Warm season weather hazards

From hail and heat waves, to thunderstorms and tornadoes, severe weather in Canada takes many different forms during the warm months of the year, particularly the summer months.

Air Quality Health Index (AQHI) (Cote air santé)

The Air Quality Health Index (AQHI) is a scale ranging from 1-10+. It shows you the risk air quality in your area may have on your health.

Learn more about the AQHI.

Floods (Inondations)

Most flooding occurs when the volume of water in a river or stream exceeds the capacity of the channel. Flooding also takes place along lake and coastal shorelines, when higher than normal water levels inundate low-lying areas.

Numerous factors affect streamflow, and therefore the potential for flooding. Most important are the amount and type of precipitation, the nature and condition of the drainage basin, and climate.

Flooding is essential to a healthy environment, but can cause human hardship and economic loss.

Over the past 40 years, flood forecasting and warning in Canada has evolved into a network of forecast systems across the country.

Note: Provincial governments are responsible for issuing flood forecasts and warnings. Find provincial and territorial flood information.

Get prepared for floods.

More about floods.

Fog (Brouillard)

Fog consists of tiny water droplets suspended in the air that reduce visibility to less than one kilometre. It is different from cloud only because fog touches the earth’s surface.

Driving any vehicle in low visibilities due to fog can be hazardous and speeds should be reduced accordingly.

Learn about the potential impacts of weather hazards that affect visibility.

Frost (Gelée blanche)

Frost is atmospheric moisture that crystallizes directly on the ground and on exposed objects, such as rooftops, when temperatures are below freezing. Frost forms under conditions that would normally create dew if the temperature were above freezing at the point of formation.

Although it looks very pretty, frost can cause serious harm to flowers and plants if it occurs during the growing season. As such, take care to protect frost-sensitive plants when it is in the forecast.

Learn about the potential impacts of frost.

Hail (Grêle)

Hail forms when updrafts in thunderclouds carry raindrops upward into extremely cold areas of the atmosphere, where they freeze and merge into lumps of ice. When the lumps become too heavy, they fall to the ground at speeds of up to 100 km/h or more.

Canada has recorded hailstones as large as grapefruit, but even smaller hail can be dangerous.

If you get caught in hail:

seek shelter in a solid building and stay away from windows, glass doors or skylights
in a vehicle, find a place to safely pull off roadways
if caught outdoors, crouch down, face away from the wind and protect your head and neck with your hands

Look out for flooded areas. Excessive hail, when combined with heavy rain, can plug storm drains and suddenly create local flooding.

Get prepared for hailstorms.

Heat and humidity (Chaleur et humidité)

Learn about Extreme heat and heat warnings.

Explore Humidity and the Humidex table.

Lightning (Foudre)

Lightning can deliver as much as 100 million volts of electricity and strike a target up to 16 kilometres away from a thunderstorm, making it an extremely dangerous form of severe weather.

When thunder roars, go indoors!

Learn more about lightning safety.

Use our Lightning Map to see lightning strikes in real-time across Canada.

Rain (Pluie)

There are two types of summer heavy rains:

Large-scale weather systems with long-term rainfall. These large weather systems can last several days. We issue rainfall warnings for this type of event
Short-lived thunderstorms with intense rainfall. Within a short period of time, sometimes only minutes, localized downpours from thunderstorms can create flash flooding. We issue severe thunderstorm warnings for this type of event

During heavy rains, avoid low-lying areas as they can unexpectedly flood or overflow. Do not try to drive on a flooded road, you cannot tell the condition of the road under the water.

Note: Provincial governments are responsible for issuing flood forecasts and warnings.

Get prepared for floods.

Severe thunderstorms and tornadoes (Orages violent et tornades)

Learn more about severe thunderstorms and tornadoes.

Storm surges (Onde de tempête)

A storm surge is an abnormal rise in water level where a high and forceful dome of wind-driven ocean water sweeps towards the coastline. This happens near where the eye of a tropical or non-tropical storm makes landfall, or passes close to the coast.

Often accompanied by high waves, storm surges can cross the normal high-water mark defined by the tide, and cause damage to infrastructure along coastal areas. They can also cause coastal flooding as far inland as several kilometres.

In the event of a storm surge, avoid coastal areas, particularly those prone to flooding, and seek higher ground.

Get prepared for coastal flooding and storm surges.

Learn more about hurricane forecasts and facts.

Ultraviolet (UV) index (Indice UV)

The UV index scale tells you how strong the sun’s UV rays are. The amount of UV you get depends on the sun's strength and how long you spend in the sun. The higher the UV index, the more the sun’s rays can harm your skin and eyes.

Learn more about the UV Index and sun safety.

Wind (Vent)

In a thunderstorm, “straight-line” winds, not tornadoes, are often the most destructive. They are winds that move horizontally along the ground away from thunderstorms, sometimes with tornado-like force. They are also called microbursts, downbursts, squall lines, plough winds, or derechos.

Strong winds can cause property damage or turn any loose item into a dangerous projectile. They can also create unsafe travelling conditions.

Stay safe in strong winds:

stay alert for potential projectiles or signs of wind (swirls of dust, waves on water, fast-moving clouds below thunderstorms)
secure or put away loose objects such as outdoor furniture or garbage cans. Park vehicles in garages, when possible
seek shelter for yourself, and your pets or livestock

Learn more about the potential impacts from strong winds.

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2026-07-07

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