What is the air mass that usually occurs only in winter over Canada?

The air mass that typically occurs only in winter over Canada is Continental Arctic (cA). This type of air mass originates from very high latitudes, specifically over the Arctic regions, and is characterized by extremely cold temperatures and low humidity. During winter, the cA air mass develops as the landmass of Canada experiences cold, dry conditions.

When a cA air mass settles over Canada, it can lead to frigid temperatures and clear skies, often resulting in the formation of significant cold weather patterns. The stability and cold characteristics of this air mass make it distinct from others, such as Maritime Tropical or Continental Polar air masses, which do not share the same extreme temperature profile associated with winter in Canada.

Maritime Tropical, for example, typically brings warm and humid conditions, usually found during the summer months. Continental Polar air masses are cold but are more moderate compared to the extreme cold of Continental Arctic air. Maritime Arctic air masses, while also cold, develop over the ocean and can contain some moisture, unlike the very dry conditions associated with Continental Arctic air.

In summary, the unique cold, stable, and dry characteristics of the Continental Arctic air mass are what make it primarily associated with winter over Canada.