ASPERATUS?
Clouds: Their changing shapes often occur over a time-scale and space-scale that we humans can not always fully appreciate unless we use time-lapse photography.
In the international cloud naming scheme used to describe and identify clouds, there are ten basic characteristic cloud FORMS or TYPES or genera (nouns): Cirrus, Cirrostratus, Cirrocumulus, Altostratus, Altocumulus, Nimbostratus, Stratocumulus, Stratus, Cumulus, and Cumulonimbus.
To further describe clouds there are several accepted and defined adjectives covering 14 cloud species (shape and structure), 9 varieties (arrangement and transparency), 9 supplementary features, and two words (genitus, mutatus) describing growth. Click here for a table of these words.
There has recently been a call from the Cloud Appreciation Society of the United Kingdom to ask The Royal Meteorological Society to apply to the World Meteorological Organization to officially add a new variety or species of cloud to the international scheme. The new word is “ASPERATUS”, after the Latin word for rough, and is intended to be used as an adjective to describe those clouds whose underbellies look like the surface of choppy sea.
This story was covered in the UK here and here and then in NZ by Radio NZ National on the Panel.
Photo credit to Bill Slater, taken near Hanmer Springs on 2 March 2005 and winner of the Met Society Photo competition, shows an example of what ASPERATUS implies. Bill explains “It was a fine day and we first noticed some round disc like clouds at fairly high altitude. We commented that they were like flying saucers. Then as we reached Hanmer Springs we started to see these swirls and dangling clouds, looking back towards the Lewis Pass… no rain ever fell.”
“Asperatus” clouds form when there are two (or more) layers of air of differing density, one sitting on the other. The cooler and higher layer is cloudy and the other layer is clear. The boundary between these layers may occasionally get knocked up, but will return downwards thanks to gravity and then may go further down but will return back up thanks to buoyancy. This creates a wave-like surface along the cloud base, and we call these gravity waves because the returning force is gravity and buoyancy.
Yes, the waves on the surface of the sea are a good example of this process.
Another good example is when moist air blows over a range of mountains and makes a system of mountain wave clouds. In New Zealand this often happens, and people in Canterbury call the mountain wave clouds “the northwest arch”.
At first individual Altocumulus lenticularis clouds form, but as a front approaches, upper-level moisture increases and middle and high clouds combine to produce an arch cloud comprising Altocumulus, Altostratus, and Cirrostratus. This arch cloud displays a very sharp edge near the mountains and often there is an arch of clear sky immediately downstream of the mountain divide.
We can cope with the current naming scheme and use Altocumulus lenticularis to describe the NW arch clouds, but it would also be useful to have the extra variety or species word “ASPERATUS” especially when there are undulations in the cloud base.
The MetService cloud poster already has a special photo devoted to the NW arch cloud. At present it is just classified as “Northwest Arch Ac/As/Cs”, but if the word “ASPERATUS” is officially accepted then we are ready and waiting.
If you are coming to the National Fieldays at Mystery Creek 10-13 June then pass by the MetService display in the main pavilion and ask for your own complimentary full-sized cloud poster.
Hi, since Bob McDAVITT has been kind enough to show you all my photo again four years after it won the annual competion, I thought I had better comment. I am not a ‘met’ person, simply a keen photographer and latterley an interested cloud watcher. Indeed, since reading Bobs June article on Asperatus I have joined the UK Cloud appreciation society! Soon after its capture the ‘chocolate Asperatus’ picture was shown on the BBC and I slightly mistakenly told the weather presenter that is was Altocumulus Lenticularis. She was most impressed. So if the World Met Society officially go with this new Asperatus title I can say…’Well of course I was there, dont you know!’ . Seriously with retirement looming I’m heading back to NZ (with a new camera) and I have high expectations, although Bobs chasing me for my subscriptions!
Bill Slater
Kelvin-Helmholtz waves or “gravity” waves just require gravity to be the restoring force, and so they can occur on the boundary of any two layers of sufficently different density, with one layer moving differently from the other.
The surface of the sea is a good example, with sea water being about 1000 times denser the the air above it, and the wind reacting with the sea to make waves that use gravity as their restoring force.
As for quotes– you can quote from my Blog and acknoweledge it as a source, but more credence comes from references to people with publications that have been reviewed by their peers– i haven’t written any , but here are some on this topic:
* Kelvin, Lord (William Thomson), “Hydrokinetic solutions and observations,” Philosophical Magazine, vol. 42, pages 362–377 (1871).
