Cumulus clouds range from small puffy cotton ball clouds (fair weather cumulus) to huge thunderstorm complexes of deep convection. The distinguishing feature between cumulus and stratus clouds is that a cumulus cloud is vertically developed. For a cloud to be vertically developed requires the ingredient of instability. Instability is released when a parcel of air is placed in an environment in which it is less dense than the surrounding air. The cumulus cloud is a parcel of less dense air (warmer and moister) than the surrounding air. Like a helium balloon or a bubble in a pot of boiling water, the parcel will rise. As the cloud rises it will continue to condense out more moisture since rising air cools. As air cools, moisture in the saturated air is condensed out. This builds the condensed moisture of the cloud as the cloud climbs in the troposphere.

The depth of the cumulus cloud will depend on how deep the layer of instability is. Shallow instability will lead to non-rain producing fair weather cumulus. These can commonly be seen in the summer time. As the layer of instability builds, cumulus clouds can become bigger and thus more vertically developed. When the layer of instability becomes deep enough and parcels can penetrate into this layer, then cumulus congestus that develop into thunderstorm clouds can occur. Cumulus clouds have a photogenic appearance given their towering cauliflower appearance in the sky.