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:

Upwelling

downwelling

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.

spring

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.

summer

 

The trend continues into the fall.

fall

 

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

winter