What a nice thing to see first thing. So I got my camera out and turned it on. It turned itself off within seconds. Hmm, try that again. Same thing. Right, dead batteries. Really? Has it been that long since the last time I used this camera? Luckily, I had some fresh batteries in the drawer, so I was able to get the picture. It’s not as good as it would have been if the batteries hadn’t been dead, but it’s still pretty good.
Cloud of the Day – Cumulus Flammagenitus
The International Cloud Atlas fifth edition, published in 2017, has included cloud types that were not recognized before, including types that are caused by humans and other unexpected sources. I’ve already posted on cirrus homogenitus, upper etage clouds forming from the condensation trails of aircraft. Today’s post is about cumulus flammagenitus, cumulus clouds formed by convection caused by a heat source. The heat source could be a wildfire or a volcano, for example, but the cloud can’t be just smoke or ash. It must include water droplets to qualify as a cloud. This type of cloud is also known by the name “pyrocumulus,” or “fire cloud,” which combines fire and the basic cloud type.
Wikipedia has a more in-depth article on flammagenitus than does the Cloud Atlas. As with other cumulus clouds, the convective clouds of the lower etage, flammagenitus can vary in vertical development, with bigger clouds being named accordingly. So we can have cumulus congestus flammagenitus, also known as towering cumulus flammagenitus, and cumulonimbus flammagenitus, complete with the lightning, wind and precipitation associated with thunder clouds.
Here’s another example. This flammagenitus is also referred to as pyrocumulonimbus. That is, a cumulonimbus cloud formed from fire. Tap for large original.
I have seen flammagenitus rising above forest fires, and it is impressive. The heat is so intense that the smoke, combined with the water driven out of the burning trees, can be driven rapidly high into the atmosphere. These clouds easily rival regular towering cumulus and cumulonimbus in their size and appearance.
Cloud of the Day – Cavum
Cavum is one of the new clouds that show up in the latest edition of the World Meteorological Organization’s International Cloud Atlas. I reported on the release of the new edition in this post. Cavum is really just a new name for a cloud type previously known as a fall streak hole, which I reported on here. There are more great pictures in that post. The full name for the example shown in this post is altocumulus stratiformis perlucidus translucidus cavum. That is, the middle etage cloud altocumulus (my previous post on altocumulus) which is in a layer thin enough to allow light through, and which has gaps between its elements and a great big hole with virga in it. Here’s how cavum is described in the International Cloud Atlas.
A well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of Cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval shaped when viewed from a distance.
When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail.
Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.
And here’s the description of the image from the International Cloud Atlas.
This thin, translucent and extensive layer of cloud is Altocumulus stratiformis translucidus. In the top part of the picture it also displays the variety perlucidus, as there are the gaps between the cloud elements. However, the most striking feature is the large, roughly circular hole beneath which there is virga. The large hole is the supplementary feature cavum, popularly known as a “fallstreak hole” or “hole-punch cloud”. The full classification for the cloud is therefore Altocumulus stratiformis perlucidus translucidus cavum.
Also of note is a linear gap in the cloud between the fallstreak hole and the horizon. This is an aircraft dissipation trail, or distrail, formed as a result of an aircraft flying through the cloud layer. Informally this is sometimes known as a “canal cloud”. It later transformed into a circular-type hole.
The supplementary feature cavum is formed when glaciation occurs in a thin cloud layer consisting of supercooled water droplets that are in a liquid state and at a temperature below 0 °C. As the supercooled water drops glaciate, the resulting ice crystals fall from the cloud layer to a lower level as virga, or fallstreaks. The resulting cloud hole typically grows larger with time while the glaciation process continues.
There’s not much I can add to that, except to invite you to visit the International Cloud Atlas website.
I’ve just learned of a website dedicated to hobbies — HobbyHelp. They describe themselves as, “a small team of enthusiasts, looking to share our knowledge and experience with anyone looking to start a new hobby.” Some of the most popular posts are, 10 Most Popular Hobbies in the World, 10 Hobbies That Look Great on Your Resume, and The Ultimate Beginner’s Guide to Astrophotography. But what brought me to the site was a message from Jenny, one of the bloggers at HobbyHelp. She had seen my post, NASA Image and Video Library, about NASA’s consolidation of its scattered resources into a single archive, and reached out to me with a link to a post of hers, The Ultimate Beginner’s Guide to Astronomy.
I think HobbyHelp is selling itself short by limiting its audience to beginners “looking to start a new hobby.” I think their site would be useful to people who are already involved in a hobby, too. People who are trying to expand their knowledge, or who are contemplating taking the next step in their experience. That is certainly true of Jenny’s post mentioned above. The depth of her research is plain to see. The care in the writing is obvious. While useful to someone taking their first steps in the hobby of astronomy, her piece would also be useful to people further along.
August 5, 2019
I got a note from Charles Wade of the NightSkyPix website asking if I would like to add a link to his site to offer my readers an additional source of information. From the site:
NightSkyPix is dedicated to learning and teaching all aspects of astrophotography and night-time imagery.
I had a look at the site and could see no obvious reason why I shouldn’t link to it. It looks like a thorough and professionally-crafted site dedicated to astrophotography.
So, there it is.