What causes Canadian heat waves?

Canadian heat waves are caused by large, slow-moving ridges of high pressure in the upper atmosphere. Air sinks beneath these ridges, compresses and warms, while clear skies let the sun bake the surface day after day. When the jet stream buckles and stalls, the ridge can sit over the same area for days or even weeks, and temperatures climb far above seasonal norms: during the June 2021 event that Environment and Climate Change Canada (ECCC) called a "heat dome", daytime highs in western Canada ran 15 to 25 °C above normal.

What actually is a heat wave in Canada?

There is no single national temperature that defines a heat wave, because "hot" means something different in Windsor than in Whitehorse. ECCC calls hot weather an extreme heat event, its official name for a heat wave, when daily temperatures reach heat warning thresholds on 2 or more days in a row with no relief overnight, sometimes with high humidity. Those thresholds differ by region; ECCC sets them with Health Canada and provincial and territorial health authorities.

ECCC's last published criteria table (2025, before its Weather Impact Guides replaced it) shows how far apart the regions are. Each value had to be met on 2 or more consecutive days:

• Most of southern Ontario: highs of 31 °C and lows of 20 °C, or a humidex of 40 (around Windsor, in Essex and Chatham-Kent: lows of 21 °C or a humidex of 42).
• Okanagan and Kootenays, BC: highs of 35 °C and lows of 18 °C.
• Edmonton, Red Deer and Calgary: highs of 29 °C and lows of 14 °C.
• Far northern Northwest Territories and northern Nunavut: highs of 22 °C.

Southern Quebec's rule differed: a humidex of 40 or more with at least 30 °C for at least an hour, or 40 °C. In most regions the rule needed a warm night as well as a hot day. In July 2026 the Middlesex-London Health Unit still listed the same southern Ontario trigger and warned that when temperatures do not cool significantly overnight, the risk of heat-related illness increases.

Which heat alerts does Environment Canada issue now?

Heat warnings. Since 26 November 2025 every heat warning carries a colour: yellow, orange or red. ECCC picks it from the expected impact, described in its Heat Impact Guide, and from how confident forecasters are that the heat will happen. A red heat warning means extreme impact with very high confidence. A yellow one can mean moderate impact that forecasters are confident about, or a more serious event they are not yet sure of, so read the impact level in the warning text. The guide describes three impact levels:

Two other terms often turn up next to heat warnings. Neither is an ECCC heat alert:

• Special weather statements are information only: ECCC says they are not an alert and have no colour, and for low-impact weather it may issue one instead of an alert. Ontario's heat plan (2024) also has ECCC issue one, at its discretion, for the season's first heat event up to the Canada Day weekend when conditions fall short of a warning.
• "Extended heat warning" is a local public health term in Ontario. Under the province's heat warning system, an ECCC heat warning that runs 3 or more days becomes an extended heat warning at the health-unit level, and the system states ECCC does not issue it.

ECCC aims to issue heat warnings 18 to 24 hours ahead and does not usually send one for heat expected to last only one day. In July 2026, the same health unit relayed a yellow heat warning for London and Middlesex County from 13 to 16 July, with highs of 30 to 37 °C and lows of 20 to 24 °C. Our guide to Environment Canada's yellow, orange and red alerts shows the full colour matrix.

Why does the jet stream matter so much?

The jet stream is a narrow band of strong wind high in the atmosphere that follows the boundary between hot and cold air. It drifts north as the sun climbs: by summer, NOAA says, the polar jet usually sits near the Canada-US border. When the jet stream weakens and buckles, a northward bulge (a ridge) can get stuck. In the classic case, which the Royal Meteorological Society calls an omega block, high pressure ends up sandwiched between two lows in the shape of the Greek letter Ω, and the block can persist for days, weeks or even months while the sinking air beneath it keeps heating up.

