Lake Breeze
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On summer days when the temperature soars, a special wind system forms along the Great Lake shorelines, and it is known simply enough as the lake breeze. Hi, I'm Dave Thurlow and this, simply enough, is The Weather Notebook.

The lake breeze is born during summer months because Great Lake waters do not warm as quickly as does the surrounding land. Air cooled by contact with this cold water is denser, therefore heavier than air over the warmer land. The heavier exerts more or relatively higher pressure over the lake. As the sun heats the land, the air above it warms, forming a wide region of lower pressure. With high pressure over the lake the air there sinks, and with low pressure over the land the air there rises. This creates a circular airflow that near the ground blows from the cool lake to the warm land.

The strength of a lake breeze depends on the air temperature contrast between lake and land. So, the strongest lake breezes, above 15 mph, generally occur during early season heat waves when lake waters still retain much of their winter cold. Since lake water temperatures change little day to day, daily sunshine creates the temperature differential necessary to form the lake breeze around mid-morning. Clear morning skies and light winds are the most favorable weather conditions.

When the sun's heating diminishes in late afternoon, so does the lake breeze. However, around large cities such as Chicago, heat stored in urban concrete and steel may permit the lake breeze to blow into the evening hours. The lake breeze brings welcome, cool relief from the heat which gave it life.

Thanks to contributing writer Keith Heidorn and engineer Sean Doucette. Support for The Weather Notebook comes from Subaru.