Why Cumulus Clouds Have Flat Bases

Anyone who knows me well knows that I am a total weather geek!  Driving into work Tuesday afternoon I was in awe at just how quickly the clouds developed following the sunshine we had earlier in the morning.  The quick development of those cumulus clouds was due to a couple of factors.

The first had to do with what was happening above our heads.  There have been several 'mini' low pressure systems rotating around one large low over southern Canada the past couple of days.  Each passage of the mini low causes air to rise in the atmosphere, forming clouds and eventually spotty showers.

Second, temperatures in the middle of the atmosphere are very cool.  As the sun heats up the surface of the earth, little tiny air parcels rise into the cooler section of the atmosphere.  This rising motion helped create a little instability in the atmosphere, which in turn allowed showers to develop.

If you looked closely at the clouds Tuesday afternoon you may have noticed they all had one thing in common: the bases of the clouds were all flat and appeared to be forming at roughly the height in the atmosphere.  Ever wonder why that is? 

This picture from Ann Kenney in Scarboro, IL captures what's going on very well.  Notice how the building cumulus clouds have flat bases to them at roughly the same height.  This is a result of the air temperature and air pressure coming together at just the right point in the atmosphere to allow rising water vapor to condense and form clouds.  As the sun heats the surface of the earth, tiny air parcels begin to rise.  These air parcels carry water vapor with them.  Water vapor is something we can't see, but is all around us - all the time.  Once that air parcel reaches a point in the atmosphere, known as the LCL (Lifted Condensation Level), it becomes saturated and forms the cloud base.  Because this point in the atmosphere is relatively uniform, the clouds appear to have their bases starting at the same height.  After this occurs, the parcels of air become more buoyant and continue to rise allowing those puffy clouds to form.  The more unstable the atmosphere is, the higher the cloud tops rise.  Notice these clouds don't have a great deal of height to them, but just enough to produce those rain showers underneath the bases.  Pretty cool!  There is also another point in the atmosphere called the CCL (Convective Condensation Level).  The CCL is usually higher up in the atmosphere than the LCL and is primarily achieved when there is no lifting mechanism in the atmosphere occurring.  For example, there are no approaching fronts, dry lines or upper level features. 

Where the base of the cloud forms also depends on how dry or moist the atmosphere is.  The drier the atmosphere, the higher the cloud base will be.  The more moist the atmosphere is, the lower the cloud base will be.