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.


Great Moments in Geographic Illiteracy, Part 4

Bullfrogs in the USAWe’ll take it easy on this one since it’s from a kid’s book… but I can’t stop laughing at how poorly the Great Lakes region is depicted on this map.  Also, the fact that it looks like giant smirking frogs are taking over the country doesn’t hurt either.

Monday News and Links


  • The above picture is courtesy of Earth Science Picture of the Day and it documents the phenomenal “ice balls” that accumulate on Lake Michigan’s shores when weather and water conditions are just right.  Be sure to read their description in the link, it’s fascinating.
  • Here’s another spectacular Lake Michigan photo from the same site.
  • If you want to learn more about soil then you ever thought there was to know, check out this Michigan State University site detailing the soils of the Great Lakes region.  Who would have thought the taxonomy of soil would be so strangely named… Incepticols, Udalfs, Psamments, Glossaqualfs??  (Sounds like warring tribes in a Dune novel.)
  • Last week Slate highlighted this curious map of Whole Foods and Wal-Mart locations in the San Francisco Bay Area, illustrating what one would assume is the stark economic disparity between the San Francisco Peninsula and the East Bay.
  • LBx Journal (“location in the language of business”) has a special section in their Winter 2013 issue devoted to women in the location industry.  Of the 18 women profiled, two are our very own: Celeste Fraser and Jillian Elder.  Nice work!

Five Lakes in One

The NASA satellite image below shows the five Great Lakes on a rare cloudless day (August 28, 2010). The lake basins were gouged out by a series of continental glaciers comprised of ice sheets more than a mile thick in places. Over the past 2.4 million years, repeated episodes of glacial advance and retreat have scoured the region. The most recent ice sheet vacated the northern edge of the Great Lakes watershed about 9,000 years ago (see Larson and Schaetzl). As the ice age came to an end, the lake basins filled with glacial meltwater. Over the next 5,000 years lake levels, lake shapes, and drainage patterns fluctuated as the elevation of the land rose with the weight of ice removed, new outlets were uncovered by the melting ice, and erosion uncovered new drainage streams. For example, 5,000 years ago the current site of the City of Chicago was under more than 20 feet of water (see Michigan State University). At that time the bulbous southern end of what is now known as Lake Michigan drained southwest to the Illinois/Mississippi river system. This outlet of Glacial Lake Chicago was abandoned about 4,000 years ago as the current level of Lake Michigan was reached.

The lakes that remain today represent the largest group of freshwater lakes on the surface of the planet. Collectively, the Great Lakes represent roughly 18% of Earth’s surface fresh water that is not frozen. The five lakes form a truly interrelated system due to their watery connections. Although new channels have been dug and natural channels have been modified by man, the Great Lakes remain connected to the sea in much the same configuration as the glaciers left them.

Lake Superior is the highest in the system (see TEACH for a profile). It empties through the St. Marys River into Lake Huron which is 20 feet lower. Lake Michigan is at the same elevation and is connected to Huron by the deep Straits of Mackinac. The three upper lakes drain through the St. Clair River, Lake St. Clair and the Detroit River into the first of the lower lakes only 8 feet below, Lake Erie. The largest drop (325 feet) is over Niagara Falls in the Niagara River connecting Erie with Lake Ontario. From there the system drains into the Atlantic Ocean through the St. Lawrence River.

As the name implies, Lake Superior is the largest of the five on nearly every measure of size (see NOAA). By volume, Lake Superior contains more water than the other four combined due to its large surface area and quarter-mile depth. Lake Michigan is the second largest by volume but Lake Huron is second by a small margin in surface area. Of the two smallest lakes, Erie has the least volume due to its shallow depth even though it is quite a bit larger in surface area than Ontario.

Ready access to this huge water supply has encouraged the growth of large population centers. The region encompassed by the image includes 25% of the Canadian population and 10% of the U.S. population. Nearly 34 million people live in the basins of the five lakes, led by Lake Erie and Lake Michigan, each having over 12 million people living in their tributary areas.

Humans use this water for drinking, recreation, agriculture, transportation, industrial processes and waste disposal. As one might expect, conflicts among these uses abound. Environmental threats include uncontrolled pollutants from urban and agricultural runoff, effluent from industrial processes, and the introduction of invasive species.

The opening of the St. Lawrence Seaway and the construction of the Welland Canal around Niagara Falls were major contributors to the introduction of invasive species into the Great Lakes. These constructions allowed ocean-going vessels to penetrate the Great Lakes all the way to Lake Superior. Ballast water in these ships contained many exotic species from around the world. Sea lampreys, round gobies, zebra mussels, quagga mussels, and Eurasian ruff are some of the most disruptive species that have invaded via this route.

Due to their voracious appetites and rapid breeding, Asian Carp pose a serious threat to Great Lakes' ecosystems

The possible introduction of Asian carp into the system represents a threat that has received a lot of attention lately. This exotic species has worked its way up the Mississippi and Illinois rivers into the Chicago region. Under natural conditions, these rivers had not connected to the Great Lakes since the days when Glacial Lake Chicago drained in that direction. With the completion of the Illinois and Michigan Canal in 1844 and the Chicago Sanitary and Ship Canal in 1990, man-made connections between the Great Lakes and the interior river systems became well established. Though useful for transportation and wastewater removal, these artificial waterways present another threat to the Great Lakes system by providing an additional conduit for the introduction of invasive species.