Shears and "Stripes"
Posted February 11, 2008
Those of you who read Bill Leslie's blog or checked the link on our main weather page to the "Winter Skies" photo contest winners may have seen the winning entry (the first picture above) from Kelly Hinkle, called "Stripes." Like many of the other photos that were entered, it was shot beautifully and made for an eye-catching scene. Being a meteorologist, it also caught my attention for the weather conditions that lead to that kind of appearance in the sky.
While this kind of cloud pattern isn't exceptionally unusual, it also isn't an everyday occurrence to see such a nicely organized series of neatly banded clouds. The trick to forming these clouds is a "just right" balance of stability, vertical wind shear, and a shallow layer of moisture that exists at an altitude where there is a fairly sharp vertical change in density (which traces back to vertical variations in temperature and humidity) that leaves a less dense layer of air just above a notably more dense layer. When the winds through the upper layer are a good deal stronger than those just below as well, this scenario can result in waves developing at the interface between the two layers, not unlike wind (much less dense) inducing waves on water (much more dense) when it blows across a lake, river, or ocean. When these waves occur within a single fluid (up in the air or down within a body of water) they are called internal waves.
In the absence of significant moisture, these waves will be invisible, but may be felt as clear air turbulence if you happen to fly through them in an airplane or helicopter. If there is a great deal of moisture and a thick layer of clouds, the waves may go unseen or unnoticed due to nearly overcast skies that obscure the pattern. However, if there is a fairly shallow layer of moisture in place at the altitude of the internal wave pattern, then the air flowing though the waves will move upward as it approaches a crest and downward as it moves into a trough. Under the right circumstances, this can lead to cooling and condensation as the air flows upward and then warming and evaporation as the air sinks, which can produce the eye-catching pattern with long lines of cloud separated by long lines of clear air in between, with the cloud lines more or less perpendicular to the wind direction at their altitude. In general, these kinds of clouds are referred to as "billows" or "undulatus" clouds, and you can see a discussion regarding some slightly different-looking clouds of this sort in an older post of mine.
It was one of those patterns that Kelly caught such a nice photo of, and it brought to mind that one of the broadcast technicians on our crew (who is also a very good photographer), Jeff Reeves, had sent around a similar photo that he shot several months ago. I've included that one as the second image above. The third and fourth images attached are ones I took a couple of years ago, and discussed in the older blog I referred to above. Enjoy them all, and be sure to check out the other great shots that our viewers/visitors sent in for the contest!