Lake breezes are a common Spring phenomena in southeast Wisconsin. They typically develop in the late morning or early afternoon when daytime solar heating warms the land surface. Spring is an optimal time for these features, because Lake Michigan is still cold, but there can be good warming of inland locations.
Click here to check out a radar loop from Sunday afternoon. The lake breeze moved slowly to the east at roughly 5 mph during the afternoon. Winds out ahead of the lake breeze were westerly and the winds behind the boundary were easterly. This creates an area of convergence. Because of this convergence, small particles (such as dust and vegetation debris) along with biological targets (like insects) get collected right along the lake breeze. Our radar can sometimes see these targets, and thus the lake breeze shows up as a thin line of reflectivity. In addition, the air mass behind the lake breeze has different properties than the air out ahead of it. This creates a density discontinuity along the lake breeze, which can affect the radar beam and return energy back to the radar. These combined effects are why we can often see lake breezes with our radar.
Click here to check out a visible satellite and surface observation loop from Sunday afternoon. Notice from the observations how temperatures cool and winds shift to the east during the afternoon. So, even though we can't really see the lake breeze with the naked eye or with visible satellite, we know it's there.
Conceptual Model of a Lake Breeze Circulation
At the surface, the sun warms both the ground and Lake Michigan. However, the ground is not very good at maintaining heat, so it returns heat and warms the air above the ground. The warmed air, with its decreased density, begins to rise (1). The rising air creates a weak low pressure area (called a thermal low) due to a decrease in air mass at the surface (2). Typically, from 3,000 to 5,000 feet (1,000 to 1,500 meters) above this low pressure, as the air cools, it begins to collect resulting in an increase in pressure, creating a "high" (3). On the other hand, water has a high specific heat capacity, which keeps the lake from warming as fast as the land. Thus, the air over the water is much cooler.
These differences in pressures over land, both at the surface and aloft are greater than the differences in pressures over water at the same elevations (4 and 5). Therefore, as the atmosphere seeks to reestablish the equal pressure both onshore and offshore, two high pressure to low pressure airflows develop; the offshore flow aloft (6) and surface onshore, called the lake breeze (7).