Where do continental air masses typically form?

Continental air masses typically form over land, which is why this choice is correct. The characteristics of continental air masses are defined largely by their source region, which is primarily land. Since they originate over large expanses of land, these air masses tend to have lower humidity compared to maritime air masses, which form over water. This is because land does not retain moisture in the same way that bodies of water do.

When air masses form over land, they are classified as "continental," as opposed to "maritime," which would indicate a formation over the ocean. This distinction is critical in understanding how different air masses can influence weather patterns.

In the context of the other options, air masses that form in deserts are also considered continental, but not all continental air masses come from desert regions specifically. Similarly, air masses that originate over water would be categorized as maritime and would feature higher humidity levels. Thus, when considering the broader characteristics of continental air masses, the key factor is their formation over land, making this the accurate choice.