Speaking in "extreme" generalities
If its a cloudy start to the day, fish tend to be caught with deeper lures. As the sun rises and warms the surface water, I get more hits off the lighter lures.
With this being said, you can see why Greg begins with two deep umbrellas and moves at least one umbrella to a board line.
Check some of the buoys on the Chesapeake Bay interactive buoy system, it will give two water temperatures, one for the surface and one for the bottom.
The MSSA meeting at the Broadneck Chapter (Monday Night) will give you information about thermoclines
5th