Lake Superior's temperature has been falling recently. Here's why.

Lake Superior in Minnesota. (Melissa Turtinen/FOX 9 / FOX 9)

The surface water temperature in Lake Superior is getting colder recently, despite some recent warm water around Duluth, Minnesota, and Superior, Wisconsin, the National Weather Service (NWS) in Duluth said Wednesday. 

The surface water temperature across the whole lake has dropped about a degree in the last week, despite seeing a steady warmup throughout the month of July, NWS tweeted. At the nearshore near McQuade Harbor in Duluth, the area of the lake has lost more than 20 degrees since July 19, when the surface water temperature was nearly 70 degrees. 

So why is this happening along the North Shore? The NWS explains: 

"First, we have to take a side trip into fluid dynamics. Due to something known as Ekman transport, water in deep bodies of water like the ocean or Lake Superior move to the right of the wind direction. With a southwesterly wind along the North Shore, for example, water moves easterly away from the coast, and deeper water rises to replace it. In Lake Superior, deeper water stays around 40 degrees year-round, and when that mixes in, surface water temperatures can drop quickly."

Since July 19, water temperature and wind direction in the area of the McQuade Harbor nearshore buoy showed a prolonged period of westerly winds that correlate with dropping surface water temperatures. Brief periods of northerly winds allowed warmer water to make a short-lived appearance, NWS said. 

The decrease in water temperature correlates with the last week that saw predominantly southwest to west winds, according to weather officials.

West-southwest winds are expected to continue over western Lake Superior, with a water temperature forecast showing cold water along the North Shore from July 25-July 29. A vertical water temperature forecast also shows cold water stays in place from Cornucopia, Wisconsin, to Little Marais, Minnesota. Cold water stays in place with a shallow layer of warmer water along the South Shore. 

You can find a marine water forecast here and information about cold water safety here.