The cool, stable air that follows behind a cold front usually serves to kill any clouds or haze, leaving the skies looking crystal clear. Sometimes, though, the sun heats the surface enough to create a shallow level of warmer, more unstable air than what lies above it. The warmer air will rise up into the colder air above and condense into fields of cumulus clouds that sometimes cover hundreds of thousands of square miles like we see today.
The difference between the rate at which land heats up and water heats up reflects in the cumulus fields.
Satellite imagery from this afternoon shows that the air is almost completely clear above most of the bodies of water in the Upper Midwest, along with areas downwind of the lakes, thanks to the water staying cooler than the land around it.
For the full effect, here's a six-image animation showing the development of the clouds with daytime heating:
[Images via GOES]