Which two air masses are considered very rare in Canada?

The correct answer highlights the rarity of the cT (continental Tropical) air mass and its combination with cP (continental Polar). The cP air mass is characterized by cold and dry conditions typically found over the Canadian interior during winter months. While cP is relatively common in Canada, the cT air mass is much rarer because it originates from warm, dry regions like deserts and is less likely to penetrate into Canada due to the country's northern latitude and associated climatic conditions.

Understanding the characteristics of these air masses is crucial for pilots, as they affect weather patterns, visibility, and overall flight safety. The cT air mass, when it does occur, brings warm and dry conditions, but the polar air masses significantly dominate Canada's weather, especially in the colder months, making occurrences of cT rare.

In contrast, the other air mass combinations listed involve either air masses that can be more common in specific areas or times of the year in Canada. For instance, mT (maritime Tropical) is more associated with wetter weather that can happen in southern regions, but still isn't as impactful in broader Canadian geography, while cA (continental Arctic) can occur but typically relates to cold Arctic air influxes which do not reflect the rarity assessed in the question