The residents of Cape Town have an endless source of spooky entertainment in Table Mountain, the imposing plateau overlooking the city. This flat-topped hunk of rock, featured recently in a viral Reddit video, is often covered by a “tablecloth” of clouds that vanish mysteriously off its cliffs. Legend has it that they’re the spoils of a smoking contest between the Devil and a pirate named Van Hunks. The explanation that atmospheric scientist David A. Kristovich, Ph.D. gave Inverse avoids the supernatural — but is no less magical.
The time-lapse video, posted to Reddit’s r/blackmagicfuckery (“Anything that clearly has no other explanation but no good voodoo black magic fuckery”), shows a thick layer of clouds rolling over Table Mountain’s plateau that dissolve as they dive off its sharp cliffs. The caption reads “Not a cloud in the sky of Cape Town” — which is not always true, though the uncanny disappearance of the clouds in this video, shot by photographer Eric Nathan, may make you think otherwise.
Kristovich, an adjunct associate professor at the University of Illinois’s Department of Atmospheric Sciences, explained in an email to Inverse that these disappearing clouds illustrate the dramatic effect that changing temperatures have on the air.
“One of the most common ways that clouds form is when air with high water vapor content is lifted upward to higher levels above sea level,” he says. “In this case, it looks like the wind is blowing moist air from the Atlantic Ocean upward to the top of Table Mountain. As the air moves upward toward the peak, it cools, and water vapor in the air condenses into the thick clouds you can see in the video.”
The disappearance of the clouds, however, has to do with the “stability” of the atmosphere — that is, whether there’s vertical movement due to warming and cooling air. On this day, Kristovich says, the atmosphere was “statically stable.”
“When the atmosphere is “stable”, any time the wind causes the air to rise to the peak of a mountain, it becomes denser than its surroundings – so, the air sinks back down to a lower height after passing by the peak,” he explains. “The cloudy air that was blown up to the top of Table mountain is encountering a stable atmosphere, so the clouds sink back down on the other side of the peak. That is why the clouds look like they are spilling over the top of the mountain.”
Those spilling clouds appear to vanish when that moist, cool air crashes into the warm air lower down the mountain. “If cloudy air is forced downward, like we see in the video, the air warms and the small water droplets evaporate back into invisible water vapor in the air,” he says. “So, the reason the clouds all seem to disappear is because the cloud droplets are evaporating as the air sinks and warms. The bottom of the clouds, in this case, is simply the point at which the cloud droplets have evaporated.”
In short, there are no devils, no pirates, and no black magic fuckery involved in this phenomenon, but who needs the supernatural when nature itself is already so weird? As Kristovich notes, “Mountain ranges can make huge changes in the types and amounts of clouds that nearby communities experience.” Perhaps no one knows it better than the residents of Cape Town: In 2015, disc-shaped lenticular clouds over Table Mountain convinced people UFOs had finally touched down on Earth.