The mothership of all umbrella clouds formed over thunderstorms in southern Alabama over the weekend.

This cap cloud, or pileus cloud, is likely made of ice particles. Such clouds form above quickly growing vertical thunderstorms. 

Typically not too deep or tall, pileus clouds develop as air is jettisoned above new cumulus or cumulonimbus clouds that are exploding in height below. As air is pushed out of the way, it freezes and becomes the pileus cloud. 

Pileus clouds also form above volcanic eruptions, as seen in this Sarychev eruption near Japan a decade ago. 

This cloud formation process is similar to pushing your finger through a moderately thick plastic that won't break but will stretch as you apply a force from below. 

Sometimes during rapid thunderstorm growth, parent thunderstorms can catch up to or even penetrate the pileus clouds. You can see this occurring in the image above on the left side of the pileus dome. If there's enough upward force from the thunderstorm a secondary cap cloud can form atop the primary pileus cloud. 

These atmospheric hoods usually thin and dissipate as the thunderstorm below stops growing. 

It appears that the cap cloud that formed in Alabama on Saturday may have gotten some extra support from above as well. 

As thunderstorms grew, they ran into an unordinarily southern dip in the jet stream for July. Winds aloft at the height of these pileus clouds were likely 50 to 65 mph. 

Winds like this coming from the jet stream would have needed to divert around or over the growing thunderstorms. 

It's likely that this cap cloud grew in part due to this wind, making it also a lenticular cloud. 

Lenticular clouds are common around mountainous areas where wind is diverted up and over or around the terrain.