What Is Extreme Heat?
Extreme heat occurs when temperatures become unusually high for a particular location and time of year, creating conditions that can place significant stress on the human body, infrastructure and the environment. Extreme heat can occur during a prolonged heat wave or as a shorter period of exceptionally high temperatures.
What qualifies as extreme heat varies by region. Temperatures that are relatively common in one part of Canada may be unusual or dangerous somewhere with a cooler climate. Humidity, nighttime temperatures, duration and exposure to direct sunlight can also influence how hazardous a period of hot weather becomes.
Extreme heat is one of the most significant warm-season weather hazards because the effects can build over time, particularly when temperatures remain high overnight and people have limited opportunities to cool down.
Quick Facts About Extreme Heat
Main Characteristic: Unusually or dangerously high temperatures
Common Season: Late spring through early fall
Peak Season: Summer
Major Human Risk: Heat-related illness
Important Factors: Temperature, humidity and duration
Nighttime Effect: Warm nights can increase heat stress
Related Alert: Heat Warning
What Causes Extreme Heat?
Extreme heat develops when atmospheric conditions allow unusually warm air to remain over a region.
Several weather patterns can contribute.
Large areas of high atmospheric pressure can suppress cloud formation and encourage sunny conditions. Strong sunshine heats the ground, which then transfers heat into the lower atmosphere.
Atmospheric circulation can also transport hot air from warmer regions into Canada.
If the weather pattern changes slowly, hot air may remain over the same area for several days. This can allow temperatures to remain unusually high and potentially develop into a heat wave.
Dry soil conditions can sometimes intensify heat because less incoming energy is used to evaporate moisture from the ground.
What Temperature Is Considered Extreme Heat?
There is no single temperature that defines extreme heat everywhere.
Climate varies substantially across Canada, so heat-related thresholds differ by location.
A temperature that is unusual in a normally cool coastal or northern community may be relatively common during summer in another part of the country.
Meteorologists therefore consider local climate, daytime temperatures, nighttime temperatures, humidity and the duration of the heat when assessing the severity of an event.
Official heat-warning criteria can also vary geographically.
Extreme Heat vs. Heat Wave
Extreme heat describes unusually or dangerously hot conditions.
A heat wave describes a period of unusually hot weather that persists for multiple days.
The two frequently occur together, but they are not interchangeable.
A single exceptionally hot day can produce extreme heat without necessarily constituting a prolonged heat wave. Conversely, a heat wave is characterized partly by its persistence.
An extended heat wave can become particularly dangerous because repeated hot days and warm nights allow heat stress to accumulate.
Extreme Heat vs. Hot Spell
A hot spell is a period of warmer-than-normal weather that may be relatively brief or less severe than a major heat wave.
Extreme heat specifically emphasizes temperatures capable of creating significant heat-related risks.
A hot spell can develop into extreme heat if temperatures become sufficiently high, but not every period of unusually warm weather reaches dangerous levels.
How Does Humidity Affect Extreme Heat?
High humidity can make hot weather more difficult for the human body to tolerate.
The body primarily cools itself by producing sweat. As sweat evaporates from the skin, heat is removed from the body.
When the atmosphere contains large amounts of moisture, sweat evaporates less efficiently.
As a result, the body may have greater difficulty releasing heat even if sweating increases.
This is why humid summer weather can feel considerably hotter than the measured air temperature suggests.
In Canada, the humidex is commonly used to describe how hot conditions feel when temperature and humidity are considered together.
What Is Humidex?
Humidex is an index used in Canada to express the combined effects of heat and humidity.
It is not an actual air temperature.
Instead, it provides an indication of how hot conditions may feel to people by considering both temperature and atmospheric moisture.
Higher humidex values generally indicate greater discomfort and potentially greater heat stress.
During periods of extreme heat, humidex values can therefore provide additional context beyond the temperature alone.
Why Are Hot Nights Dangerous?
Nighttime temperatures are an important part of extreme heat events.
Normally, temperatures decrease after sunset, allowing buildings and the human body to release some of the heat accumulated during the day.
During extreme heat, overnight temperatures may remain unusually high.
This reduces the opportunity for people without adequate cooling to recover before the next hot day begins.
When warm nights occur repeatedly during a heat wave, heat stress can accumulate over several days.
Urban areas can be particularly affected because buildings, pavement and other surfaces release heat stored during the daytime.
What Is Overnight Heat?
Overnight heat occurs when nighttime temperatures remain unusually high instead of providing substantial relief from daytime heat.
It can be especially important during prolonged heat events.
Warm nights can keep homes and apartments hot, particularly buildings without air conditioning or adequate ventilation.
If high daytime temperatures return the following day, indoor temperatures may rise even further.
