In August, we posted a photograph of some odd, rare clouds known as Morning Glory clouds without providing an explanation for how they form. In response to reader interest, we followed up with meteorologist Roger Smith of the University of Munich, who has studied their formation.

“Over the years we’ve developed a good understanding of them,” Smith said. “It’s no longer a mystery, but still very spectacular.”

The Morning Glory phenomenon is the result of the particular configuration of the land and sea on the Cape York Peninsula, in a remote part of Australia. The peninsula tapers off from about 350 miles wide to 60 miles as it extends north between the Gulf of Carpentaria to the west and the Coral Sea to the east. The easterly trade winds push the sea breeze across the peninsula during the daytime, which meets the sea breeze from the west coast in the late evening. The collision produces a wave disturbance moving inland to the southwest that is a key part of the cloud formation.

As moist sea air is lifted to the crest of the waves, it cools and condensation forms a cloud. Sometimes there is just one wave, but Smith has seen as many as 10 together in a series.

“If you look at the clouds, it looks as if they are rolling backwards,” Smith said. “But in fact the clouds are continuously formed at the leading edge and continuously eroded at the trailing edge. That gives a rolling appearance.”

These clouds do occur elsewhere, including Munich, where they form about once in a decade. Cape York is unique because they happen regularly in the fall above the small town of Burketown. And they can also be particularly impressive there as well, growing up to 600 miles long. Pilots fly into the area every year, hoping to see the intriguing clouds.

Not many scientists study them, or really any weird clouds, because their very rarity makes them relatively unimportant for studying precipitation or climate. So, oftentimes, their formation is poorly understood.

"It's hard to get funding to study something that's neat looking," said cloud physicist Patrick Chuang of the University of California, Santa Cruz.

On the following pages, we've gathered photos of some of the strangest, most beautiful cloud types and asked scientists to help us understand how they form.

Images: Above: Ulliver/Wikimedia Commons. Below: Mick Petroff/APOD

Known as mammatus clouds, these strange, pouchy formations often occur in association with a thunderstorm. They have been seen in lots of places, including the midwestern United States.

"They are some seriously weird looking clouds," said cloud physicist Patrick Chuang.

But scientists do have some theories about mammutus clouds. Another cloud physicist, Daniel Breed of the National Center for Atmospheric Research, says buoyancy and convection of air is the key.

"They're kind of like upside-down convection," Breed said.

Convection is like a buoyant bubble, he said. In mammutus clouds, evaporation causes pockets of negative buoyancy as it cools the air inside the cloud. This makes the clouds puff downward instead of up like cumulus clouds, and they end up being like upside-down bubbles.

The reason they are smooth is the thermal structure right below them. The speed at which the temperature drops with increased height, known as the "lapse rate," needs to be close to neutral, he said. In other words, if you put a warm little bubble of air in a particular spot, it won't go up or down much at all — no heat goes in or out. This is typical of the thermal structure of thunderstorms. Without these circumstances, you'd get more common ragged-looking clouds or cloudy wisps coming out.

"Anywhere you get thunderstorms it can certainly happen," Breed said. "You don't have to have thunderstorms, but you have to have atmospheric conditions with some buoyancy to it."

Images: Above: Flickr/3d_king. Below: Flickr/turbojoe. 2) Flickr/coreburn. 3) Flickr/nebraskasc.

Sometimes likened to UFOs, lenticular clouds are usually created by gravity waves. Chuang evokes loose shock absorbers to describe what gravity waves are.

"You take your grandma's Cadillac and drive it over a speed bump, and after that it goes up and down for a while," he said. "The reason you are going down is because of gravity, and then there are springs in the suspension that push you back up."

In the case of lenticular clouds, the speed bump is usually some kind of topography, like a mountain, that gets in the way of air flow. As the air comes down the side of the mountain, it tends to overshoot and then springs back up. It oscillates like this for a while, and on the upward part of the waves, clouds form as rising air cools.

"Clouds mark the highest part of the oscillation," Chuang said.

Lenticular clouds can also be caused by other speed bumps, such as tall thunderclouds, but because they often form on the downwind sides of mountains, they are also known as lee clouds, wave clouds or lee wave clouds.

