WHAT IS A LAKE SHADOW?

Written by: Bob Metcalfe

Continuing from Brian's Thursday open forum, one reader asked about the lake shadow.

Pardon my horrifically crude editing on the picture, but I have highlighted the "lake shadow" area in red, along with the general wind direction on a typical summer day in western New York. This is a visible satellite image.

Notice the distict line between clear skies closest to the shore, and then a sudden development of clouds? The clear area is the lake shadow.

Here's how it works: On a warm summer day, the sun heats the surface to high temperatures. As we all know, a warm airmass will rise. On hot days, this rising motion is strong enough for clouds to form (fair weather cumulus as they are so often called.) This rising motion is termed instability. This happens in an unstable airmass.

Now, remember that the lake is nearly always colder than the ground during the summer. In this picture, the wind was of a northwesterly direction. So the air crossing Lake Ontario is modified to a cooler state. This cooler air on these warm days is deemed "stable" and therefore doesn't rise. The colder the air or the stronger the wind (or the combination of them both), the more pronounced and farther reaching the lake shadow can develop. That's why on a summer day it can be clear near the lake, but you can have tons of cumulus even a few miles away.

This effect can also occur off of the Finger Lakes, but to an obviously smaller extent. Great question!