'UFO Clouds' Are Real. Here's How They Happen

Technically called “lenticular clouds,” the weird phenomenon seen over Cape Town has a simple explanation.

Social media users in Cape Town, South Africa, posted photos over the weekend that call to mind classic alien invasion movies. The pictures feature eerie, saucer-shaped puffs that seem to hang over the sky like UFOs.

But these so-called “UFO clouds” are nothing to fear.

Meteorologists call them lenticular clouds, which form when strong, moist winds blow over rough terrain, such as mountains or valleys. Picturesque Cape Town is framed by such features, including the 3,500-foot (1,066-meter) Table Mountain.

(See more cloud pictures and learn about the science of mirages.)

As the wind flows over large features it may cool, causing it to condense into disk-shaped clouds that develop perpendicular to the direction of the air flow.

The clouds in the latest viral photos are called stratocumulus standing lenticularis, because of their height and shape. Lenticular is a Latin word that means lens-like, another reference to their shape. (See more photos of “UFO clouds” from National Geographic’s Your Shot community.)

A number of past reports of UFO sightings have been linked to lenticular clouds, which can form in many places around the world. (Other UFO sightings have been attributed to hole-punch clouds, which are formed by miniature snowstorms in thin, subfreezing cloud layers.) 

“The aliens have been moniting [sic] cape town for years now,” Instagram user monre44 quipped over the weekend.

There are more Instagram pics of the "UFO clouds" below. What do you think the clouds look like? 

Follow Brian Clark Howard on Twitter and Google+.

<p>A supercell thunderstorm strikes in South Dakota. Among the most severe storms, supercells can bring strong winds, hail, and even tornadoes. (<a href="https://news.nationalgeographic.com/2015/04/150411-pictures-weather-storm-climate-change-hurricane-tornado-lightning/">See more extreme weather pictures</a>.)</p>

Lightning Strikes

A supercell thunderstorm strikes in South Dakota. Among the most severe storms, supercells can bring strong winds, hail, and even tornadoes. (See more extreme weather pictures.)

Photograph by Jim Reed, National Geographic

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