Extreme heat events and heat warnings

About extreme heat

Extreme heat events are commonly known as "heat waves". We call hot weather an extreme heat event when daily temperatures reach heat warning thresholds on 2 or more days in a row with no relief overnight, sometimes with high humidity.

During heat events, make sure to protect your health:

stay safe before, during, and after an extreme heat event
know the health risks of extreme heat
learn more about UV index and sun safety
find out about sunburn and heat illness
understand what the Humidex reading means

Keep in mind, it is important to take precautions to protect your health on hot days, even if there is no heat warning in effect.

When we issue heat warnings

Heat warnings are part of our weather alert system. We aim to issue heat warnings 18-24 hours in advance of an extreme heat event.

Get heat warnings and weather alerts:

directly through our mobile app WeatherCAN
on your desktop at Canada.ca/weather

We send out heat warnings when air temperature and/or Humidex forecasts reach levels that could impact your health. Heat warnings can be issued at different temperature values depending on where you live.

If we expect a heat event to last only one day, we do not usually send out a heat warning.

Parts of a heat warning

Our weather alerts are based on expected impacts. Alerts focus not only on what weather conditions we will expect to see, but also on the impacts of that weather.

A heat warning weather alert lets you know when the extreme heat event will happen, how the heat can affect you, and how to keep yourself safe.

Details on when and what

the type of alert (i.e.: Heat Warning)
dates and times: the beginning and expected end of the heat event (i.e.: From: Now, To: Friday May 8, 11:00 am PST)
description of weather: temperature levels, potential humidity, etc.

Impacts of the weather

how extreme heat will impact you and your community
who is at high risk and how extreme heat can affect them
the potential risks and effects to your health

What to do next

take steps to protect your health and those at risk from extreme heat

Heat warnings and Humidex

We include the Humidex in a heat warning only in areas that have high humidity with extreme heat. You can use the Humidex calculator to estimate the Humidex level for your area.

Terms we use in heat forecasts

Our meteorologists use specific terminology to discuss heat events, such as:

Get the full definitions in our weather and meteorology glossary.

Our role in extreme heat

At the federal level we provide scientific information and support public health and emergency management officials across Canada. For extreme heat events we:

send heat warnings to Canadians
collaborate with Health Canada and Provincial/Territorial health authorities to establish evidence-based heat warning criteria
collaborate with Health Canada to support Heat Alert Response Systems (HARS) across Canada
calculate the role of human-caused climate change in recent extreme weather events

Extreme heat and air quality

Extreme heat events and poor air quality can happen at the same time because air pollution concentrations can be higher in hot weather. Light wind, high air pressure, and low air movement can trap air pollution near the surface of the ground. Extreme heat events can be worse in urban areas (cities) because they are often made worse by the presence of air pollution.

In Canada, your air quality can be affected when wildfire smoke and extreme heat events happen at the same time. It is important to know how to protect your health during a wildfire when this happens.

When there is an extreme heat event when wildfire smoke is present, prioritize keeping cool, because overheating can be more dangerous to your health.

Check the Air Quality Health Index (AQHI) to learn about the air quality in your area.

You can also use the WeatherCAN app to set up custom Extreme Temperature notifications and Air Quality notifications.

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2026-05-13

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