Even on quiet nights out for dinner with my wife, weather geekdom can find its way to the surface. Friday was one of those evenings. The Italian restaurant was packed so I dropped my wife off to get us on the wait list. While parking the car, I took out my phone camera and snapped a picture of the cumulonimbus cloud above. There are some textbook features associated with it. As such, it became the subject of my latest meteorological “101” series.
I noticed the cloud while driving and was secretly hoping my vantagepoint at the restaurant would be optimal for a picture. Of course, my wife gave me the serious eye roll when I walked in. She said, “OMG, you were out there taking pictures.” After twenty-six years of marriage, she’s kind of use to these weather shenanigans. Let’s dive into this cumulonimbus cloud.
The storm was located roughly 35 miles southwest of my location in Snellville, Georgia. It was a classic cumulonimbus cloud in so many ways. In fact, if you look at the chart below, the picture resembles the symbol for a cumulonimbus (Cb) that has an anvil shape at the top. By definition, the United Kingdom Meteorological Office website describes cumulonimbus clouds as, “Menacing looking multi-level clouds, extending high into the sky in towers or plumes…. commonly known as thunderclouds, cumulonimbus is the only cloud type that can produce hail, thunder and lightning.” The American Meteorological Society Glossary formally defines cumulonimbus as, “A principal cloud type (cloud genus), exceptionally dense and vertically developed, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions.”
How do they form? They represent atmospheric convection and can be associated with significant instability or surface heating. Under certain conditions, some type of forcing (convective heating, a front, mountain, or other boundary) may lift air. If the upper air is colder, then like a hot air balloon, the parcel of air becomes buoyant and rises. You see this commonly with other types of convective clouds including cumulus and cumulus congestus. The AMS Glossary goes on to say, “Because of its great vertical size and of the magnitude and variety of forces that act within and upon it, cumulonimbus is a vertical cloud factory.” Its vertical extent also indicates that there are very strong updrafts.
Let’s go back to my photograph at the beginning of the article. Do you see the cauliflower-shaped clouds ringed at the top by an expansive fan-shaped cloud feature? That’s the anvil. The anvil shape happens because as clouds reach the tropopause, a relatively stable area that acts like an atmospheric “lid,” they fan out horizontally. There is also something else going on in my picture. In the center, the clouds extend above the anvil. When a meteorologist like me sees that, we know there is a very strong updraft with a lot of energy such that it “overshoots” the tropopause. Anvils and overshooting tops are commonly visible in satellite imagery too. How much energy can these powerful storms contain? The UK Met Office website writes, “They get taller and taller until they represent huge powerhouses, storing the same amount of energy as 10 Hiroshima-sized atom bombs.”
I want to draw your attention to last feature in my picture. There is a smaller ”cap-like” cloud above the overshooting top. It is called a pileus cloud. Such clouds are defined by the International Cloud Atlas as, “An accessory cloud of small horizontal extent, in the form of a cap or hood above the top or attached to the upper part of a cumuliform cloud that often penetrates it.” They can be rather small or quite extensive. They form because air is rising so quickly that the water vapor is rapidly converted to an ice cloud. Because these clouds contain ice, iridescence (or bright rainbow colors) can occur.
There are actually different types of cumulonimbus clouds. If you want to take your weather “snobbery” up a notch at the next dinner party, you might ask what the differences are among cumulonimbus calvus (puffy, water droplets at the top), cumulonimbus capillatus (fibrous appearance, some freezing), and cumulonimbus incus (anvil or “incus”, fibrous).
