What Is a Hot Spell?
A hot spell is a period of unusually warm or hot weather that lasts for several days but may be shorter, less intense or less persistent than a major heat wave. Temperatures during a hot spell are typically noticeably above normal for a particular location and time of year.
Hot spells can occur throughout Canada’s warmer months and are not limited to the middle of summer. An unusually warm stretch in late spring or early fall can also be considered a hot spell when temperatures remain well above seasonal averages.
Unlike an official weather alert, there is no single temperature or duration that universally defines a hot spell. The term describes a period of unusually hot weather relative to the local climate.
Quick Facts About Hot Spells
Main Characteristic: Several days of unusually warm or hot weather
Typical Duration: A few days or longer
Common Season: Spring through early fall
Temperature: Above normal for the location and season
Common Causes: Warm air masses and persistent high pressure
Possible Hazards: Heat stress, dehydration and worsening dry conditions
Related Conditions: Heat wave and extreme heat
What Causes a Hot Spell?
Hot spells develop when atmospheric conditions bring unusually warm air into a region and allow it to remain there for several days.
High-pressure systems commonly contribute to these conditions.
Under high pressure, sinking air can suppress cloud formation, allowing more sunlight to reach and heat the Earth’s surface.
Atmospheric circulation can also transport warmer air northward from lower latitudes.
If the weather pattern remains relatively stationary, above-normal temperatures can continue for several consecutive days.
What Temperature Is Considered a Hot Spell?
There is no universal temperature that defines a hot spell.
Instead, temperatures are considered relative to what is normal for a specific location and time of year.
For example, a temperature that would be ordinary during July in Southern Ontario could represent unusually hot weather if it occurred much earlier in spring.
Similarly, northern Canadian communities may experience a significant hot spell at temperatures that occur more frequently in southern regions.
This relative definition allows hot spells to occur in many different climates.
How Long Does a Hot Spell Last?
A hot spell generally lasts for several consecutive days.
There is no universally accepted minimum duration, and the term is often used more broadly than heat wave.
A brief period of one unusually warm afternoon would generally not be considered a significant hot spell.
When above-normal temperatures persist for several days, however, the term becomes more appropriate.
If temperatures become particularly intense or remain unusually high for an extended period, the event may also be described as a heat wave.
Hot Spell vs. Heat Wave
Hot spells and heat waves both involve periods of unusually warm weather, but a heat wave generally implies a more significant or prolonged event.
A hot spell can describe a shorter or less severe period of above-normal temperatures.
A heat wave generally refers to unusually hot weather persisting for multiple days, particularly when the heat becomes significant enough to create increased impacts.
There is no universal boundary separating the two terms, so their use can vary between organizations and regions.
For Snowstorms.ca, treating hot spells as the broader or less intense condition keeps the two pages distinct.
Hot Spell vs. Extreme Heat
Extreme heat describes unusually or dangerously high temperatures capable of creating significant health risks.
A hot spell does not necessarily reach extreme levels.
For example, a community could experience several days that are considerably warmer than normal without temperatures becoming dangerously high.
However, a sufficiently intense hot spell can produce extreme heat.
The difference is primarily one of severity: a hot spell describes an unusually warm period, while extreme heat emphasizes hazardous temperatures.
Can a Hot Spell Occur in Spring?
Yes.
Hot spells can occur during spring when unusually warm air moves into a region earlier than normal.
Temperatures may rise substantially above seasonal averages for several days before returning to more typical levels.
Spring hot spells can be particularly noticeable because they may occur shortly after much colder weather.
Vegetation may also respond quickly to an early period of warmth, particularly if the hot spell lasts for several days.
Can a Hot Spell Occur in Fall?
Yes.
Periods of unusually warm weather can occur during early or even mid-autumn.
A persistent ridge of high pressure or a flow of warm southerly air can temporarily push temperatures well above seasonal averages.
Because average temperatures decline through autumn, conditions do not necessarily need to reach midsummer levels to represent an unusually significant warm period.
A fall hot spell ends when the atmospheric pattern changes and cooler seasonal air returns.
Can a Hot Spell Happen in Winter?
Unusually warm periods can occur during winter, although terms such as warm spell or unseasonably warm weather are generally more appropriate when temperatures are not objectively hot.
Warm southerly winds can occasionally produce dramatic temperature increases during winter.
However, for a weather conditions taxonomy, Hot Spell is most useful for periods occurring during the warmer portion of the year.
