What Is a Heat Wave?
A heat wave is a prolonged period of unusually hot weather that lasts for several consecutive days and is significantly warmer than normal for a particular location and time of year. Heat waves may involve high daytime temperatures, warm overnight temperatures, elevated humidity or a combination of these conditions.
There is no single temperature that defines a heat wave everywhere. A heat wave is generally considered relative to the normal climate of a region, meaning the temperatures associated with one can vary considerably across Canada.
Heat waves can become dangerous when intense daytime heat continues for several days without sufficient nighttime cooling. Prolonged exposure can increase the risk of heat-related illness and place additional stress on infrastructure, agriculture and the environment.
Quick Facts About Heat Waves
Main Characteristic: Prolonged unusually hot weather
Typical Duration: Several consecutive days or longer
Most Common Season: Summer
Primary Hazard: Heat-related illness
Important Factors: Temperature, humidity and duration
Nighttime Effect: Warm nights can increase heat stress
Related Condition: Extreme heat
Related Alert: Heat Warning
What Causes a Heat Wave?
Heat waves develop when atmospheric patterns allow unusually warm air to remain over the same region for an extended period.
Persistent areas of high atmospheric pressure are commonly associated with prolonged hot weather.
Under high pressure, sinking air can suppress cloud formation and precipitation. With fewer clouds, strong summer sunshine can repeatedly heat the ground.
Atmospheric circulation may also transport warm air from lower latitudes into Canada.
When these patterns move slowly or remain in place, temperatures can stay elevated for several consecutive days.
How Long Does a Heat Wave Last?
A heat wave typically lasts for multiple consecutive days, but there is no universal duration that defines every event.
Some heat waves may last only several days, while persistent atmospheric patterns can produce hot weather lasting a week or longer.
Duration is important because the effects of heat can accumulate.
Several hot days followed by warm nights provide less opportunity for people, buildings and infrastructure to cool before temperatures rise again the following day.
What Temperature Is Considered a Heat Wave?
There is no single temperature threshold that defines a heat wave across Canada.
Normal summer temperatures differ substantially between regions.
For example, temperatures considered exceptionally hot in a northern or coastal community may occur more frequently in parts of southern Canada.
Because of these regional differences, heat waves are generally considered in relation to the climate of the affected location.
Humidity, nighttime temperatures and the length of the event can also affect its severity.
Heat Wave vs. Extreme Heat
A heat wave and extreme heat are closely related but describe different aspects of hot weather.
Extreme heat refers primarily to the severity of unusually high temperatures.
A heat wave emphasizes the persistence of unusually hot weather over multiple days.
An exceptionally hot day could produce extreme heat without necessarily constituting a heat wave.
A prolonged heat wave, meanwhile, may include several days of extreme heat and warm overnight temperatures.
Heat Wave vs. Hot Spell
A hot spell generally describes a period of warmer-than-normal weather.
The term can be used for shorter or less intense periods of heat that may not reach the severity or persistence associated with a significant heat wave.
A hot spell can develop into a heat wave if unusually high temperatures persist.
The exact distinction is not universal, so the terms may sometimes be used differently between weather organizations, researchers and everyday conversation.
Heat Wave vs. Heat Dome
A heat wave describes a period of prolonged hot weather.
A heat dome describes an atmospheric pattern associated with persistent high pressure and exceptionally warm air.
A heat dome can therefore contribute to the development of a heat wave, but the two terms describe different things.
The heat wave is the resulting period of unusually hot weather, while a heat dome refers to an atmospheric setup that can help produce or intensify it.
Why Can High Pressure Cause a Heat Wave?
High-pressure systems are often associated with sinking air.
As air descends through the atmosphere, it can warm through compression and make cloud formation less likely.
Fewer clouds allow more solar radiation to reach the surface during the day.
If the high-pressure pattern remains in approximately the same location, this heating can occur repeatedly for several days.
Light winds may also reduce the movement of cooler air into the region.
Together, these conditions can allow heat to build and persist.
What Role Does the Jet Stream Play?
The jet stream helps steer weather systems across North America and influences the movement of warm and cold air masses.
Large bends in the jet stream can allow unusually warm air to move northward.
If the atmospheric pattern becomes slow-moving, the warm air can remain over the same region for an extended period.
The position and strength of the jet stream can therefore influence where heat waves develop, how widespread they become and how long they last.
Why Are Heat Waves Worse With High Humidity?
Humidity can increase the stress that hot weather places on the human body.
Sweating is one of the body’s primary cooling mechanisms.
