There are so many different types of clouds that we see that it’s hard to imagine that they’re all primarily water vapor in the face of different atmospheric conditions. Most of the common clouds – cumulus, stratus, and cirrus, are easily identifiable and their causes are well-known.

But, occasionally odd cloud formations arise, leaving skywatchers stupefied as to how on Earth they came to be. Here are explanations for a few rare, but intriguing cloud types:

Cloud #1: Kelvin-Helmholtz wave clouds

A Kelvin Helmholtz cloud (NASA)
A Kelvin Helmholtz cloud (Benjamin Foster/NCAR)

Kelvin-Helmholtz Wave clouds (also called billow clouds) are clouds that form an almost uniform line across the sky, and take the visual form of a breaking wave.

These clouds are extremely short-lived, and break in the same fashion as a wave on a shore – the bottom layer of water moves slower than the top layer, and the top billows over and crashes.

This behavior is caused by wind shear, when two vertically adjacent layers of air are moving at different speeds.

“This type of atmospheric condition normally occurs on windy days where there is a difference in densities of the air, such as in a temperature inversion,” notes Rachelle Oblack of in her article about Kelvin-Helmholtz clouds.

These clouds are indicative of extreme turbulence – while it may be scenic from a plane window, it also marks a good time to remain seated.

Video of Kelvin-Helmholtz wave clouds breaking here.

Cloud #2: Pyrocumulus clouds

Via NASA, from Idaho: “The photo, originally published on the Elk Complex Incident site, shows the white (pyrocumulus) cloud rising above the gray smoke plume on the afternoon of August 10 (2013).”

Also known as a “fire cloud,” a pyrocumulus cloud looks like a normal cumulus cloud, but is formed by intense heating of the surface below it by wildfire or a volcanic eruption rather than solar energy (NASA, The Weather Guys). This is obvious by the large tower of smoke leading up to the bottom of the cloud.

Many pyrocumulus clouds formed this summer with the intense wildfires in Colorado and New Mexico. NASA has an overhead image of the pyrocumulus cloud resulting from the New Mexico Silver fire, and The Weather Channel has an incredible ground view photo of the Patch Springs Wildfire’s pyrocumulus in Utah.

If the pyrocumulus occurs in an area where the atmosphere contains a lot of water vapor, the cloud can develop into a more dangerous storm cloud just like a regular cumulus cloud. It is then called a pyrocumulonimbus (The Weather Guys).

There was an incidence of pyrocumulonimbus on August 13th in Boise, ID from the Beaver Creek Fire that is still raging. Satellite imaging is available here.

 Cloud #3: Lenticular clouds

Lenticular cloud over the Big Island of Hawaii, 2003 (NOAA)
Lenticular cloud over the Big Island of Hawaii, 2003 (NOAA)

Lenticular clouds take the visual form of a flying saucer, which gives them their nickname of “UFO clouds.” These clouds form in mountainous regions (this particular picture is from Mauna Kea, HI).

When stable, moist air streams over a mountain and is heated,  it condenses into this lens-shaped cloud.

Via the Gemini Observatory, November 25, 2003: “Taken from Hilo, HI this image of a lenticular cloud formed by winds over Mauna Kea.” (Gemini Observatory)

The wind that brings the air over the mountain then cools and sinks in a wave pattern, which can lead to a line of lenticular clouds extending away from the mountain that build on the crests of the airwave and diminish at the troughs.

These clouds can also stack vertically when there is a layer of dry air between two layers of moist air moving in the same direction over a mountain. It leads to an image like this (National Geographic).

Two other bizarre cloud forms we’ve written about recently are mammatus and noctilucent clouds.

See these posts for more information:

Stunning mammatus clouds south of Washington, D.C. Thursday evening

Noctilucent clouds and aurora: all in one view (VIDEO)

Have you seen any of these rare cloud forms in person?

(The author of this post, Amanda Morgenthal, is a Capital Weather Gang summer intern. She is an atmospheric science major at the University of Virginia. More info.)