Asperitas

Asperitas image courtesy of Jim Montanus May 4th, 2021 5:28 pm with permission.
Visit https://www.facebook.com/MontanusPhotography 

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. This is the basis for understanding all clouds including asperitas. Clouds are the vocabulary of the weather with novel text freshly printed in the sky every day for all to read and appreciate.  

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. Climate has considerable inertia and the uncontrolled warming will not stop there. The long-range forecast is for thousands of years of a "Super-Interglacial" period as predicted by James Black in 1977 to the Exxon Oil Executives. 

But let's return to the science behind these dramatic and very 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. 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. Normally, atmospheric temperatures decrease with height but this tendency is reversed when the atmosphere warms instead as one climbs through a stable 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. 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." Our ground-based view can only observe the undulating bottoms of these dramatic clouds as they ride the inversion. Some science is required to interpret those shapes in terms of what is happening within the clouds!

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 and
details where one is more likely to find aspertas clouds.

Asperitas clouds are associated with a strong flow of warm and moist air - the warm conveyor belt (WCB in the above graphic). Higher elevated asperitas are found above the frontal inversion within the labelled area enclosed by a solid yellow line in the above graphic. The cooler and drier air of the cold conveyor belt lies under that 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 shifting northward, it is a warm frontal inversion with a more shallow slope. If the cold air is neither advancing or retreating, the associated front must be nearly stationary. 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 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. 

This cooling from below can even result in low-level inversion conducive to asperitas without the aid of the front as within the yellow dashed area above labelled as "B" for cooling from Below. 

A 2007 Graphic Illustrating Swells and the Warm Frontal Zone
from an unpublished COMET Module

Atmospheric swells might also contribute to the turbulent shaking of the inversion. Large in both 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 can really shake up the low-level inversion contributing to the characteristic asperitas clouds. 

Any instability in the air above the inversion can also dramatically accentuate the 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.

My Coriolis Hand illustrates a cyclonic 
swirl with the thumb pointing upward in the
atmosphere in the direction of the ascending
air. Meteorologists also refer to this as 
positive vorticity. 

The strong winds essential to create the sensational shapes of the asperitas also generate wind shear. 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. 

The vortices that typify asperitas tend to be chaotically distributed in all directions which makes these clouds so very distinctive.

As a result of the rotation of Greenhouse Earth and the Coriolis Force, the right hand is your Coriolis Hand in the northern hemisphere. Mimicking the cloud curls you witness with the fingers of your Coriolis 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 (as pictured above). 

         Planar satellite view of  warm air advection      
and the associated swirls and lines created 
within the moisture patterns.         

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 pointing upward at the red "X" in the accompanying graphic, the sense of rotation within those swirls must be in the direction of your fingers. Any lines (deformation zones) 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 warm frontal asperitas as a moderate to strong low-pressure system approaches. 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

The strong winds required to produce the most dramatic examples of asperitas are often  associated with Langmuir Streaks. Aligned streets of cloud interact in helical circulations contributing to the apparent chaos of the asperitas cloud patterns. Langmuir Streaks were first identified in the Sargasso Sea in 1927 but the same processes can be witnessed within the atmospheric ocean. See "Langmuir Streaks – Take the time to Observe and Learn from Nature".

Oceanic Langmuir Streaks

Asperitas clouds do not produce much if any precipitation and appear at lower levels in the atmospheric ocean (the low etage beneath 6500 feet above ground level). The lack of precipitation is consistent with the preferred location of these clouds under the anticyclonic companion of the warm conveyor belt - the leading portion of the warm conveyor belt which curls anticyclonically within the atmospheric ocean.  

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 achieved means that the atmosphere can currently hold about 12 percent more water vapour than before. That is a lot of water! More clouds must be the result. 

In addition, a warmer air mass is more likely to be significantly chilled by passing over a cold surface thus creating a significant low-level inversion. This later effect of Climate Change will only be realized until the cold ocean currents falter or large interior lakes stop freezing in the winter. 

Of course, more observations of asperitas might also be the result of a dramatic increase in the number of people with cell phones. 

Do not be concerned! None of the above meteorology will appear on any exam. It is enough to just appreciate the natural world of Greenhouse Earth. Asperitas cloud might be more simply summarized without any science as:

Chaotically undulating gravity wave clouds within a weakly stable frontal inversion characterized by strong and turbulent wind shear within the clouds.  (I am still thinking about these words. The clouds might appear violent and chaotic but at some time and space scale, nature has asperitas neatly organized.)

Artists interested in the sky have witnessed and recorded these clouds before. John Constable (1776-1837) was the first of such artists but did not produce an asperitas painting to my knowledge. His art did contribute significantly to the science of cloud observations that followed but that is another very interesting story about how art and science are one. 

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

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 quasi-stationary frontal weather situation with low-pressure centres of moderate intensity far to the southwest. Quasistationary fronts are typically located east of the low-pressure area where warm fronts are found. The cold air on the north side of the front was neither advancing nor retreating so the surface front remained stationary. 

The Great Lakes were still cold after winter. Even Singleton Lake was too chilly for swimming in early May! The light easterly surface winds were part of the cold conveyor belt drawn into the approaching low-pressure system. The satellite top-down view of the weather did not reveal the beauty of the sky witnessed gazing upward from the ground. 

The weather observation of the afternoon of May 4th, 2021. Singleton Lake is
located at the red star on the map in the lower right of the graphic.

Jim Montanus, a Facebook friend took the following dramatic image of the asperitas over chilly Lake Ontario at about the same time that I was painting the clouds over Singleton Lake about 60 kilometres to the northeast. 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 recent observations of asperitas.

Note that my painting depicted higher-level asperitas riding above the warm frontal surface while Jim Montanus' photograph shows clouds that appear to be closer to the chilly waters of Lake Ontario. These observations are consistent with the conceptual model proposed above.  

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

Jim Montanus' image also illustrates how the wind shear oriented at a multitude of different angles can create swirls and rotating tubes with all kinds of intriguing 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. 

Science is curiosity. Science is a process of investigating, learning more and eliminating errors in our understanding of nature. This story of asperitas is a start and might be improved as we discover more. Nature is always right!

Warmest regards and keep your paddle in the water,

Phil Chadwick