Ask a Nerd

July 3rd, 2002

RD of Brunswick, ME asks:

What causes "lake effect"?

The Nerd Responds:

I had to look this one up, and it was pretty interesting. "Lake Effect" snow, like you see in places like Chicago, Illinois or Buffalo, New York, occurs when cold, dry winds come down from the artic and sweep over the large bodies of water that are the Great Lakes. Here is what happens:

Large bodies of water like the Great Lakes collect warmth from sunlight and are typically a few degrees warmer than the air above them. When cold, dry air moves across these lakes, the air mass acts like a giant sponge and soaks up large quantities of moisture off the lake.

Don't believe it? I actually wanted to see if I could duplicate the above process in my kitchen. I took a clear glass and set it in my freezer for about half an hour. I covered the glass with a lid to keep the cold air trapped inside. Meanwhile I took a glass bowl and filled it with warmish water. After half an hour, I took the glass out and quickly brought it over to the bowl. Turning the glass upside down, I took away the lid and then very slowly drew it across the surface of the water. Sure enough, the cold air started to draw in the the warm water, as I could see when ice crystals began to appear on the inside of the glass. I had made a miniture snow storm! Try it out!

Back to the lake effect. Having sucked in all that warmer water, the air is now warmer than the rest of the air around it. Being warmer, it naturally rises up higher into the atmosphere. As it rises through the cooler air, this warm air starts to cool down again, and all the water it picked up starts to condense and forms clouds capable of producing snow. (the subject of exactly how snow forms is something I will tackle later if anyone is interested). The areas downwind of these clouds are likely to receive a great deal of precipitation in the form of snow. Sometimes the temperature difference between the warmer lakes and the bitter artic air is so extreme that thundersnow is actually created. When this occurs, you can actually see lightening and hear thunder while it is snowing.