Chicago lies in the zone of prevailing westerlies, but what does that really mean?

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Dear Tom,
The wind seems to be coming from the east more often than it used to, so what is happening to prevailing westerlies?
— Cher Stallman, Dwight, Ill.
Dear Cher,
Chicago lies in the zone of prevailing westerlies, but what that really means is weather systems (low- and high-pressure areas) generally move from a westerly (northwest, west or southwest) direction to an easterly (southeast, east or northeast) direction.
Surface winds associated with those systems blow in a counterclockwise sense around lows and in a clockwise sense around highs. Therefore those winds do not reflect the actual motion of the pressure systems causing them. Also, remember that Chicago’s lake breeze tends to blow in warm weather. Chicago has experienced warm conditions this autumn quite often and so the lake breeze has been frequent.

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