5 February 2009

Flying Saucer Clouds

Posted by Dan Satterfield


Ask most Meteorologists (including me) what their favorite cloud is, and you will probably get the same answer



Imagine a blob of air riding along in the wind. Suddenly, you run into a big mountain. As the air stream you are in, rides over the  mountain, the pressure drops and so does the temperature. If it gets cold enough, the moisture in the blob of air can no longer stay in vapor form, and will condense to tiny water droplets. Cloud droplets to be exact.

A cloud forms!


As the air descends the mountain, it warms,  and the pressure increases. The cloud droplets can now go back to invisible vapor, and the cloud disappears. If you think about it, this will lead to a cloud forming near a mountain range that just sits there! Even if the wind is blowing 80 knots across the mountain range, the cloud does not move.

Lenny clouds can develop far away from mountains too.

If you throw a rock into a pond, ripples will will move out in concentric rings far from where the rock fell. The same thing happens in the atmosphere. Wind blowing over a mountain range will start an up and down oscillation. Each time the air rises to the top of the wave, a cloud will form. These “Gravity Waves” can cause lenticularis clouds to form thousands of miles from a mountain range. I once observed lennys in Florida, caused by strong winds across the mountains of Central Mexico!


A very rare type of Lenny that develops only when conditions are just right is called a Flying Saucer Lenticularis. I’ve never seen of these, but I sure hope to someday! There was a SPECTACULAR display of them last Fall in Seattle. One of these shots made the Astronomy Picture of the Day!

You have to see it, and you can RIGHT HERE.

Isn’t that GREAT?