* Helmholtz, Hermann Ludwig Ferdinand von, “Über discontinuierliche Flüssigkeits-Bewegungen [On the discontinuous movements of fluids],” Monatsberichte der Königlichen Preussische Akademie der Wissenschaften zu Berlin [Monthly Reports of the Royal Prussian Academy of Philosophy in Berlin], vol. 23, pages 215 ff (1868).
Hi Bob,
Appreciate your comments and explanation on the Iowa and Scotland cloud images…
Just one further query re the Kelvin-Hemholtz waves.
Presumably that implies two opposing adjacent layers of air, in speed and possibly direction, with the presumably lower cloud free layer (with presumably, the greater windspeed) “digging into” the upper slower moving cloud layer, and so creating these unbroken waves or undulations we see on the underside of the stratocumulus.
Would you also mind me quoting my source of technical advice on this interesting topic, by mentioning your name and designation (Bob McDavitt, Meteorologist, N.Z. Met Service Weather Ambassador).
I intend posting a revised (corrected)comment on these unusual cloud formations on the website previously mentioned , to clear up any possible confusion amongst readers of that site!
Your insights have been most invaluable and appreciated.
Thanks again,
Cheers
David
Intersting comments David That Iowa cloud (http://www.guardian.co.uk/environment/gallery/2009/jun/01/2?picture=348217732) looks to me to be in a stable environment , and mamma implies instabilty as found in a Cumulonimbus , so I agree that it looks more like Stratocumulus undulatus, as does the image from Scotland… these look to be non-breaking (Kelvin-Hemholtz) waves on the underbelly of the cloud between the cloudy layer and the cloud-free layer. The word Asper and its derivatives seemd to imply “breaking waves” in the undulations.
As for using Autumn colours, or the time of the first frost, or first daffodil, or a prolific flowering of a plant or tree (e.g.) as indicators of the possible outcome of the coming season–this is called phenology. It has some merit. Long term phenologal readings may also be a useful proxy measure of trends in climate change.
Hi Bob,
This particular blog caught my eye and especially after I read your comments in the NZ Herald on these unusual cloud formations, in which some individuals (RMS) are trying to persuade meteorologists to introduce a new classification for these clouds using the term “Asperatus”, named after the latin word for rough.
Your analysis of the possible or was it probable processes involved in this particular sky, the sky shown in the Iowa photo, was particulary interesting and seems quite plausible.
I don’t think you actually put a name to the Iowa cloud formation but referred to the Altocumulus
lenticularis formations observed in Hamner Springs.
Now, I have recently posted a comment on a particular site http://www.mostholyfamilymonastery.com which was discussing this very thing and decided after consulting Ludham and Scorer and a few other sources besides, I ‘stuck my neck out’, and proclaimed that the sky in the Iowa photo best fitted the description Stratocumulus cumulogenitus mamma, obviously belonging to the low cloud family.
After reading your anaysis of this Iowa sky, I have had second thoughts, and perhaps I should have allowed the possibility of it fitting the description Stratocumulus undulatus…of course, knowing the physical processess involved in its formation would no doubt go a long way in deciding which classification best suits it!
The second photo (Hamner Springs) is obviously Altocumulus lenticularis, and the Scotland photo, in my mind is Stratocumulus undulatus…
Incidentally, “Asperatus” is not the latin word for rough. The latin word for rough, an adjective, is “asper,aspera, asperum, being the masculine, feminine and neuter forms respectively. If they are going ahead with this crazy idea, at least call it asperarum, so the Iowa cloud, according to their new classifiction might be, say, stratocumulus asperarum. [o.k. I could be accused of 'splitting hairs' on this one]
Of course. I strongly disagree that there needs to be any new classification for any of these clouds, as they are already ‘well taken care of ‘.
Anycase your “call” on the Iowa sky would be appreciated…
Finally, in conclusion, [sorry this is so long!] your article on Autumn colors was very interesting. I wonder if it is possible to use autumn colors in N.Z. to forecast the type of winter that lies ahead for that particular year or even subsequent seasons?
I would imagine several years of data collecting would be indispensable for this purpose; perhaps someone somewhere has or is doing this right now…it would make a facinating study, a thesis perhaps for someone, the results and conclusions , of which, would no doubt be of economic and social benefit to all N.Zers…
Cheers
David S.