, sets a national humidex record of 52.1.
• July 25, 2007: Carman, Manitoba, breaks the record with a humidex of 52.6.
• Summer 2021: A historic heat dome over British Columbia causes hundreds of deaths, highlighting extreme heat as Canada's deadliest weather.
• August 2025: A southern Quebec heat wave, made more likely by climate change, is linked to possible heat-related deaths.
• July 2026: A multi-day heat event brings humidex values into the mid-40s across southern Ontario and Quebec.

Public Health Impact and Warnings

Extreme heat, often quantified by high humidex values, is the deadliest form of extreme weather in Canada. The 2021 heat dome in British Columbia, which shattered temperature records, was linked to hundreds of sudden deaths and is considered the deadliest weather event in Canadian history. Health professionals warn that heat exhaustion and heat stroke are direct risks during high-humidex events, with symptoms including headache, nausea, dizziness, intense fatigue, and, in severe cases, confusion and loss of consciousness. Vulnerable populations such as older adults, young children, people with chronic illnesses, and those who are socially isolated face the greatest risk.

Environment and Climate Change Canada issues colour-coded heat warnings when forecast humidex values indicate dangerous conditions. A yellow heat warning is commonly used to alert the public to impending multi-day heat events where humidex values are expected to reach 40 or higher. During such events, public health authorities and municipalities activate response plans. These typically include opening cooling centres and public pools, extending hours at splash pads and wading pools, and public messaging urging people to check on vulnerable neighbours, stay hydrated, and limit outdoor exertion during the hottest parts of the day. The health impacts are tracked, though often underreported.

During six heat waves in the summer prior to 2026, Toronto emergency rooms recorded 74 heat-related visits. Montreal's public health department reported three possible heat-related deaths during an August 2025 heat wave. In response to the growing threat, some municipalities like Toronto and Hamilton have explored bylaws to require landlords to maintain rental units below 26 degrees Celsius. New Ontario regulations also prohibit landlords from preventing tenants from installing air conditioners, provided proper notice is given.

Humidex in Recent Forecasts and Climate Context

Recent weather forecasts consistently highlight the humidex as a key metric for communicating summer heat risk. In 2026, a major multi-day heat event was forecast for southern Ontario and Quebec, with humidex values expected to make it feel like more than 40°C. Toronto braced for its hottest day of the year with a humidex in the mid-40s, while similar warnings were issued for communities across the Prairies, the Great Lakes region, and southern Quebec. These events are often driven by hot, humid air masses pushing north from the United States. The occurrence of high-humidex events is increasingly analysed through the lens of climate change.

An analysis of a heat wave over southern Quebec suggested it was at least two to ten times more likely due to climate change. As climate change causes hotter weather, humidex values reach higher levels. This trend suggests humidex values in the 40s and the associated public health risks may become more common. The trend can extend into fall, as seen in forecasts where humidex values made temperatures feel like the low to mid 30s.

Public Response and Adaptation

Municipalities have developed specific heat response plans activated when Environment Canada issues humidex-based warnings. In Toronto, during a 2026 heat warning, the city provided a 24-hour cool space at a designated community centre and kept more than 170 splash pads operating daily from 9 a.m. to 9 p.m. Montreal similarly ensured its water parks and cooling centres were operational, with particular attention to areas frequented by homeless and vulnerable populations. Public advisories consistently stress behavioural adaptations during high-humidex periods.

People are advised to drink plenty of water, seek air-conditioned spaces like shopping malls or libraries, take cool showers, wear loose-fitting and light-coloured clothing, and avoid strenuous outdoor activity during the peak heat of the afternoon. The importance of checking on elderly neighbours, friends, and family who may be at risk is a fundamental part of public safety messaging. These collective adaptation measures, from personal choices to municipal services, are critical for reducing the health impacts of extreme humid heat as it becomes a more persistent feature of Canada's climate. Cities also coordinate with health agencies to monitor emergency room visits and deploy outreach teams during prolonged heat events.

Questions about Humidex

What does a humidex of 40 mean?

A humidex of 40 falls into the 'great discomfort' category on the standard scale. At this level, the combined heat and humidity make it feel extremely hot, and the public is advised to avoid exertion. Prolonged exposure can lead to heat exhaustion, with symptoms like headache, nausea, and dizziness. Environment Canada typically issues heat warnings when the humidex is forecast to reach 40 or higher.

How is the humidex different from the temperature?

The humidex is an index number that describes how hot the weather feels by combining the effect of heat and humidity. The temperature is a measure of the actual heat in the air. The humidex is a nominally dimensionless quantity based on the dew point. For example, an air temperature of 30 °C with a dew point of 25 °C results in a humidex of 42.

What was the highest humidex ever recorded in Canada?

The highest humidex ever recorded in Canada is 52.6. This was measured in Carman, Manitoba, on 25 July 2007. This value broke the previous record of 52.1, which was set in Windsor, Ontario, on 20 June 1953. A humidex above 45 is classified as dangerous, with heat stroke quite possible.

Why does Canada use humidex instead of the U.S. heat index?

Canada uses the humidex, an index developed by Canadian meteorologists. The humidex is derived from the dew point, while the U.S. heat index is derived from relative humidity. Both dew point and relative humidity are directly related to atmospheric moisture. The humidex was specifically developed for use in Canada.

What should I do during a high humidex warning?

During a high humidex warning, you should watch for signs of heat exhaustion. You should seek out cool or air conditioned spaces. Environment Canada asks people to check on older or at risk people. A humidex above 45 is dangerous, and exertion should be avoided.

Is the humidex affected by climate change?

Yes, climate change is increasing the frequency and intensity of high-humidex events. Warmer air holds more moisture, leading to higher dew points and more oppressive humid heat waves. Analysis of past events, like a 2025 Quebec heat wave, has shown they are made significantly more likely by human-caused climate change. This trend means dangerous humidex values are becoming more common in Canadian summers.

Sources and further reading

• Humidex (Wikipedia)
• Toronto braces for hottest day of the year with mid-40s ... (toronto.citynews.ca)
• Environment Canada warns of high heat and humidity for ... (nanaimonewsnow.com)
• Fall offers southern Ontario a final summer heat boost, ... (theweathernetwork.com)
• Moving day heatwave: Humidex could soar to 45 today (montrealgazette.com)

Checked by the Your Space Hamilton desk. Last updated 2026-10-24.