For most of the day on May 28, a dreary sky hung over southern New Jersey as a result of winds from the east off of the chilly Atlantic Ocean. As I looked out the window during the evening, I was shocked to see that the monotonous gray sky had been taken over by billowing, wavy features on the bottom of the clouds. These were none other than the breathtaking Undulatus clouds, which were formally recognized as a cloud type by the World Meteorological Organization back in March.
To be more precise, these are Undulatus Asperatus clouds, whose defining features include the wavy motions at the cloud base. The ingredients needed for these clouds to occur are rising air (to help with the formation of overcast skies) and some type of wind shear or turbulence. A developing area of weak low pressure offshore of Virginia Beach at that time may have provided enough of the required rising air motion.
To find the wind shear ingredient, it would be easiest to look at a sounding. Meteorologists use soundings to get a snapshot of processes going on in different levels of the atmosphere at a specific time. For reference, the left side represents the pressure level, or height above the surface, and the right side shows the direction and speed of the wind at each level (the bottom represents temperatures in Celsius, but we don’t have to worry about that for this situation).
The sounding below (courtesy of the College of DuPage), is from the initialization of the 00z May 29 GFS model run for Atlantic City, where the cloud picture and video was taken. While the winds change direction with height – from easterly at the surface to southerly at 850mb, where the cloud layer was located – something else caught my eye. The winds actually slow down quite a bit as you move from the surface up to 700mb, where the white circle on the right-hand side indicates that winds were stationary.
Typically, winds increase with height as the force of friction from the ground becomes less apparent. The decrease of the winds with height in this unusual situation may have provided the kind of wind shear necessary for the Undulatus Asperatus to form.
Although the picture and video may look fascinating, trust me when I say that it was much more stunning to be there in person. I truly hope you all have a chance to view these clouds one day!
WeatherBolt 