I have noticed that winds are frequently southwesterly, even southerly. If this is occurring across the entire Northern Hemisphere, why doesn’t air accumulate in the polar region?
—Paul Jenson, Champaign
The tendency toward southwest or south winds is a mid-latitude thing, but it’s not universal even there. Southwest or south winds dominate much of the United States, especially April through October. The same trend shows up here in Chicago.
Cold dense air spills southward from the polar regions, warms as it moves south and begins to rise. This rising motion, about a third of the way between the North Pole and equator, encourages mid-latitude air to surge north into the region of ascending air, twisting prevailing westerly winds there to a southwesterly direction.