This photo shows what I initially thought was virga falling over Lake Pleasant, some 10 miles west of me. It was just after sunset on Nov 24, 2006. Canon 20D settings: 55 mm, f/6.3, 1/100 sec, ISO 400. The image languished in my “Clouds” folder until just last week. I was looking for something else when I discovered it, and immediately recognized it as not virga, but a rare cloud type called mammatus.
I didn’t learn about mammatus clouds until many years after I took that photo. Technically, it is not a cloud type, but a supplementary cloud feature according to the International Cloud Atlas. If you follow this mammatus link to Wiki you’ll see many fine examples, most of which were photographed from under the cloud. In this case we are seeing the cloud nearly edge-on.
The name “mammatus” comes from the “udder-like” structures hanging down under the cloud. They are formed by pockets of air that have cooled, increased in density, and fallen through the normally level cloud base where air temperature increases to above the dew point. Click on the photo to see a larger version. Note how the mammati are roughly hemispherical — characteristic for these features. Virga, on the other hand, appears as elongated streaks.
There may actually be a mix of mammati and short-range virga happening below that cloud. Both phenomena can be triggered by the same cause: cooler denser descending humid air. And it looks like that might be happening to the leftmost two mammati.
Of course, after discovering this photo, I had to check the entire folder (containing hundreds of cloud photos) to see if I had captured any other mammatus. No joy — as I said, they’re a rare meteorological phenomenon. I was lucky to get this single photo, and it was serendipity to re-discover it.
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