Chicago Before Chicago

Surface Temperature Differences in the Great Lakes

By Derek Kaden

Have you ever heard someone say that the water along Illinois’ or Wisconsin’s beaches is colder compared to Michigan’s? How could that even be possible? I mean, the air temperature in Chicago and Benton Harbor in Michigan could be the exact same, but the lake temperatures in these two places could be completely different. Why?

The answer has everything to do with geography.

All water is propelled by the wind. In the Great Lakes region, the dominant winds – called the Prevailing Westerlies – generally move from the west to the east. They travel in this direction because the Earth rotates counterclockwise. Therefore, the Westerlies push lake water away from the western shore and toward the east.

One important characteristic of water is that the colder it gets in temperature, the heavier it gets as well. Warm water is lighter, less dense, which means its molecules are more spread out. Therefore warm water rises to the surface, while cold water sinks to the bottom. Fresh water is at its densest when it is at a cold 39.2°F. This means that the water at the bottom of the Great Lakes – or any lake that extends deeper than the pycnocline (1,000m) – is always going to be 39.2°F! Learn more about lakes, differences between fresh and salt water, and the ocean in this blog post.

When the wind pushes water away from Chicago’s shore, the water it pushed needs to be replaced. At the same time, the water being pushed toward Michigan’s shore needs somewhere to go. This movement of water is called upwelling and downwelling.

Chicago’s shore experiences upwelling, meaning the water being pushed away by the wind gets replaced by the dense cold water from the bottom of the lake. Downwelling is the reverse of this. In Michigan, the warm surface water gets shoved to the bottom, leaving no chance for the cold water at the bottom to rise.

Take a look at these pictures I drew which help to illustrate the point:



The fluctuation in temperature is greatest between late spring and early fall. In these months, the surface temperatures on Lake Michigan can vary by as much as 15 or 20 degrees between the western and eastern shores. The same goes for any of the other four lakes. During the winter, the lakes’ surface temperatures are pretty much as cold as at the bottom. It’s either frozen (32°F) or just covered in cold dense water. Take a look at these temperature maps produced by the National Oceanic and Atmospheric Association (NOAA).

The first one is from spring of 2014. Notice the warmer surface temperatures beginning in the middle of Indiana on Lake Michigan. They go on past Muskegon.


The same fluctuation can be seen in the summer. The biggest difference is on Wisconsin’s shoreline, between Milwaukee and Green Bay (light green), compared to shoreline north of Muskegon (brown and red). Both of these regions are at basically the same latitude, but the difference in water temperature is up to 15 degrees! This is upwelling and downwelling in full effect.



The trend continues into the fall.



And by winter, the fluctuation subsides and we’re left with a combination of cold dense water and…ice.


Cool Summer Experience presents… The ECO-Times

Baseball & Skate ParkA few months ago we reported on the highly creative and advanced work of one of our youth programs called Cool Summer Experience (CSE) based in Waukegan. Well, they’re back with Issue 2 of the ECO-Times, a comprehensive wrap-up of everything they got into this past summer. Highlights include a tour around Lake Michigan on the W. G. Jackson aquatic research vessel, a trip to the Dokum Mskoda Nature Preserve to track down the elusive Eastern Prairie Fringe Orchid, and an encounter with enormous butterflies at the Chicago Botanic Gardens. Read all about it… The ECO-Times, Summer 2013

Lake Michigan’s Ice Formations: Part 7

The following post is the latest installment of an ongoing series here at the Backyard Geographer documenting seasonal ice formations on Lake Michigan.  For previous coverage, see parts 1 and 2 from 2009-2011, and parts 3, 4, 5, and 6 from winter 2012-2013.


The following series of pictures was taken on January 27, 2013.  The air temperature is in the high 20s after similar conditions yesterday.  Lake Michigan water temperature is about 37⁰F.

January 27, 2013, looking east along lakeshore, Wilmette, Illinois, Gillson Park

January 27, 2013, looking east along lakeshore, Wilmette, Illinois, Gillson Park

Even with air temperatures below freezing, water temperatures above freezing are winning the battle at the shoreline.  The impressive 12-foot tall ridge at the waterline only three days ago has melted back to leave these remnants stranded offshore by open water.

January 27, 2013, looking northeast along lakeshore, Wilmette, Illinois, Langdon Park

January 27, 2013, looking northeast along lakeshore, Wilmette, Illinois, Langdon Park

Signs of ice demise are everywhere.  The right side of this ice volcano is collapsing as relatively warm water melts the ice walls of the volcano’s plumbing system.  One slab has collapsed forming the window through which the lake can be seen.  The large block forming the ice bridge seems on the brink of collapse due to developing cracks.

Temperatures are forecast to reach well into the 50s in a few days.  Time will tell if any ice remain until the next cold snap predicted for later in the week.


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