WRAL WeatherCenter Blog

Cloud Chaos!

Posted January 21, 2009

Chaos may be a poor choice of words for the title, because there is some apparent organization and an underlying principle at work in the accompanying photos, but at the same time there is an interesting combination of sheetlike smoothness and otherworldly disturbance about them as well.

The photos were taken in the Low Gap, NC area by a viewer from Cary named Beth Farrell, who calls them "the strangest clouds ever" and wondered if we could offer an explanation. I'll give it my best shot, briefly, but have to add the caveat that I did not have access to a weather observation or sounding for the exact location these were seen for that exact time.

However, when I saw the images I immediately wondered if there wasn't a notable low level temperature inversion a few thousand feet off the ground, and in fact I found such a structure in vertical soundings taken with radiosonde balloons st other sites in the region that day, along with significant amounts of moisture both at the height of the inversion and a few thousand feet above. There was a notable shift in wind direction atop the inversion as well. Finally, she took these in an area of the North Carolina foothills where the terrain is still rather uneven.

All that being the case, I would surmise that a shallow layer of moisture sufficient for cloud development existed within and at the base of the strong low level inversion. An inversion with this structure is a very stable layer of air, one which resists displacement, whether upward or downward, and when displaced, can begin to oscillate vertically above and below its level of equilibrium. When displaced upward into notably less dense air above, it encounters a restoring force (gravity) that pushes it downward, and when it overshoots the equilibrium level moving downward, into notably denser air, it encounters buoyancy as an upward restoring force.

In many cases, this general form of organization Can result in billows, or wave clouds, in which an organized series of waves (like those on the surface of the ocean, another instance of a dense fluid with a much less dense fluid above in the form of air,  when there is a steady wind blowing across it) results in lines or arcs of denser clouds where air movement through the wave is upward and thinner, or completely dissipated, lines or arcs in between where the motion of air through the waves is downward. See http://weathervortex.com/sky-ribbons.htm for some nice photographic examples of these kinds of clouds, and you'll even find one well down the page is somewhat similar to the attached photos Beth sent in.

In the case of her images, it appears that the wind was not flowing in a coherent manner that created well-formed waves, but instead was being turbulently lifted and lowered as it flowed over terrain obstacles in the area. In addition, there may have been showers in the area or other disturbances that set up wavelike motions in the stable layer that constructively and destructively interfered with others. The analogy to be made with water on the ocean would be to suppose that you had a modest wind blowing across the ocean surface creating some light wave action, but then also had a couple of thunderstorms nearby that created outflows that sent other waves moving in from different directions to create somewhat chaotic appearing undulations of the water's surface.

Here the thin layer of clouds that happened to exist within the stable sheet of air acted to make visible how that sheet was being disturbed, as if two or three people were holding a sheet at the edges and shaking it up and down in an uncoordinated way. Another analogy might be to consider what you might see looking up from below at the surface of a pool as a bunch of people were jumping in at random times in different locations, as opposed to tossing a stone into an otherwise undisturbed pool so as to send out rings of nicely organized ripples.

That's a ll a little speculative and off the cuff. If anyone out there has additional insights about this kind of cloud formation, feel free to chime in!

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