What Is Dry Heat?
Dry heat describes hot weather that occurs when atmospheric humidity is relatively low. During dry heat, the air contains less moisture than during humid hot weather, allowing sweat to evaporate from the skin more efficiently.
This can make dry heat feel different from humid heat, but it does not make high temperatures harmless. The body can still become dehydrated or develop heat-related illness during prolonged exposure to very high temperatures.
In Canada, dry heat can occur when hot, dry air moves into a region or when persistent high pressure produces sunny weather with limited rainfall. Extended periods of dry heat can also contribute to declining soil moisture, drought conditions and increased wildfire danger.
Quick Facts About Dry Heat
Main Characteristic: High temperatures with relatively low humidity
Common Season: Summer
Humidity: Relatively low
Primary Human Risks: Dehydration and heat-related illness
Evaporation: Faster than during humid conditions
Environmental Effects: Increased drying of soil and vegetation
Related Conditions: Extreme heat, drought and heat waves
What Causes Dry Heat?
Dry heat develops when high temperatures occur within an air mass containing relatively little moisture.
Several atmospheric patterns can create these conditions.
Persistent high pressure can suppress clouds and precipitation, allowing strong sunshine to repeatedly heat the ground.
Atmospheric circulation can also transport hot, dry air from continental regions into other parts of Canada.
When dry conditions have already persisted for an extended period, reduced soil moisture can further intensify daytime heating.
What Does Low Humidity Mean?
Humidity describes the amount of water vapour present in the atmosphere.
Relative humidity compares the amount of moisture currently in the air with the maximum amount the air could hold at that temperature.
Warm air can contain considerably more water vapour than cold air.
During dry heat, relative humidity is generally low enough that evaporation occurs readily.
This is one reason perspiration may disappear from the skin quickly during hot, dry weather.
Dry Heat vs. Humid Heat
The primary difference between dry heat and humid heat is the amount of moisture in the atmosphere.
During dry heat, lower humidity allows sweat to evaporate relatively efficiently.
During humid heat, the atmosphere already contains more moisture, which reduces the rate at which sweat evaporates.
Because evaporation is one of the body’s main cooling mechanisms, humid heat can feel more oppressive at the same air temperature.
However, very high temperatures can still be dangerous under dry conditions.
Does Dry Heat Feel Cooler Than Humid Heat?
Dry heat can sometimes feel more tolerable than humid heat at the same temperature because sweat evaporates more effectively.
Evaporation removes heat from the skin and helps regulate body temperature.
However, this can create a misleading sense of comfort.
Because sweat may evaporate rapidly, a person may not notice how much fluid they are losing.
Dehydration can therefore develop during prolonged exposure even when the skin does not feel excessively sweaty.
Can Dry Heat Be Dangerous?
Yes.
Low humidity does not eliminate the risks associated with high temperatures.
During very hot weather, the body must continually release heat to maintain a safe internal temperature.
Physical activity, direct sunlight and prolonged exposure can increase the amount of heat the body must manage.
If the body’s cooling mechanisms cannot keep pace, heat-related illness can develop.
Dry heat can also accelerate dehydration because water is continually lost through perspiration and respiration.
Dry Heat and Dehydration
Dehydration is an important concern during dry heat.
Low humidity allows sweat to evaporate quickly from exposed skin.
Because the moisture disappears rapidly, people may underestimate how much fluid they are losing.
Physical activity further increases perspiration and fluid loss.
Drinking water regularly is therefore important during prolonged periods of hot, dry weather, particularly for people working or exercising outdoors.
Dry Heat vs. Extreme Heat
Dry heat describes the combination of high temperatures and relatively low atmospheric humidity.
Extreme heat describes temperatures that become unusually or dangerously high for a particular location and season.
The two conditions can occur together.
A period of dry heat may be relatively moderate, or temperatures may rise enough to produce extreme heat.
Extreme heat can also occur under very humid conditions.
Dry Heat vs. Heat Wave
A heat wave is a prolonged period of unusually hot weather lasting several consecutive days.
Dry heat instead describes the moisture characteristics accompanying the hot weather.
A heat wave can therefore be either dry or humid.
If a persistent heat wave occurs during a prolonged period of low humidity and limited rainfall, dry conditions may become increasingly severe.
Can Dry Heat Occur in Canada?
Yes.
Although parts of Canada frequently experience humid summer weather, dry heat also occurs.
Interior and western regions can experience particularly hot and dry conditions during summer.
