Asperitas

We live at the bottom of the atmospheric ocean. Clouds and water vapour are the visible evidence of the currents and waves within that relatively shallow fluid. Science can deduce the relative motions of the components of the atmospheric ocean from the characteristics of those cloud tracers. That science is meteorology and when combined with thermodynamics, humans have learnt to forecast the weather. The climate is the time integral of that daily weather. 

Weather and climate have always been in a state of flux but what we have been witnessing since the start of the Industrial Revolution is the Anthropocene. Greenhouse Earth has already raced past the 1.5 Celsius line in the sand with 3.0 Celsius dead ahead. Brace yourself...

Now for some interesting clouds!

Asperitas clouds over Burnie, Tasmania, Australia. 
 Credit: © Gary McArthur, Cloud Appreciation Society Member 5353

Actually, asperitas clouds are not that rare. My life surrounded by nature has been spent with my head in the clouds so to speak. I have observed these patterns many times and painted them at least twice. As a meteorologist, asperitas tell a weather story of a stable layer, wind shear and an unstable atmosphere above within the relatively warm and moist air where those clouds reside. Typically, atmospheric temperatures decrease with height but this tendency is inverted when the atmosphere actually warms instead as one climbs. This is why the stable layer is commonly referred to as an inversion.

Asperitas clouds form in Kanata, 2024 (Cindy Broderick/X)

Asperitas is a cloud formation first popularized and proposed as a type of cloud in 2009 by Gavin Pretor-Pinney of the Cloud Appreciation Society. Asperitas was added to the International Cloud Atlas as a supplementary feature in March 2017 and it is the first cloud formation added since cirrus intortus in 1951 - before I was even born! Asperitas means "roughness" in Latin. A Latin teacher friend told Pretor-Pinney to use the Latin verb "aspero" to name this new cloud, Pretor-Pinney said, because the poet Virgil used it to describe the sea "roughened by the northern winters' gales."

Large expanses of asperitas are more common with warm frontal inversions which only have a slope of 1 (vertical) to 200 (horizontal). Cold fronts are much steeper with a slope of 1:50. 

The Conveyor Belt Conceptual Model of Mid-Latitude Weather Systems
This graphic illustrates how weather systems harness energy from the jet stream.

The asperitas clouds are in the warm and moist air above the frontal inversion - part of the warm conveyor belt (WCB) in the above graphic. The cooler and drier air of the cold conveyor belt lies under the inversion. If that cooler air is moving southward, it is a cold front with the slope of the frontal surface dependent on the speed of the front. Fast-moving cold fronts are steep. If the cold air is moving northward, it is a warm frontal inversion with a more shallow slope. The winds in the cold conveyor belt (CCB in the above graphic) are a function of the intensity and the speed of the approaching system.  

The stability of the inversion is enhanced when the surface is cold like one of the Great Lakes in spring (after a winter) or over cold ocean currents like the Californian or Subpolar Gyre that abut the west coast of North America. The low levels of the atmospheric ocean are chilled from beneath thus contributing to the temperature contrast across the inversion and its associated stability.

A 2007 Graphic Illustrating Swells and the Warm Frontal Zone

Atmospheric swells might also contribute to the turbulent shaking of the inversion. Large both in amplitude and wavelength, oceanic swells travel long distances from the storms that generate them. The strong winds found at the source of the swells are not experienced far away where the atmospheric swells might really shake up the low-level inversion thus creating the characteristic asperitas clouds. 

Any instability in the air above the inversion can also accentuate the dramatic gravity wave patterns of the asperitas clouds. Updrafts must also produce downdrafts. These vertical motions would dent and shake the inversion like a giant walking on a taunt bedspread.

Wind shear is when the wind direction and/or speed changes relative to neighbouring flows. Shear is responsible for all swirls witnessed within any fluid. The shear can be oriented in any direction but to keep things simple, meteorologists generally refer to horizontal relative winds as they impact vertical motions in the atmosphere. For example, when cyclonic swirls are advected over air that is not twirling as quickly, air rises, the surface pressure decreases and a relative low-pressure forms. The actual science is a bit more complicated but this is the gist of the process required to make weather. 