So forecast confidence in a heat wave comes down to how long the ridge holds. Our Canadian city pages put Environment Canada's GEM, the European ECMWF and the American GFS side by side and flag each day as "models agree" or "split", with the gap in degrees. Early on 13 October 2026, our Toronto forecast had the models agreeing that day but split by ±5° on the Wednesday and ±3° on the Saturday. In a heat wave, a split on the day the ridge should break down is the honest signal that the end date is uncertain.

What is a heat dome?

"Heat dome" is a plain-language term for an especially strong, persistent area of high pressure that traps hot air underneath it, rather like a lid on a pot. Below it, the ground dries out, making it easier to heat even more. The most extreme Canadian example came in late June 2021, when a sprawling high-pressure ridge parked itself over western North America in what ECCC calls the hottest week in Canadian history:

• On 29 June 2021 the village of Lytton, BC, reached 49.6 °C, the highest temperature ever recorded in Canada and nearly 24 °C above normal. Two days earlier it had broken the old national record of 45 °C, which had stood for 84 years.
• At night the temperature could not fall much below 20 °C, and from 24 June to 4 July more than 1,000 daily temperature records fell.
• The day after the record, a wildfire burned 90% of Lytton and killed 2 people.
• In Vancouver alone, the heat was a contributing factor in the premature deaths of almost 600 people, and Alberta reported 185 heat-related deaths.

How does humidity make it feel worse? (Humidex)

Air temperature alone undersells Canadian summer heat, especially in the east. ECCC's humidex describes how hot, humid weather feels: sweat cools you as it evaporates, and Health Canada explains that high humidity slows that process. At 32 °C, ECCC's table gives a humidex of 40 ("great discomfort; avoid exertion") at 50% relative humidity but 33 ("some discomfort") at 25%. ECCC includes the humidex in a heat warning only where high humidity comes with the heat; extremely high readings are rare except in the southern regions of Ontario, Manitoba and Quebec. In its last published criteria, Alberta, British Columbia and the territories used temperature only. More in our humidex guide.

Do cities make heat waves worse?

Yes. In the urban heat island effect, roofs, paved roads and parking lots absorb the sun's heat by day and release it into the air at night. Health Canada says North American cities are on average 1 to 3 °C warmer than the surrounding countryside, and dense areas can be up to 12 °C warmer in calm, clear conditions; the higher night-time temperatures limit the body's ability to cool down during extreme heat. In 2021, ECCC counted 14 nights above 20 °C in Toronto, against an average of 4. Heat islands also vary by neighbourhood, so a station on the edge of town may read cooler than a downtown street.

Is climate change part of the picture?

Yes. The Intergovernmental Panel on Climate Change (IPCC) says it is virtually certain that hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s, with human-induced climate change the main driver, and ECCC says human activities, mainly greenhouse gas emissions, are causing more extreme heat events. A World Weather Attribution study of the June 2021 heat around Seattle, Portland and Vancouver, later peer-reviewed, found it would have been virtually impossible without human-caused climate change, and about 2 °C hotter than if it had happened at the start of the industrial revolution. Whether warming also makes stalled ridges more likely is less settled: research points to a weaker summer jet stream, but the study found it unclear whether that played a role in 2021. ECCC now runs its own rapid attribution system for extreme weather, starting with heat waves: its first results, for June 2024, found that human-caused climate change made the heat waves in eastern Ontario, Quebec and Atlantic Canada much more likely.

How can you stay ahead of a heat wave?

Get ECCC's heat warnings through its free WeatherCAN app or at Canada.ca/weather, and read the overnight forecast, not just the afternoon high. ECCC also reminds people to take precautions on hot days even without a heat warning. Because everything hinges on how long the ridge holds, look at model agreement, not a single number: our Canadian forecast pages show where GEM, ECMWF and GFS split, and our accuracy scoreboard shows which model has been closest on the daily high, maximum wind and rain over the last 30 days (it does not grade overnight lows). The heat warning itself, and its colour, is ECCC's call.