What Is a Heat Warning?
A Heat Warning is issued by Environment and Climate Change Canada when temperatures and/or Humidex values are forecast to reach levels that could affect human health.
Heat warnings are generally associated with an extreme heat event lasting at least two consecutive days, often with little relief from the heat overnight. The exact temperature and Humidex thresholds vary across Canada because people in different regions experience different climates.
Prolonged heat can increase the risk of heat-related illnesses, including heat exhaustion and heat stroke. The risk can become greater when high daytime temperatures are combined with high humidity and warm overnight temperatures.
Related Warnings
| Warning or Alert | Description |
|---|---|
| Air Quality Warning | Extreme heat and poor air quality can occur together, particularly when hot, stagnant conditions allow air pollution to accumulate near the surface. |
When Is a Heat Warning Issued?
Environment and Climate Change Canada issues a Heat Warning when forecast temperatures and/or Humidex values reach regional thresholds associated with increased health risks.
Unlike many weather warnings, there is no single temperature threshold that applies everywhere in Canada. Heat Warning criteria vary by region based on local climate and heat-health research.
For example, in Southern Ontario, a Heat Warning can be triggered when:
- Daytime maximum temperatures are forecast to reach 31°C or higher and overnight temperatures remain at 20°C or higher for at least two consecutive days, or
- The Humidex reaches 40 or higher for at least two consecutive days.
In Northern Ontario, the thresholds are lower: daytime temperatures of at least 29°C combined with overnight lows of at least 18°C, or a Humidex of at least 36, for two or more days.
Environment and Climate Change Canada generally aims to issue Heat Warnings 18 to 24 hours before an extreme heat event begins.
What Weather Conditions Should You Expect?
During a Heat Warning, expect very hot daytime temperatures that persist for multiple days. Depending on the location and weather pattern, high humidity may make conditions feel considerably hotter than the actual air temperature.
Heat waves can also bring unusually warm nights. When temperatures remain elevated overnight, people and buildings have less opportunity to cool down before temperatures rise again the following day.
Conditions can vary between heat events. Some events involve dry heat, while others combine high temperatures with substantial humidity and high Humidex values.
Hot, stagnant weather can also contribute to deteriorating air quality. During periods of extreme heat, check both the weather forecast and any active Air Quality Warnings in your area.
FAQ
A Heat Warning means temperatures or Humidex values are forecast to reach levels associated with an increased risk of heat-related health effects.
There is no single Canada-wide temperature threshold. Heat Warning criteria vary by region based on local climate and health impacts. In Southern Ontario, for example, temperatures of at least 31°C during the day and 20°C overnight for two or more days can meet the warning criteria.
Yes. Heat Warnings can be based on air temperature alone or a combination of temperature and humidity, depending on the regional criteria. Some extreme heat events are relatively dry.