Periods of dry heat can also develop elsewhere when atmospheric patterns produce high temperatures, low humidity and limited precipitation.
Canada’s large geographic size means the frequency and intensity of dry heat vary considerably between regions.
Why Can Dry Ground Make Hot Weather Worse?
Moist soil can moderate daytime temperatures through evaporative cooling.
Some incoming solar energy is used to evaporate water from the soil and vegetation rather than directly heating the surface.
When the ground becomes very dry, less water is available for evaporation.
More of the available solar energy can then heat the land surface and surrounding air.
This relationship can reinforce hot and dry conditions during prolonged periods without significant rainfall.
Dry Heat and Drought
Dry heat and drought are closely connected, although they are not the same condition.
Drought develops from prolonged shortages of precipitation and water availability.
Dry heat can accelerate moisture loss from soil, vegetation and bodies of water through increased evaporation.
When hot, dry weather occurs during an existing drought, soil moisture can decline even further.
Drought conditions can also contribute to higher daytime temperatures by reducing evaporative cooling from the land surface.
Dry Heat and Wildfires
Periods of hot, dry weather can contribute to increased wildfire danger.
High temperatures and low humidity can remove moisture from vegetation and other fuels.
If dry conditions persist, grasses, leaves, branches and forest debris may become increasingly susceptible to ignition.
Wind can further increase wildfire danger by accelerating drying and helping existing fires spread.
However, temperature alone does not determine wildfire risk. Precipitation, vegetation, soil moisture, wind and ignition sources are also important.
Does Dry Heat Affect Plants?
Yes.
Plants lose water through a process known as transpiration.
Hot, dry air can increase the rate at which moisture is lost from leaves and surrounding soil.
Plants may respond by reducing water loss, but prolonged hot and dry conditions can still cause stress.
If soil moisture becomes insufficient, vegetation may wilt, stop growing or become damaged.
Agricultural crops can be particularly vulnerable when dry heat occurs during sensitive stages of development.
Does Dry Heat Affect Roads and Infrastructure?
Prolonged high temperatures can affect infrastructure regardless of humidity.
Road surfaces can become extremely hot under direct sunshine, while materials used in roads, bridges and railways expand as temperatures increase.
Electricity demand may also rise as air conditioning use increases.
Hot, dry conditions can additionally increase water demand for agriculture, landscaping and other uses.
Why Can Temperatures Drop Quickly at Night in Dry Conditions?
Dry environments can sometimes experience relatively large differences between daytime and nighttime temperatures.
Water vapour and clouds can help absorb and re-radiate infrared energy toward the Earth’s surface.
When skies are clear and the atmosphere is dry, more heat can escape after sunset.
As a result, temperatures may decline more quickly overnight than they would under humid, cloudy conditions.
This does not happen during every dry heat event, but it can contribute to larger daily temperature ranges.
How Is Dry Heat Forecast?
Meteorologists forecast dry heat by examining expected temperatures and atmospheric moisture.
Numerical weather prediction models forecast temperature, relative humidity, dew point, wind, cloud cover and atmospheric pressure.
Weather stations provide real-time measurements of temperature and humidity near the surface.
Meteorologists can also examine atmospheric profiles from weather balloons to determine how much moisture is present at different elevations.
These observations help identify when hot, dry air is expected to affect a region.
What Is Dew Point?
Dew point is the temperature to which air must cool for it to become saturated with water vapour.
It provides useful information about the actual amount of moisture in the atmosphere.
Lower dew points generally indicate drier air, while higher dew points indicate greater atmospheric moisture.
During dry heat, dew points are typically lower than during hot and humid conditions.
Does Humidex Matter During Dry Heat?
Humidity has less influence on humidex when atmospheric moisture is low.
In Canada, humidex is used to describe how hot conditions feel when temperature and humidity are considered together.
During very humid weather, the humidex can be considerably higher than the measured temperature.
During dry heat, the difference may be much smaller.
However, a lower humidex does not mean extremely high air temperatures are safe.
How to Stay Safe During Dry Heat
Drink water regularly during prolonged hot weather, particularly when spending time outdoors.
Do not rely solely on how sweaty you feel to determine whether you are losing fluids because perspiration can evaporate quickly in dry air.
Limit strenuous activity during the hottest part of the day and seek shade or a cooler indoor environment when necessary.
Lightweight clothing and protection from direct sunlight can reduce heat exposure.
Monitor official weather forecasts and alerts when temperatures are expected to become unusually high.