Understanding the Different Types of Fog

Fog is a common weather feature in many parts of Canada, but not all fog forms in the same way.

Fog is classified based on the processes that cause air near the surface to become saturated. Understanding those differences can help explain why some areas experience dense fog while nearby locations remain clear.

Radiation fog develops when the ground cools rapidly, usually overnight under clear and calm conditions. The air near the surface cools to its dew point, allowing fog to form. It is often most noticeable during the overnight and early morning hours and can dissipate after sunrise as temperatures rise.

Advection fog is particularly important in coastal areas. It forms when warm, moist air moves over a colder surface, causing the air to cool to its dew point.

Steam or evaporation fog occurs when cold air moves over relatively warm water. Moisture evaporating from the water quickly saturates the colder air, producing fog that can sometimes appear like steam rising from a lake, river or ocean.

Upslope fog forms when moist air is forced to rise over hills or mountains. As the air rises, it expands and cools, potentially reaching its dew point and producing fog along elevated terrain.

Fog can also develop during periods of precipitation. Precipitation fog occurs when rain evaporates into the air below a cloud, increasing moisture and sometimes cooling the air enough for saturation to occur. This can be particularly common around frontal systems.

Another important type is freezing fog, which occurs when fog droplets remain liquid despite temperatures being below freezing. When those supercooled droplets come into contact with surfaces, they can freeze and produce rime, creating hazardous conditions for motorists and aircraft.

Valley fog can also develop, particularly in low-lying areas. It is often associated with radiation fog, as colder, denser air settles into valleys overnight.

While fog can occur at any time of year, its formation depends on the combination of temperature, moisture, wind and surface conditions.