Cloudy Perceptions

by Dianna Hrabovsky, Treasurer MAST

I really enjoy observing the world around me and learning about the whys and hows through scientific discovery.  This column is designed to share those discoveries and promote discussion about varied and unusual topics.  The Curious Eye will, hopefully, become a useful scientific research tool.
    My topic for this week I will humorously entitle, "Cloudy Perceptions."  Saying that my perceptions often have me looking up is not that far from the truth. In the daytime I enjoy looking up at the sky and clouds and at night the stars and moon.  Lately, because of getting very excited about storm-chasing and looking at different cloud formations--trying to discern the reasons for their type and possible meaning, I have found myself observing strange atmospheric activity.  This is activity that would most likely go unnoticed unless you were specifically looking at the clouds in depth.
    On 21 May, at about 7:15 PM (2315Z) I observed some very interesting rotation atop pronounced cumulus clouds.  As cloud rotation is of some interest to storm chasers, I continued to watch this rotation.  The strange thing about it was that as the cloud moved slowly to the southeast, there was significant rotation about a horizontal axis across the top of this cloud in two places.  This rotation moved in the same direction as the cloud itself, and made several complete rotations while I watched.
    This is a sketch of what I saw,

There was another cloud a bit behind the one with the rotation and the rear rotation was drawing in the leading portion of the second cloud.
    The rear rotation eventually tipped over about a vertical axis and sank into the cloud, leaving a definite indentation before it disappeared.
    I went inside shortly after this and told my husband what I had seen.  He told me it was very unusual and, of course, scolded me for not having taken any video of this event.  I got the video camera and went back outside to see if I could catch this on tape.  Much to my dismay there was little more to be found as the clouds had moved on and only a few stragglers were left to tape.  I did get some slight rotations on tape, however, and we have viewed these together.
    We tried to formulate some ideas of what caused this occurrence and I paged through some wind and pressure data for times near my observation. The surface winds were much as I expected.

At 950 MB (millibars, a unit of pressure also used as a vertical coordinate system) and 850 MB the winds were northwesterly at about 25 knots.  Note that I am referring to the direction the wind is coming from.  Higher up though, in the 700 MB to 500 MB range, the speeds were nearly 50 knots and the direction became southwesterly.

Something significant to note was the difference in wind direction over the next 12 hours.  The winds changed from northwest to south.  This drastic change of direction may have resulted from a convergence (air coming together).  I think this might also have had something to do with this strange cloud activity.  Or, perhaps, it was an upper level trough (a region where a counterclockwise wind pattern develops).
    What could have caused these distinct and swift rotations?  Why didn't all the clouds have these rotations?  Why did the horizontal rotation in the back of the one cloud draw in the lead portion of a second cloud?  Why did it then go vertical?
    As you can see, there are many questions to ponder about this event.  Perhaps others have observed the same event and related data can be pooled.  In any case, to me this was a significantly unique event to be noted and examined.  Feel free to share and comment, Di.

Converted by Mathematica  (May 23, 2003)