For this reason, meteorologists consider both daytime highs and nighttime lows when assessing potentially dangerous heat events.
What Is Dry Heat?
Dry heat refers to hot weather accompanied by relatively low atmospheric humidity.
Sweat can generally evaporate more efficiently in dry air than in very humid conditions, which can make the same air temperature feel different.
However, dry heat can still be dangerous.
Very high temperatures can overwhelm the body’s ability to regulate its temperature, particularly during prolonged exposure or physical activity.
Dry conditions can also increase evaporation from vegetation and soils, potentially worsening drought and wildfire conditions when hot weather persists.
How Is Extreme Heat Forecast?
Meteorologists use numerical weather prediction models to forecast temperature, humidity, cloud cover, wind and atmospheric pressure.
Weather balloons measure temperature and moisture through different levels of the atmosphere, while surface weather stations continuously monitor conditions near the ground.
Satellite observations provide additional information about cloud cover, atmospheric patterns and surface conditions.
Forecasters can often identify the potential for unusually hot weather several days in advance.
As the event approaches, forecasts become more precise regarding expected maximum temperatures, nighttime minimum temperatures, humidity and duration.
Why Do Cities Get Especially Hot?
Cities can become warmer than surrounding rural areas because of the urban heat island effect.
Roads, rooftops, concrete and other built surfaces absorb solar energy during the day and release that heat gradually.
Urban areas may also contain less vegetation than surrounding regions, reducing cooling from shade and evaporation.
The difference can become particularly noticeable overnight because buildings and pavement continue releasing stored heat after sunset.
As a result, densely developed areas can experience higher nighttime temperatures during extreme heat events.
Can Extreme Heat Occur Without High Humidity?
Yes.
Extreme heat can occur in either humid or dry conditions.
High humidity can increase heat stress by making evaporative cooling less effective, but exceptionally high temperatures can be dangerous even when the air is dry.
This means humidity should not be used as the only measure of heat risk.
Temperature, duration, sunlight, wind, activity level and access to cooling all influence the effects of extreme heat.
How Does Extreme Heat Affect the Human Body?
The human body attempts to maintain a relatively stable internal temperature even when the surrounding environment becomes hot.
Blood flow to the skin increases and sweating helps release heat.
During extreme conditions, these cooling mechanisms may become insufficient.
Prolonged heat exposure can contribute to dehydration, heat exhaustion and, in severe situations, heat stroke.
People performing strenuous outdoor activity can generate additional internal heat, increasing the challenge of staying cool.
How Does Extreme Heat Affect Infrastructure?
High temperatures can place additional stress on infrastructure.
Electricity demand may increase substantially as homes and businesses use air conditioning.
Road and rail infrastructure can also be affected by prolonged high temperatures because materials expand when heated.
Buildings without adequate cooling can retain heat well into the night.
Extreme heat can also increase demand on water systems and other public services during prolonged events.
Extreme Heat and Drought
Extreme heat and drought can reinforce one another.
When soil contains adequate moisture, some solar energy is used to evaporate water.
During drought, less moisture is available for evaporation, allowing more energy to directly heat the ground and surrounding air.
At the same time, extreme heat increases evaporation and water demand, potentially worsening existing dry conditions.
Persistent hot and dry weather can also increase wildfire risk by drying vegetation.
How Long Can Extreme Heat Last?
Extreme heat can last from part of a day to many consecutive days.
A brief surge of hot air may produce exceptionally high temperatures before a cold front brings relief.
Other atmospheric patterns can remain relatively stationary, allowing hot conditions to persist.
When unusually high temperatures continue for several days, the event may be classified more broadly as a heat wave.
The longer extreme heat persists, particularly when nights remain warm, the greater the potential for cumulative impacts.
What Is a Heat Warning?
A Heat Warning is an official weather alert issued when forecast heat conditions are expected to reach established criteria for a particular region.
Warning criteria are designed around the expected health impacts of high temperatures and can differ across Canada.
A Heat Warning is therefore different from extreme heat itself.
Extreme heat is the weather condition. A Heat Warning is an alert communicating that potentially dangerous heat conditions are expected or occurring.
How to Stay Safe During Extreme Heat
Monitor forecasts and official weather alerts when unusually hot weather is expected.
Stay hydrated and spend time in air-conditioned or otherwise cool environments when possible.
Avoid strenuous outdoor activity during the hottest part of the day, particularly when temperatures and humidity are very high.
Lightweight, loose-fitting clothing can help the body release heat.
Never leave children or pets unattended inside parked vehicles, where temperatures can rise rapidly.
People should also watch for symptoms of heat-related illness in themselves and others during prolonged hot weather.