Lake breezes that bring some relief on a scorching summer afternoon are thought to move more slowly through Chicago than through the surrounding suburbs. Scientists at the Illinois State Water Survey have discovered that this is often not the case and have gained new insights into the mysteries of how cities affect winds off a lake.

Looking at 44 lake-breezes from Lake Michigan in the Chicago region from March to November 2005, Jason Keeler, a graduate student in the University of Illinois Department of Atmospheric Sciences, and David Kristovich, head of the Center for Atmospheric Sciences in the Illinois State Water Survey, found that in many instances, the lake breeze acted as if the city wasn’t there. Nearly 30 percent of the lake breezes moving through the city reached as far as 30 miles inland.

Two factors explain why urban areas might slow breeze movement. Urban areas have many taller structures than more rural areas. Flow around these structures disrupts the lower portions of the lake breeze, causing it to slow.

A second important reason for slowing of lake breezes is winds set up by “urban heat islands.” Urban heat islands, which describe the tendency for cities to be hotter than surrounding suburbs, can change the wind speeds and directions in the area. In theory, the lake breeze should slow down due to the onslaught of winds coming from various directions.

“We found that quite often, the lake breeze doesn’t follow the theoretical pattern,” Kristovich said.  Keeler could not find a relationship between the afternoon urban heat island intensity and how quickly the lake breeze moved through the urban area.

“What really shocked us is that there was a strong relationship between lake breeze movement and the city heat the night before.”

High temperatures in the city center are most pronounced at night, but lake breezes typically develop in the afternoon.  It appears that the temperatures in the city night, many hours before lake breezes form, may influence the movement of lake breezes the next afternoon.

“The exciting aspect of these outcomes is that if this finding is confirmed, it may give us a strong prediction tool,” Kristovich said.  “We can measure the temperatures the night before and have a strong hint of whether the lake breeze will move through or stay in the city the next day.”

Research findings of this type are valuable because lake breezes can have a large impact on summer temperatures and resulting use of electricity for air conditioning, the location where it rains, and even the direction that planes must land and take off.  Lake breezes can even change the amount of air pollution in the city.

This and other research on lake breezes also can be applied to sea breezes, Kristovich said.  Lake and sea breezes affect millions of people living in ocean coastal cities and along the Great Lakes.

The Illinois State Water Survey at the University of Illinois at Urbana-Champaign, a division of the Prairie Research Institute, is the primary agency in Illinois concerned with water and atmospheric resources.