Humidex Explained: Canada's Version of the Heat Index

The humidex is Environment and Climate Change Canada's (ECCC) measure of how hot, humid weather feels to the average person. It combines the air temperature with the moisture in the air into one number: a humidex of 40 means that 30 °C in humid air feels about as hot as 40 °C in dry air. ECCC rates 30 to 39 as some discomfort, 40 to 45 as great discomfort, and anything above 45 as dangerous.

What is the humidex, exactly?

The name joins "humidity" and "index". The term was already in Toronto weather forecasts in the mid-1960s, and Environment Canada's Atmospheric Environment Service set out the current formula in a 1979 report by J.M. Masterton and F.A. Richardson, Humidex: a method of quantifying human discomfort due to excessive heat and humidity. It answers a simple question: when the air is sticky, how hot does it feel? Your body sheds heat partly through sweat, which cools you as it evaporates, and Health Canada explains that high humidity slows that process. The same 30 °C therefore feels far more oppressive on a humid day than on a dry one, and the humidex puts a number on the difference.

Treat the humidex as an index, not a temperature. ECCC's glossary calls it an index, and its humidex tables print the result as a plain number next to air temperatures in °C. Writing "the humidex is 40" rather than "40 °C" keeps it apart from the thermometer reading.

How is the humidex calculated?

From the air temperature and the dew point. The dew point is the temperature at which the air would become saturated, so it rises with the amount of moisture in the air. ECCC turns the dew point into the water vapour pressure, e, a measure of how much water vapour the air holds, and adds part of it to the temperature. Its standard formula is:

Humidex = T_air + 0.5555 × (e − 10)

where T_air is the air temperature in °C and e is the water vapour pressure in hectopascals. The higher the dew point, the larger e becomes and the further the humidex climbs above the thermometer reading. ECCC's humidex page and public tables start from relative humidity instead, which together with the air temperature carries the same information: at 30 °C and 65% relative humidity the humidex is 40, and at 25 °C and 80% it is 34.

Note: ECCC works out e from the dew point in kelvins: e = 6.11 × exp[5417.7530 × (1/273.15 − 1/dew point)].

When does Environment Canada show the humidex?

Only when it adds something to the temperature. ECCC includes it:

• In forecasts when the temperature reaches 25 °C or higher and the humidex is at least one degree higher than the temperature.
• In current conditions when the temperature is 20 °C or higher and the humidex is at least one degree higher than the temperature.
• In heat warnings only in areas where high humidity comes with the extreme heat.

How do I read the humidex scale?

ECCC groups humidex values into four ranges by level of discomfort and health risk, and notes that some people are more affected than others:

ECCC's advice follows the same steps. Above 30, limit or modify outdoor exercise depending on your age, any physical limitations, your clothing and the other weather conditions. Anything over 40 is an extremely high reading, and ECCC says to reduce all unnecessary physical activity. Readings that high are rare except in the southern regions of Ontario, Manitoba and Quebec.

Is the humidex the same as the US heat index?

No. Both answer "how hot does it feel?", but they are built differently, so they rarely give the same number for the same air.

In typical Canadian summer heat, the same air gives a higher humidex than heat index. The humidex values below are from ECCC's table; the heat index values are our calculation with the NWS equation:

In these three cases the humidex is 5 to 6 points above the heat index, but that is not a rule for all air: in extreme humid heat the US equation can come out higher. Neither is wrong; they are two national scales with different assumptions. The NWS adds that its heat index was devised for shade and light wind, and that full sunshine can raise it by up to 15 °F (about 8 °C). When you compare a Canadian and an American forecast for the same heatwave, expect the feels-like numbers to differ because the formulas differ, even when the forecasts agree.

When does Environment Canada issue a heat warning?

