OK, this might seem a little trivial and of course you get clouds at night. But for this article we are concerned with cumulus clouds which build up during the day and then dissipate at night. So, where do they go?
Clouds are simply made up of water droplets which are suspended in the air in an environment which is totally saturated with 100% relative humidity. They form when the air has been cooled enough to its dew point and becomes saturated and unable to hold any more water vapour.
Clouds will break up and dissipate when any or all of these three things happen :-
- The temperature increases. If this happens then evaporation occurs as the cloud is warmed and the air is then able to hold more moisture. This scenario happens when low cloud is "burnt" off in the morning as the sun warms the land. The warming of the land/cloud increases the temperature and therefore lowers the relative humidity.
- Dry air mixing. When a cloud builds during the day it is because the parcel of air rising is warmer than its surroundings and will continue to rise. You can tell if this process is occurring when cumulus clouds have well defined "clear cut" edges to them near the top. As soon as this process stops and the source of heat is removed then the drier air surrounding the cloud will begin to mix with the cloud edges in a process called entrainment. When entrainment starts to occur, mixing between the cloud and the surrounding air causes evaporation of the cloud into its surroundings.
- Sinking air. This process is the most common for cloud dissipation. It can happen in a variety of ways including when the heat source is removed in the evening as the sun sets and the air which has risen during the day cools and descends back to the surface. High pressure moving in will cause upper level convergence of the air or Negative Vorticity Advection which will create sinking air at upper and medium levels which will dissipate clouds. Air sinking to the lee of mountains or hills will warm the air and cause cloud dissipation too. Basically, any process where the air is sinking will dissipate clouds.
So, cumulus clouds don't go to sleep at night, but they simply return back to the boundary layer as moisture ready for the next day of warming.