EXPLOSIVE. That was the first word that came to mind when I first saw this picture of a tornadic supercell taken Tuesday.
These stacks are what are known as pileus clouds. Pronounced like “pill-ee-us,” pileus is latin for “cap.” No surprise, another name for these thin clouds is “cap clouds.” They are actually in the same cloud family as lenticular clouds, as they are high-level clouds composed of ice crystals.
Like lenticular clouds, they form when air is pushed upward quickly. However, there is one huge difference between the two cloud types. With lenticulars, the force, or physical barrier, that typically forces the air upward that then cools and condenses into a cloud is a mountain or mountain range. With pileus clouds, that force is instead a tall cumulonimbus cloud.
As the cumulonimbus cloud explodes vertically into the atmosphere, the upper level airflow around it is forced upward so abruptly that any moisture in the air immediately condenses directly into an ice fog as the air rises and cools. Thus, pileus clouds provide a visual cue that a thunderstorm has a rapidly rising updraft, and thunderstorms with strong updrafts tend to be the ones most likely to turn severe and even tornadic.
Now, back to the photograph above. It is very unusual to have more than one pileus cloud over a thunderstorm at one time. Due to the ice-crystalline nature of the cloud, they tend to be short-lived and dissipate quickly. However, the severe thunderstorm above was so explosive, with an updraft so strong, it formed multiple pileus clouds. The updraft was probably so intense that it was forming new pileus clouds faster than the old ones could dissipate.
The stacked pileus clouds almost look like a shock wave being set off by the storm, traveling high up into the atmosphere.
Unfortunately, this supercell thunderstorm did in fact produce a confirmed EF-1 tornado that tracked over Scott, Rankin, and Leake counties in Mississippi. This was just one of dozens of tornadoes that have been confirmed by National Weather Service survey teams after Tuesday’s tornado outbreak.
The 44 tornado reports Tuesday made it the second-highest tornado day of 2016, surpassing tornado outbreaks that happened much earlier in the year, including those over the more typical tornado alley and during the more traditional spring tornado season.
Tornado surveys are ongoing across the Southeast, and we probably won’t know the final tornado count until the weekend.
Here is another photo taken Tuesday of an exploding storm over Scott County, Miss., also showing a pileus cloud hovering above the cumulonimbus.
So if you ever see a pileus cloud (or several) capping an approaching thunderstorm, know that that storm means serious business and take shelter until it has passed.
#cwgpicoftheweek
