Did You Notice the Clouds Yesterday?

Did you notice the clouds yesterday?  They were altocumulus castellanus and a treat for cloud spotters (especially in Western Washington).   

“Alto” (for mid-level clouds), “Cumulus” (vertical clouds that form from rising bubbles of air) and “Castellanus” (castle-like). 

My wife, Helen, took this picture with her cell when we were on a flower delivery.  It’s a cloud that we don’t often see in Enumclaw, or for that matter in Western Washington. 

Cumulus clouds are most often caused by rising bubbles of warm air that come from the sun heating the ground.  The air cools as it rises and, if lifted far enough, forms clouds.  Check out my recent blogs (Warm Air Rises and Cold Air sinks--Doesn’t It?) for an explanation of how rising and sinking air in the atmosphere works differently from rising air in a room.

Altocumulus castellanus are a small-scale cumulus that forms in the mid-levels.  They form, not from bubbles of warm air rising from the surface, but from air from the south that has been lifted over long distances into the mid-levels of the atmosphere.  When the lower parts of the lifted air becomes warmer than the air just above it, that air raises and altocumulus castellanus may form.

Altocumulus castellanus clouds often are a precursor of afternoon thunderstorms east of the Rockies. 

However, in Western Washington, we don’t have the extreme warm and humid air at the surface and the stronger heating from the sun that is found east of the Rockies in the summer.  Also, east of the Rockies the ground is already almost a mile high, which helps bubbles of warm air reach the level of the altocumulus castellanus.