Published: 2009-10-12 10:09:00
Updated: 2013-10-27 13:34:24
Posted October 12, 2009 10:09 a.m. EDT
Updated October 27, 2013 1:34 p.m. EDT
By Mike Moss
A former viewer who recently moved away from the Raleigh area sent us a group of photos taken by her son when some thunderstorms passed through through Cumming, GA a while back. She asked if we could identify the type of cloud seen in the two attached images.
The clouds that are most interesting in the photos are associated with thunderstorm outflow boundaries called gust fronts. These fronts consist of rain-cooled air that rushes downward in the storm and then flows outward when it reaches the ground. Warm, humid air surrounding the storm is then forced upward and over this expanding "slab" of denser air. As the air flows upward, it cools sufficiently to condense out moisture and form a cloud (called an "arcus," and also called a "shelf cloud" if it is attached to the thunderstorm producing the outflow) that takes on the shape of the upper surface of the gust front. These are often arc-shaped and rather smooth, but when the gust front is especially strong, resulting in very rapid upward motion of the ambient air, or if there has previously been some rain prior to the arrival of the gust front, making the surrounding air a little more humid, then some lower clouds can form on the way upward. These sheared clouds can take on the appearance of streaks, shards, or almost a curtain of cloud, and are called fractus. All of this is illustrated in both of the images, but is most neatly displayed in the second shot. You can clearly see the fractus clouds just above the white building and the power poles, being formed as air rushes up the front of the outflow boundary, and can also make out a more "classic" arcus cloud (probably of the shelf variety) curving across the top of the image.
When you see a cloud like this, especially one with the fractus flowing upward near the front, it is common that very gusty winds and a notable temperature drop will set in at about the time the clouds pass your location. In effect, a small scale cold front is moving through.
If you do some web searches for arcus clouds, shelf clouds, and the like, you'll see a variety of images that will give you more examples of these cloud types. They aren't really uncommon if you keep your eye out for them, and are especially noticeable in flat open terrain like the plains states or near the coast. Another cool feature with these clouds is a kind of eerie turbulent appearance of the cloud base just behind the arcus cloud once it passes over, a feature that has been called the "whale's mouth." Our thanks to Carolyn Eisenman for sharing the pictures!