WRAL WeatherCenter Blog

Billow Talk...

Posted January 23, 2006

The title of this week's post was inspired by a very nice display of stratocumulus undulatus clouds that appeared in the skies outside my truck window on the way in to the station last Tuesday morning, the 17th of January. You might recall that was one of the days we were building into a very windy pattern, with a cold front and deep upper level trough approaching from the west, and there was concern about potentially severe thunderstorms Tuesday night into Wednesday morning. In the end, the atmosphere was sufficiently stable overall to inhibit thunder and prevent the strongest gusts that might have occurred. Just the same, the wind peaked with a gust of 46 mph at RDU, and Seymour Johnson AFB in Goldsboro reported a gust to 54 mph. There were some scattered instances of downed trees and power outages associated with the event, along with a pretty decent rainfall.

On the morning in question however, we were still in the stage in which moist air was flowing in from the south, with a rapid increase in winds with height, and a continued temperature inversion just a few thousand feet above the ground. Winds at the time were blowing from the south-southwest at the surface, veering a bit to the southwest at higher levels (veering is a term that describes a clockwise turning of wind direction - it can apply to a horizontal or vertical shear, as in this case, or a change of direction over time at a single point) and increasing sharply in velocity. The atmospheric sounding taken in Greensboro about 90 minutes prior to my drive in is shown below. Note that the lower atmosphere indicates a saturated layer (temperature and dew point profiles come together) and a temperature inversion (warming with height) not far off the surface. This sounding wouldn't apply perfectly in Raleigh 90 minutes later, where surface winds were more from the south-southeast, but gives a good idea of the general scenario for the morning.


It doesn't always work out this way, and is usually rather transient when it does, but the situation that morning generated a very nice example of statocumulus billows. Billow clouds are arranged in well organized rows, usually near an inversion level, with the rows oriented more or less perpendicular to the wind direction. The best examples form when an internal shear-gravity wave develops and when the relative humidity at the level of the gravity wave is within a narrow range that allows for cloud to be formed over a fairly short vertical distance as air flows upward and cools, and to dissipate over the same distance as the air sinks and heats. You can think of these waves somewhat like waves on a lake being generated by faster moving air blowing across the surface of the more dense water. In this case, the temperature inversion leads to a notably denser sheet of air just below a less dense layer, and vertical wind shear provides for faster wind speeds just above the more dense layer of air.

When all these conditions are just right, the interface between the denser and less dense air ripples like the surface of a lake. If the air is too dry, these ripples are invisible, and if the air is too moist, the clouds may be so thick as to make the organization difficult to see. Last Tuesday, though, for a short time, just the right amount of moisture was present over a shallow layer. Unfortunately, at the point when the cloud display was at its very best, I was in traffic and could not stop. Also, I didn't have a high-quality camera with me. Just the same, I was able to pull into a strip mall 8 or 10 minutes later, and had a PDA with a camera function handy, and took the pictures below. The two that show the many parallel cloud rows off into the distance were shot toward the north northwest, with the other image shot toward the west, more or less along a couple of the rows. By the time I shot that one, the pattern was starting to break down and dissipate on its west flank, and you can see the rows giving way to a more disorganized sheet of stratocumulus clouds toward the horizon. The bases of these clouds were around 3500-4000 feet above the ground. (Well, after lots of effort, image manipulations, and so on, for no apparent reason the Blogger system accepted and displayed the sounding image above, but refuses to display the cloud photos - if I can figure out a reason and fix this later, I will - if anyone out there has run into problems wherein some images/photos post just fine, but others with apparently identical file types and properties will not, please let me know if you've run across a workaround - thanks...) UPDATE: I've now created a web page for posting photos that do not work properly through blogger. So, you should be able to see the images I described above HERE.

By the way, if you ever run across meteorological terminology like "shear-gravity wave," "billow clouds" or "undulatus," a good place to get a quick rundown on the meaning is the online AMS Glossary of Meteorology.
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