Thursday afternoon's visible satellite imagery was very cool as two holes in a large area of cumulus clouds persisted over south central Illinois. To understand why these holes appeared, the process of cumulus cloud formation must be understood. Below is a schematic showing the earth's surface and air near the surface being heated by the sun. As the temperature increases rapidly, air parcels begin to rise because they are warmer than their surroundings. If you combine the rising air with enough moisture, clouds are created because the air slowly saturates as it rises and cools. Cumulus clouds are formed! The reason there were no clouds over the large lakes is because water does not heat up as fast as the land does during daytime heating, and therefore air parcels were not rising high enough to create clouds over the lake. The satellite loop is also below.
The sun heats the ground and the air near the surface rapidly. Air begins to rise because it is warmer than its surroundings. As it rises, it begins to cool again however the air becomes more saturated. Clouds are then created. Water does not heat as rapidly during the day as land. On the other hand, it does not cool as fast at night.
Notice the two persistent holes in the imagery over south central Illinois. Two arrows pointing out the location of the lakes appear on the last image in the loop. Another large area void of cumulus clouds is evident over central and southeastern Indiana as well as central Kentucky. This is because there was not enough moisture to produce clouds.
Pictured below is a pyrocumulus cloud. The cloud at the top is caused by the heat plume from the fire below. This picture is great for illustrating how the warmer air at the surface rises to create a cloud.