May 8, 1998 transcript #: 232-5  
Subject(s): clouds, condensation level  
Title: LISTENER QUESTION: SHEARED CLOUDS

Hi this is Dave Thurlow for The Weather Notebook.  Today I have the answer well at least my answer to a question about clouds.   

“I’m calling from Omaha, NE.  My name is Doris Wallace.  I just heard The Weather Notebook on KVNO.  And it was about getting a window seat in an airplane so that you can see…look down on the clouds.  And I’ve often wondered, why is it sometimes in the distance it looks like the clouds have all been sheared off right at the bottom of the cloud?  They all have been sheared off in an even line.  I’ve often wondered what it was that caused that.”  

Clouds do have flat bottoms.  This is especially noticeable in low clouds.  You see, for a cloud to form air has to rise and as it does it cools.  At some point the water vapor in the air will condense, meaning it turns into liquid drops of water and becomes a cloud. Now this happens at one level  in the atmosphere on any given day.  That level is called the condensation level and it varies depending on temperature and moisture.  So as air rises, it eventually hits that level and voila, a cloud forms, starting at the bottom and that's the line we see at the bottom of the cloud. The air keeps rising and randomly pushes moisture higher and higher so that the top of the cloud goes up to any place it wants depending on the amount of water vapor in the air.  The exact elevation that the clouds start to form is at the same level on a given day so that each cloud will always have a flat bottom.  Check it out, it’s true.   

We welcome questions at our toll free line 1-888-724-6001.  The Weather Notebook is underwritten by Subaru the beauty of all wheel drive with major funding provided by The National Science Foundation.