When sweat evaporates from the skin, it removes heat.
High humidity slows evaporation because the surrounding air already contains substantial moisture.
The body can therefore have more difficulty cooling itself during hot and humid conditions.
This is why a humid heat wave can feel considerably hotter than the measured air temperature alone suggests.
Why Are Warm Nights Important During a Heat Wave?
Warm overnight temperatures can significantly increase the effects of a heat wave.
Normally, temperatures fall after sunset, giving the body and buildings an opportunity to release heat accumulated during the day.
During some heat waves, nighttime temperatures remain unusually high.
Buildings without air conditioning may stay warm throughout the night, particularly in densely developed urban areas.
When another hot day follows, temperatures can begin rising from an already elevated starting point.
Repeated warm nights can therefore increase cumulative heat stress.
What Is the Urban Heat Island Effect?
Cities can experience higher temperatures than surrounding rural areas because of the urban heat island effect.
Concrete, asphalt, rooftops and other built surfaces absorb solar energy during the day.
These materials gradually release stored heat after sunset.
Cities may also have less vegetation and shade, reducing natural cooling through evaporation.
The effect can become particularly important during heat waves because urban neighbourhoods may remain warmer overnight.
How Are Heat Waves Forecast?
Meteorologists can often identify the potential for a heat wave several days in advance.
Numerical weather prediction models forecast atmospheric pressure, temperature, humidity, wind and the position of the jet stream.
Forecasters examine whether unusually warm air is expected to move into a region and, importantly, whether the atmospheric pattern is likely to remain in place.
Expected daytime highs and overnight lows are monitored throughout the event.
Weather stations provide real-time observations that allow forecasters to compare actual temperatures with earlier predictions and local climate conditions.
Can Heat Waves Occur in Spring or Fall?
Yes.
Although heat waves are most common during summer, periods of unusually hot weather can occur during late spring or early fall.
Because a heat wave is defined partly in relation to normal conditions, temperatures do not necessarily need to reach midsummer levels to be unusually hot for the season.
Early-season heat can also be significant because people may not yet be acclimatized to hot weather.
Can Heat Waves Occur in Northern Canada?
Yes.
Heat waves can occur anywhere temperatures become unusually high relative to the local climate.
Northern Canadian communities generally have cooler climates than southern Canada, so the temperatures associated with a significant heat event may differ.
This is one reason a single national temperature cannot be used to define every heat wave.
Can a Heat Wave Cause Drought?
A heat wave alone does not necessarily cause drought, but prolonged hot weather can worsen existing dry conditions.
Higher temperatures increase evaporation from soils, lakes and vegetation.
Plants may also require more water during periods of intense heat.
When prolonged heat occurs alongside limited rainfall, soil moisture can decline rapidly.
Persistent hot and dry weather can therefore contribute to worsening drought conditions.
Do Heat Waves Increase Wildfire Risk?
Heat waves can contribute to conditions favourable for wildfires, particularly when they occur alongside drought and low humidity.
High temperatures increase evaporation and can dry vegetation and surface fuels.
However, heat alone does not determine wildfire risk.
Recent precipitation, soil moisture, wind, humidity, vegetation and ignition sources are also important factors.
Prolonged hot and dry conditions can nevertheless create an environment in which fires may ignite or spread more readily.
How Do Heat Waves Affect Infrastructure?
Prolonged heat can place substantial stress on infrastructure.
Electricity demand may rise as homes and businesses use air conditioning.
Road surfaces and railway tracks can expand during high temperatures.
Water demand may also increase.
Buildings without effective cooling can retain heat for extended periods, particularly when nighttime temperatures remain elevated.
Long-lasting heat can therefore affect transportation, utilities and public services in addition to human health.
How Do Heat Waves End?
Heat waves usually end when the atmospheric pattern responsible for the hot weather changes.
A cold front may move through and replace the hot air mass with cooler air.
Changes in the jet stream can also push a persistent high-pressure system away from the region.
Thunderstorms sometimes develop near the transition between hot and cooler air.
Temperatures may fall quickly after a strong front or decline more gradually as the broader weather pattern changes.
How to Stay Safe During a Heat Wave
Monitor weather forecasts and official alerts when prolonged hot weather is expected.
Stay hydrated and spend time in cool or air-conditioned environments when possible.
Limit strenuous outdoor activity during the hottest part of the day.
Lightweight and loose-fitting clothing can help the body release heat.
Pay particular attention to extended periods of warm nighttime temperatures, when homes may not cool sufficiently between hot days.
Never leave children or pets unattended in parked vehicles.