What Role Does High Pressure Play?
Persistent high pressure can create favourable conditions for a hot spell.
Sinking air associated with high pressure often reduces cloud formation and precipitation.
Clearer skies allow more solar radiation to reach the surface.
If the high-pressure system moves slowly, this pattern can repeat for several days.
Depending on the strength and duration of the system, temperatures may gradually rise as heat builds.
What Role Does the Jet Stream Play?
The jet stream influences the movement of weather systems and air masses across Canada.
When the jet stream bends northward, warm air from lower latitudes can spread farther north.
If this atmospheric pattern becomes slow-moving, the warm air may remain over the same region for several days.
Changes in the jet stream can eventually allow cooler air and weather systems to move back into the region, ending the hot spell.
How Does Humidity Affect a Hot Spell?
Hot spells can occur under either humid or dry conditions.
When humidity is high, sweat evaporates from the skin less efficiently, reducing the body’s ability to cool itself.
As a result, humid hot weather can feel considerably warmer than the measured air temperature.
In Canada, humidex is commonly used to describe the combined effects of temperature and humidity on how hot conditions feel.
A dry hot spell may feel different but can still create health risks if temperatures become sufficiently high.
What Is a Dry Hot Spell?
A dry hot spell combines unusually high temperatures with relatively low humidity and limited precipitation.
These conditions can increase evaporation from soil and vegetation.
If dry weather was already occurring before the hot spell, soil moisture can decline further.
Repeated or prolonged periods of hot, dry weather can contribute to drought development and may increase wildfire danger when vegetation becomes sufficiently dry.
Why Can Hot Spells Be Worse in Cities?
Urban areas can become particularly warm because of the urban heat island effect.
Roads, buildings, rooftops and other artificial surfaces absorb solar energy during the day.
These surfaces then gradually release stored heat.
Cities may also have less vegetation than surrounding rural areas, reducing shade and cooling through evaporation.
The effect can be especially noticeable overnight, when urban neighbourhoods may remain warmer than surrounding areas.
Can Hot Spells Cause Drought?
A single short hot spell does not necessarily cause drought.
However, hot weather increases evaporation from soils, vegetation and bodies of water.
If a hot spell occurs during an existing period of low precipitation, it can accelerate the loss of soil moisture.
Multiple hot spells combined with prolonged rainfall deficits can contribute to worsening drought conditions.
Can Hot Spells Increase Wildfire Risk?
Hot weather can contribute to wildfire-conducive conditions by increasing evaporation and drying vegetation.
However, temperature is only one part of wildfire risk.
Humidity, wind, recent rainfall, soil moisture, vegetation conditions and ignition sources are also important.
A hot spell accompanied by dry air, strong winds and existing drought can create more concerning wildfire conditions than hot weather occurring after substantial rainfall.
How Are Hot Spells Forecast?
Meteorologists use numerical weather prediction models to track expected temperature, atmospheric pressure, wind, humidity and the movement of air masses.
Forecast models can identify when warmer air is expected to move into a region and estimate how long it may remain.
Forecasters also compare predicted temperatures with historical and seasonal averages to determine how unusual the upcoming warmth may be.
As the event approaches, weather stations provide observations that show how quickly temperatures are rising.
How Does a Hot Spell End?
A hot spell usually ends when the atmospheric pattern changes.
A cold front may move through and replace the warm air with a cooler air mass.
The high-pressure system responsible for the hot weather may also weaken or move away.
Clouds, rain or thunderstorms can accompany the transition.
In some cases, temperatures decline gradually rather than ending with a dramatic frontal passage.
Are Hot Spells Dangerous?
Hot spells can become hazardous, particularly if temperatures become very high, humidity increases or the event persists.
Prolonged heat exposure can increase the risk of dehydration and heat-related illness.
The risk can become greater when nighttime temperatures remain elevated because the body has less opportunity to recover from daytime heat.
A hot spell that intensifies significantly may eventually produce extreme heat conditions.
How to Stay Safe During a Hot Spell
Check weather forecasts when several unusually hot days are expected.
Drink water regularly and limit strenuous outdoor activities during the hottest part of the day.
Seek shade or cooler indoor environments when necessary.
If temperatures remain high overnight, pay attention to indoor temperatures, particularly in buildings without air conditioning.
Never leave children or pets unattended in parked vehicles, even for a short period.
Official weather alerts should be followed if the hot spell becomes sufficiently intense to trigger a Heat Warning.