Stratus clouds,  irrespective of altitude have a more or less constant and solid base ,  indicating stable layers in the atmosphere and lack of thermal activity.

By contrast the Cumulo Nimbus cloud betrays a high degree of vertical motion of the atmosphere and thermal activity.  Cumulus congestus and Towering cumulus display a moderate degree of vertical extent

In the curtain plot below,  the classic rapid change of cloud level characteristic of Cb cloud can be seen between 19.00 and 23.00 hours.  After that time the cloud base solidifies and rises to 6500 ft

Cb cloud gives way to Ac cloud