There is no single humidex that triggers one. ECCC sends heat warnings when the forecast air temperature, the humidex or both reach levels that could affect your health, and those levels differ depending on where you live. ECCC sets its heat warning criteria with Health Canada and provincial and territorial health authorities. Its heat page adds:

• An extreme heat event means heat warning thresholds are reached on 2 or more days in a row with no relief overnight. If ECCC expects the heat to last only one day, it does not usually send a warning.
• ECCC aims to issue heat warnings 18 to 24 hours before the event.
• Since 26 November 2025, every heat warning, like every ECCC weather alert, is yellow, orange or red. ECCC's Heat Impact Guide describes three levels of impact: an increased, a higher and an extreme risk of illness or death. The colour combines the expected impact with how confident forecasters are, so a high-impact heat event that forecasters are not yet sure of is yellow, not orange. Our guide to Environment Canada's yellow, orange and red alerts explains how the colour is chosen.

What temperature or humidex triggers a heat warning in Canada?

Until 26 November 2025 ECCC published a table of regional criteria; its Weather Impact Guides now replace that table. The last version (modified 3 July 2025) shows how far apart the regions were. Each value had to be met on 2 or more consecutive days:

So a humidex of 40 was the trigger in southern Ontario, and southern Quebec's row also used 40 together with an air temperature of 30 °C, but it was never a national rule. Most other regions with a humidex trigger used 34 to 38, Essex and Chatham-Kent needed 42, and Alberta, British Columbia, the three territories and northern Quebec had temperature thresholds only.

ECCC no longer publishes a criteria table for its heat warnings, but local health units still list the triggers. In July 2026 the Middlesex-London Health Unit in Ontario listed its heat warning triggers as a forecast high of 31 °C or more with a low of 20 °C or more for two consecutive days, or a humidex of 40 or more for two consecutive days: the same values as the old southern Ontario row. The yellow heat warning it relayed for London and Middlesex County was issued to run from 13 July to the evening of 16 July 2026, with forecast highs of 30 to 37 °C and overnight lows of 20 to 24 °C, and the health unit warned that because temperatures would not cool much overnight, the risk of heat-related illness would increase.

Why does the feels-like number vary between apps?

A humidex is only as good as the temperature and dew point forecast it is built on, and those come from weather models that do not always agree. Our Canada forecast pages put Environment Canada's GEM model, the European ECMWF and the American GFS side by side with a forecast confidence line, and every city page lists each model's own feels-like value (the model's apparent temperature, not ECCC's humidex) next to its wind and humidity. At 01:56 Toronto time on 13 October 2026, an autumn night with air temperatures of 16 to 18 °C across the models, our Toronto page showed feels-like values of 20° from GEM, 18° from ECMWF and 16° from GFS. ECCC would show no humidex at all in air that cool. On a humid summer day, a spread like that between models tells you whether to plan around the number or treat it as uncertain. Our accuracy scoreboard shows which model has been closest on the daily high temperature, the daily maximum wind and rain over the last 30 days; it does not grade humidity or the humidex. The heat warning itself is ECCC's call.

How do you stay safe when the humidex climbs?

Health Canada's advice for extreme heat:

• Drink water before you feel thirsty, and before, during and after physical activity.
• Plan around the heat. Move outdoor activities to cooler parts of the day and find an air-conditioned place nearby where you can cool off for a few hours. Fans help comfort but are ineffective at cooling the body above 35 °C.
• Check on people at risk. Risks are greatest for older adults, infants and young children, people with chronic illnesses, people who work or exercise in the heat, and people who cannot keep their home cool. Health Canada suggests regular visits, because visitors can spot signs of heat illness that could be missed over the phone. It is believed that most heat deaths in Canada happen indoors.
• Know the signs. Heat exhaustion can bring headache, nausea or vomiting, dizziness or fainting, rapid breathing and heartbeat, and extreme thirst: move somewhere cool and drink water right away. A high body temperature with confusion, lack of coordination, or very hot or red skin can mean heat stroke, which is a medical emergency: call 911.

The humidex has a winter counterpart in the wind chill index that Canada and the United States introduced in 2001, explained in how wind chill is calculated in Canada. Where the humidex adds to the temperature for humid heat, wind chill takes away for wind on exposed skin. Use the humidex to judge how hard a hot day will be on your body, and the heat warning and its colour to know when ECCC and your local health authority expect it to be dangerous.