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How the Great Lakes produce this extensive cloud cover

Posted: 3:24 PM, Dec 27, 2017
Updated: 2017-12-27 15:29:40-05

WEST MICHIGAN — At this time of year, the Great Lakes are still mostly unfrozen — even if temperatures have been below average for the month of December. That makes them a huge heat source when Arctic airmasses invade from the northwest.

That is what’s happening right now as the coldest air we’ve experienced in almost three years settles into the area.

The image above from National Oceanic and Atmospheric Administration (NOAA) satellites shows the extensive cloud cover downwind of the Great Lakes.

The cold, dry air from Canada is clashing with the (relatively) warm, moist air over the Great Lakes to create these clouds, and resultant lake effect snow showers. The airmass that is crossing the Great Lakes from Canada doesn’t have a lot of moisture carrying capacity. Therefore when water from the mostly unfrozen lakes evaporates into this airmass, clouds form very quickly.

Here’s a look at one of our live cameras offshore of Chicago this afternoon:

By contrast, here is a look from the channel at Muskegon from the same approximate time frame after the cold, dry air crosses the lake:

As you can see, the sky changes from nearly cloud-free on the west side of the lake to overcast on the east side.

The clouds also have some convective characteristics — that is, they build vertically (somewhat like a cumulonimbus cloud in a summertime thunderstorm) since the air over the lake is so much warmer and more buoyant than the resident airmass.

Obviously, lake surface temperatures are 32° or warmer since most of the lake is ice-free. This is in contrast to the bitterly cold temperatures we’re seeing on land at this hour:

That’s why this satellite image is so impressive right now.

As the lakes continue to accumulate ice in the weeks ahead, the extend of this cloud cover during subsequent Arctic invasions won’t be quite as impressive. So enjoy the thick lake effect clouds while they last!