Houston, TX – Those clouds in my cover photo from Brent Bell are a magnificent sight! Asperitas clouds, for short (Undulatus asperatus), these have been photographed for years but didn’t get their full recognition until the World Meteorological Society added them to their library in 2017. And guess who drove that idea home? Gavin Pretor-Pinney who founded the Cloud Appreciation Society, which I’ve encouraged you all to join in past blogs, like this one from Valentine’s Day!
Asperitas are closely related to Mammatus clouds, like those below, which form in the wake of strong thunderstorms--the cold sinking air causes those pockets of drooping clouds:
So what brought these undulating whipped up clouds to us last Saturday? Basically, different wind directions. The afternoon storms produced small hail indicating sinking cold air, like with mammatus clouds, and those storms rode in on a west to east wind at 30mph. The High Pressure in place to our north produced a lighter surface wind from northeast to southwest. Winds at different speeds above and below clouds are what cause the undulating part.
And you might remember that very little rain reached the ground because we had such dry air at the surface (blue morning skies). So where was that water vapor stored? In the clouds themselves and it’s water vapor that produces the darkness. The result were these fantastic swirls of light and dark grey!
Gravity waves likely played a role. Those sound fancy but imagine when you throw a rock into a still pond causing the rippling effect. Air is a fluid, also, and winds from different directions can cause a ripple effect. I found a terrific article here on these clouds. The author describes Gravity Waves as similar to four of us holding the corners of a bed sheet and snapping it--you can imagine the roiling of the sheet folds much like the roiling of the clouds.
No one much enjoyed the dampening of our Saturday skies (although we need rain south of I-10), but hopefully you got a kick our of the rare Asperitas clouds!