With a heat dome currently suffocating Canada, many parts of the country are under a heat warning issued by Environment Canada.
Millions of Canadians are at risk, according to Environment Canada, with advisories listed in Ontario, Quebec, Alberta, Saskatchewan and the Northwest Territories.
Extreme heat is defined when daytime temperatures reach the heat warning threshold for two or more consecutive days without a decrease in temperature at night.
Environment Canada also says the agency aims to issue heat warnings 18 to 24 hours before extreme heat occurs. If a heat wave is expected to last one day, no warning will be sent.
Heat warnings are also issued “when air temperatures and/or humidity forecasts reach levels that could affect your health,” Environment Canada says. It also depends on the varying temperatures across Canada.
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What do the warning colors mean?
Environment and Climate Change Canada (ECCC) introduced color-coded weather alerts on November 26, 2025, aiming to “tell you at a glance what risks the weather may pose to you.”
Weather warning colors start out yellow and change to orange and then red as the potential risk increases.
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The change to the color-coding system helps “more easily communicate the severity or severity of specific weather events to agencies that need to know about them,” said Ross Hull, a meteorologist at Global News.
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Yellow Alert: What does it mean?
According to the ECCC, the yellow alert is the most common. The effects of a yellow warning are “moderate, localized and/or short-term.”
In addition, weather classified as a yellow alert “may cause damage, disruption or harm to health.”
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Below are the potential impacts of a heat wave classified as a yellow alert, according to Environment Canada:
- Increased risk of illness or death
- Power failures and/or interruptions
- Animals at risk
- Outdoor events postponed and/or adjusted
- Workforce exposure due to higher indoor and outdoor temperatures
“A yellow heat warning would be an increase in daytime and nighttime temperatures for just a couple of days,” Hull said.
Orange Alert: What does it mean?
The ECCC says an orange warning is more rare and is issued when severe weather is “likely to cause significant damage, disruption or health impacts.”
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Any weather event designated as an orange warning is considered “major” and “widespread” and could last “several days.”
“When you go to an orange heat warning, it’s a longer-term situation because we’re dealing with this event, so there will be very high temperatures for three or more days,” Hull said. “High heat, humidity – this (ECCC orange heat warning) also alerts the public to open cooling spaces.”
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According to Environment Canada, an orange heat warning may consist of the following events:
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- Higher risk of illness or death
- Local travel disruptions
- Risk of local water shortage
- Power failures and interruptions
- Animals at high risk
- Crops, plants and gardens are at risk of damage
- Items, vehicles or materials left in direct sunlight are at risk of damage.
- Some damage to critical infrastructure
- Outdoor events postponed and/or cancelled.
- High impact on essential services
Red Alert: What does it mean?
According to Environment Canada, red alerts are considered rare and the impacts are “extensive, widespread and long-lasting.”
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The effects of the red warning are listed as “very dangerous and possibly life-threatening weather” that will “cause enormous damage and destruction.”
“A red heat warning would mean the situation escalates to an even higher level,” Hull said. “You’re talking even more than three days. It would probably be a very rare situation where, say, we would be in this stifling heat and humidity for a full week or more, and that could obviously lead to serious health problems for people.”
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According to Environment Canada, a red heat alert can result in:
- Extreme risk of illness or death
- Widespread travel disruption due to route and/or aircraft, train or vehicle disruption.
- Excessive demand and potential damage to utilities, including energy and water supplies
- Power outages and water shortages
- Animals at extreme risk
- Crops, plants and gardens are at risk of serious damage
- Items such as vehicles or materials left in direct sunlight are at risk of being damaged or destroyed.
- Damage to critical infrastructure
- Outdoor events postponed and/or canceled for several days
- Long-term and widespread impact on essential services
What is the difference between weather warnings, advisories and watches?
Environment Canada notes that Canadians are issued to “prepare” for “potentially severe weather” and are issued when “conditions are favorable for severe weather to develop.”
If conditions worsen, the watch can be set to alert.
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Hull said the guidelines are “usually not to the point where hours are clearly required.”
“This (advisory) can often be a short-term event. We often have fog warnings. That’s in terms of the severity, the hazard level or the impact level is probably not that high, but it’s still something that Environment Canada wants to highlight and make sure people are aware of,” he said.
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Environment Canada issues an advisory when Canadians need to “act immediately to protect themselves from severe weather,” and it is typically issued for snowstorms, fog, freezing rain and frost.
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To call the watch, Hull said conditions were “favorable for a certain event to occur.”
“For example, there typically aren’t heat hours. The watches are more geared – if we’re going to use a summer context – to watch for severe thunderstorms. That’s what you’ll see a lot in the summer. That will mean conditions are favorable for a particular area to see severe weather,” he said.
“It hasn’t happened yet (…) but forecasters believe that, say, this afternoon something will develop and we want people to be aware of that.”
Environment Canada says Canadians are being warned to “act now to protect themselves from severe weather.” Warnings are typically issued six to 24 hours in advance, but may be longer.
Severe weather conditions such as tornadoes can be reported in less than 30 minutes.