Picture This: You’re on the shores of Lake Ontario and skies are completely sunny. Later on that day, you drive south on I-390 and notice an abundance of cumulus clouds suddenly hovering overhead. What gives? We call this effect a lake shadow, and it can produce a satellite image like this:

This effect is pretty harmless and really just provides a fun looking satellite image and friendly clouds to many, but it can also tell us about what temperatures are like across the region. Here’s how it works:

As incoming solar radiation from the sun heats up the Earth, the ground is able to warm much faster than water due to differences in specific heat. On a day where high pressure is in control, and fair weather provides sunny skies to the region, the temperature difference that occurs causes a shift in the winds out of the north, and a lake breeze boundary sets up.

All you need to get this effect is air to flow over the cooler, more stable waters of the lake. Then once that same air flows over the warm land driven by the sun, you get clouds to bubble up from daytime heating. This produces a “shadow-like” appearance just south of the lakeshore where skies remain clear that can stretch as far down as into Rochester and along the thruway.

Where the air is still stable just south of the lake, convective cloud development is minimized while not too far to the south, you get areas of cloud development where warm air rises and condenses to form clouds. You can even get lake shadows to form downwind of the Finger Lakes on some occasions!

Lake shadows typically occur during the spring, summer, and sometimes fall months. This effect can be deceiving as well as we typically associate sunnier skies with warmth and cloudier skies with cooler temperatures. When this effect takes place and a lake breeze boundary is present, northerly winds by the lake mean much cooler temperatures than those who are farther inland even with the full sunshine.

When a lake shadow is present, you can get vastly different sky views from Rochester and Canandaigua such as the ones below:

~Meteorologist Christine Gregory