As a result of the rotation of Greenhouse Earth and the Coriolis Force, your right hand is your Coriolis Hand in the northern hemisphere. Mimicking the curl you witness with the fingers of your Coriiolis Hand will point your thumb either up or down. If it is cloudy your thumb will likely be pointing up and you are witnessing a cyclonic swirl conducive to ascending air. 

A time-lapse video of asperitas should reveal circulations consistent with the type of air that is moving above the frontal inversion within the warm and moist air. For example, dynamic warm fronts occur when warm air is advected toward the north. Such winds veer with height and must create cyclonic (counter-clockwise) swirls in the atmosphere. With the thumb of your Coriolis hand (your right hand in the northern hemisphere) pointing upward, the sense of rotation within those swirls must be in the direction of your fingers. Any lines linking these swirls must be in the shape of a "backward S". 

In my experience, surface winds associated with asperitas have been calm or even easterly. These winds are consistent with a moderate to strong low-pressure system associated with the warm front. These deductions are explained in "Weather Lessons for Everyone from the Cold Conveyor Belt Wizard". https://philtheforecaster.blogspot.com/2020/08/weather-lessons-for-everyone-from-cold.html

Asperitas clouds typically do not produce any precipitation and appear at lower levels in the atmospheric ocean (the low etage beneath 6500 feet above ground level). 

The question has been asked if the increased number of observations of asperitas might be related to the Anthropocene and Climate Change. Applying thermodynamics to the observed climate warming of 1.5 degrees Celsius already realized means that the atmosphere can currently hold about 12 percent more water vapour than before. More clouds must be the result. Of course, more observations might also be the result of a dramatic increase in the number of people with cell phones. 

Artists interested in the sky have witnessed and recorded these clouds before.

Untitled Lawren Harris painting (and weather observation) circa 1920-1930

John Constable was the first of such artists but did not produce an asperitas painting to my knowledge. Lawren Harris was the first artist that I am aware of who actually recorded asperitas perhaps as early as the 1920s. Not much is known about that particular painting included above. To my knowledge it is not even named although back then, artists tended not to give titles to their works. I have painted two apseritas weather observations so far and I am not done yet. 

#2510 "Singleton Asperitas" 16x20 by 7/8 inches
Mid-afternoon Tuesday May 4th, 2021 looking westward across Singleton Lake

The meteorology I described above was confirmed in my own weather observation of asperitas. It was a warm frontal weather situation with low-pressure centres of moderate intensity far to the southwest. The Great Lakes were still cold after winter. I was not swimming in Singleton in early May! The light easterly surface winds were part of the cold conveyor belt being drawn into the approaching low-pressure system. The satellite top-down view of the weather did not reveal the beauty of the sky witnessed from the ground. 

The weather observation of the afternoon of May 4th, 2021

Jim Montanus, a Facebook friend took the following dramatic image of the asperitas over chilly Lake Ontario at the same time that I was painting the clouds over Singleton Lake about 60 kilometres to the northeast (the red star on the map in the above graphic). Jim Montanus' post caused a flurry of comments and excitement. I could not resist adding a bit of an explanation and the science behind the unusual clouds. This post is a continuation of that effort given the observations of asperitas in 2024.

Courtesy of Jim Montanus May 4th, 2021 5:28 pm with permission.
Please visit https://www.facebook.com/MontanusPhotography 

Jim Montanus' image illustrates how the wind shear oriented at a multitude of different angles can create swirls and rotating tubes with all kinds of orientations - far more than the simplified vertical interpretation of basic meteorology. All of those patterns can be explained but not without digging much deeper than we have already. Sometimes it is best just to appreciate the beauty and complexity of nature. 

Warmest regards and keep your paddle in the water,

Phil Chadwick