A stalled dome of high pressure is trapping hot, humid air across much of Canada, placing millions of residents under government heat alerts that stretch from the Northwest Territories to the Atlantic provinces. Environment Canada has posted yellow and orange warnings for parts of Ontario, Quebec, Alberta, Saskatchewan and the Northwest Territories, reflecting a multi-day event that officials say poses direct health risks. The oppressive conditions arrived without the reprieve of cooler overnight temperatures, a key factor that determines whether a heat event is declared official.
How the alert system works
Under the federal weather service’s criteria, an extreme heat event is identified when daily temperatures meet heat warning thresholds for two or more consecutive days, with no significant overnight cooling. The agency aims to issue heat warnings 18 to 24 hours before such an event begins.
If a hot stretch is expected to last only a single day, no warning is sent out. The alerts are triggered when “air temperature and/or humidex forecasts reach levels that could impact your health,” Environment Canada states, with specific temperature values varying by region to account for local climate and how residents are acclimatized. The colour-coded system now in use was introduced on Nov. 26, 2025, by Environment and Climate Change Canada.
It was designed to “tell you at a glance what risks the weather may pose to you,” the agency said. Alerts start at yellow and escalate to orange, then red as the potential risk increases.
The change replaced older text-based bulletins and aims to “more easily communicate the severity or the seriousness of particular weather events to agencies that need to know about it,” according to Ross Hull, a meteorologist with Global News. Yellow is the most common alert, according to the ECCC.
Its impacts are described as “moderate, localized and/or short-term,” and the weather under a yellow alert “may cause damage, disruption, or health impacts.” By nightfall, residents across the affected provinces described the scene as oppressive, and the key question now is how long the alerts will persist and whether any region sees the colour shift from orange to red.
A relatively new hazard category
While Environment Canada has issued weather alerts for decades, heat warnings are a comparatively recent addition. The agency launched its first dedicated heat warning program in the early 2000s, after studies linked extreme temperatures to spikes in hospital visits and deaths. Before that, heat was treated as a routine summer condition rather than a distinct hazard.
The shift reflected a growing scientific understanding that prolonged heat, especially when combined with humidity, poses a serious health risk because the body is denied a chance to recover overnight. A heat warning is now an official alert when conditions are expected to reach or exceed thresholds that can harm human health, with thresholds varying by region.
The warning triggers only when daytime highs and overnight lows stay above those levels for multiple days; a single hot day does not meet the criteria. The system was overhauled in late 2025, when the colour-coded scale was adopted. The aim was to make the severity of a weather event instantly clear to the public and to emergency managers who activate cooling centres and outreach programs.
The significance for residents is direct and personal. Heat is the deadliest weather-related hazard in Canada, outpacing floods, storms, and cold snaps in annual fatalities.
Older adults, people with chronic illnesses, and those without air conditioning face the greatest danger. As the climate warms, heat waves are arriving earlier, lasting longer, and covering broader areas. Understanding the colour alerts is no longer optional; it is a basic safety tool for the summer months ahead, and the current event is testing how well the new system communicates risk to millions of Canadians.
