Wind direction at the ground is sometimes different than higher up

Ask Tom Why

Dear Tom,

I have noticed on occasion that the wind direction at the ground is sometimes different than higher up. For example, northeast winds at the ground but southeast winds at cloud level. How are these two events, next to each other, appearing to act independently of each other?

Margaret Robertson,
Chicago

Dear Margaret,

Different wind directions at the surface and aloft are not at all unusual. In fact, it is the case much more often than not. And it’s true of wind speeds as well; it is usually much windier aloft than at the ground. The atmosphere is complex, and that certainly applies to wind. Broadly speaking, the factors that cause wind vary considerably from place to place, especially in the vertical. Aloft, say at 2,000 feet above the surface, air usually moves in response to large-scale weather features that often have little effect at the surface, where winds respond to local surface factors.

In the situation that you described, northeast winds at the ground but southeast winds at cloud level last June 8th (the date that you mentioned in your email), the presence of Lake Michigan may have been the explanation. The lake water is still quite chilly in June, and it imparts that chill to air over the water, but land temperatures can get quite warm during the day. Cool air is heavier and more dense than warm air, and so it tends to blow under the warmer air over land: we experience this as the familiar “lake breeze.” Winds in the lake breeze are usually from the northeast, but winds in the warm air (now lifted aloft, above the lake breeze) are still blowing from the southeast.

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