What causes Canadian heat waves?
Persistent upper-level ridges of high pressure — often locked in place by a stalled jet stream — trap sinking, warming air over a region for days. That is the engine behind almost every major Canadian heat wave.
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 sits still — sometimes for a week or more — and temperatures climb well above seasonal norms. Environment and Climate Change Canada (ECCC) calls the most extreme versions "heat domes."
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 it does in Whitehorse. Instead, ECCC issues heat warnings using region-specific daytime and overnight thresholds set with provincial and territorial health authorities. In much of southern Ontario and Quebec, for example, the criteria combine two or more days of high daytime temperatures with warm, humid nights that give the body no chance to recover. The overnight minimum matters as much as the afternoon peak — that is when heat-related illness climbs.
ECCC's warning tiers run from special weather statements (heat is possible) up to heat warnings and, for prolonged, dangerous events, extended heat warnings. Each is a signal to check on vulnerable neighbours, hydrate and find cooler air.
Why does the jet stream matter so much?
The jet stream is a fast river of air high in the atmosphere that separates cold polar air from warmer air to the south. In summer it weakens and drifts north. When it develops large, lazy north–south waves — a pattern called "blocking" — a big northward bulge (a ridge) can park itself over part of Canada. Because the block is slow to move, the weather underneath it barely changes. Sunny, subsiding air just keeps heating up. Remove the block and the heat wave breaks; keep it and the numbers escalate.
This is exactly why forecast confidence for heat waves comes down to how long the ridge holds. On checkweather.io we line up ECCC's GEM model against the ECMWF (European), NOAA's GFS (American) and DWD's ICON. When they agree the ridge will stay locked for six days, our confidence score is high. When they split on the breakdown timing, that disagreement is the honest signal — and it is exactly the detail a single-source app hides.
What is a heat dome?
"Heat dome" is a plain-language term for an especially strong, persistent ridge that traps heat like a lid on a pot. The most severe Canadian example on record was the late-June 2021 event over British Columbia and western Canada. On 29 June 2021, the village of Lytton, B.C., reached 49.6 °C — the highest temperature ever recorded in Canada, according to ECCC. The heat also contributed to the wildfire that destroyed much of Lytton the following day. That event is the textbook case of a heat dome: a massive, stationary ridge, record warmth, and tragic downstream impacts.
How does humidity make it feel worse? (Humidex)
Air temperature alone undersells Canadian summer heat, especially in the east. ECCC uses the Humidex to express how hot it actually feels when humidity is factored in. Humid air slows the evaporation of sweat, which is how your body sheds heat, so a 32 °C afternoon with a Humidex of 40 is far more dangerous than a dry 32 °C on the Prairies. When ECCC issues heat warnings for Ontario, Quebec and the Maritimes, the Humidex is usually the number doing the heavy lifting.
| Ingredient | What it does | Where it dominates |
|---|---|---|
| Upper-level ridge / heat dome | Sinking air warms and dries; clear skies bake the surface | Anywhere — the core driver nationwide |
| Stalled (blocking) jet stream | Keeps the ridge from moving, so heat compounds for days | All regions during major events |
| High humidity (Humidex) | Blocks the body's cooling; raises the "feels-like" number | Ontario, Quebec, Atlantic Canada |
| Warm overnight lows | Removes nighttime recovery; drives health impacts | Cities (urban heat island) |
| Dry soils / drought | Less evaporative cooling means faster daytime heating | Prairies, interior B.C. |
Do cities make heat waves worse?
Yes. The urban heat island effect means concrete, asphalt and buildings absorb heat during the day and release it slowly at night. That keeps overnight temperatures elevated in Toronto, Montreal and other cities, which is precisely the condition that raises health risk. It also means the airport thermometer on the edge of town may read cooler than a downtown street. When you compare a forecast to what a city actually recorded, that gap is worth remembering.
Is climate change part of the picture?
ECCC and the Intergovernmental Panel on Climate Change (IPCC) are clear that a warming climate makes extreme heat events more frequent and more intense. The underlying weather pattern — a stalled ridge — is natural, but it is now loading its dice on a warmer baseline, so the same setup produces hotter peaks than it would have decades ago. The 2021 B.C. event was formally studied as one that would have been virtually impossible without human-caused warming.
How can you stay ahead of a heat wave?
Watch for ECCC special weather statements and heat warnings at weather.gc.ca, and pay attention to the overnight forecast, not just the afternoon high. Because the whole event hinges on how long the ridge holds, look at model agreement rather than a single number. On checkweather.io our forecast-confidence score and accuracy scoreboard show you whether the models actually line up on the breakdown day — no black boxes, just the honest spread.