While watching the Starlink 25 launch earlier this month, I noticed that the Florida peninsula seemed to be nicely outlined by clouds. Pulling up satellite imagery from that time showed that my impression was correct: virtually the entire southeastern United States was covered with small, puffy clouds, neatly cut off at the coastline, with clear skies over the ocean.
What causes this?
The small puffy clouds over Florida are cumulus clouds ('fair weather clouds' accompanying the launch). The sun heats the ground and the ground composition is not evenly distributed, where air is warmed more quickly it starts to rise until the moisture it contains condensates, forming clouds, while colder air in between sinks. This is called convection, in this case limited to the lower atmosphere over Florida.
The ocean is mostly free of them because it isn't heated as much as the land and warming is more uniformly distributed, so that in that specific weather situation (high pressure it seems) no convection did develop over water.