A mountain range can form a series of long wave clouds, but if the speed bump is more isolated, like a single mountain, the result can be oval-shaped clouds that look like UFOs. Sometimes multiple ovals form that look like a stack of saucers.

"I like wave clouds because I see them so often here," Breed said of Boulder, Colorado, where NCAR is. "I have a lot of favorites, but this is the one I have on my screen saver." (Below).

Images: Above: Flickr/cardiffjackie. Below: 1) Daniel Breed. 2) Betsy Mason, Wired.com. 3) NCAR/UCAR.

These crazy clouds that look like a row of crashing waves are known as Kelvin-Helmholz waves. They form when two layers of air or liquid of different densities move past each other at different speeds, creating shearing at the boundary.

"It could be like oil and vinegar," Chuang said. "In the ocean, the top is warm and the bottom is really cold. It's like a thin layer of oil on a big puddle of water."

When these two layers move past each other, a Kelvin-Helmholz instability is formed that is sort of like a wave. Parts of the boundary move up and parts move down. Because one layer is moving faster than the other, the shear causes the tops of the waves to move horizontally, forming what looks like an ocean wave crashing on the beach.

"It really is like breaking waves," Chuang said. "A wave breaks when the water on top moves so much faster than the water below that it kind of piles up on itself."

Images: Above: UCAR/NCAR. Below: 1) Mila Zincova/Wikimedia commons. 2) UCAR/NCAR

This spectacular photo of the eruption of Sarychev Volcano in the Kuril Islands, northeast of Japan on June 12 shows an interesting example of a pileus cloud. The ash plume appears to have a smooth white cap on it as it breaks through the cloud cover above.

This type of cloud is caused by strong, relatively fast upward motion. Situations where this occurs include quickly growing thunderheads, volcanic eruptions and even nuclear explosions. In each case, something pushes warm, moist air upward quickly.

"You can see them very commonly above thunderstorms storms, and that's because the air is moving so rapidly up there that the air flow gets perturbed above it," Breed said. "And as soon as it cools enough for a cloud to form, you get these cloud caps."

Chuang says pileus clouds look similar to the smooth caps sometimes seen on top of cumulus clouds, which are actually ice caps.

"If you have very strong convection, pushing quickly and very high, it forms more of an ice cloud because it is high enough for the crystals to freeze," Chuang said. "It's nothing more mysterious than things freezing."

Image: NASA

We became aware of noctilucent clouds here at Wired Science in July when the strange glowing clouds began appearing over the United States and Europe, much farther south than they were normally seen.

These "night shining" clouds are formed by ice at the boundary of Earth’s atmosphere and space, 50 miles high. They shine because they are so high they remain lit by the sun even after it has gone below the horizon. It’s not clear why these clouds have migrated down from the poles, or why more of them are appearing in the polar regions, too, and shining more brightly.

Nobody knows for sure, but most of the answers seem to point to human-caused global atmospheric change.

The clouds form at temperatures around minus-230 degrees Fahrenheit, when dust blowing up from below or falling into the atmosphere from space provides surfaces for water vapor to condense on and freeze. Right now, during the northern hemisphere’s summer, the atmosphere is heating up and expanding. At the outside edge of the atmosphere, that actually means that it’s getting colder because it’s pushed farther out into space.

“The prevailing theory and most plausible explanation is that CO2 buildup, at 50 miles above the surface, would cause the temperature decrease,” said James Russell, an atmospheric scientist at Hampton University and the principal investigator of an ongoing NASA satellite mission to study the clouds. He cautioned, however, that temperature observations remain inconclusive.

But the truth could be far more complex, and there are many other theories about these clouds.

Images: Above: Mike Hollingshead, Extreme Instability. Below: NCAR/UCAR

Morning Glory clouds are a specific and more unusual type of roll clouds, which are more common, and not usually as long.

Roll clouds typically occur in the lower atmosphere ahead of a storm front. Warm updrafts in the storm front push cold air up, which then flows down along the sides of the updraft. The cold downdraft then bounces back up a bit setting up a wave-like structure in front of the storm.

On the upswing, the cold air forms a cloud. Evaporation of the cloud causes a downdraft on the edges that erodes the cloud, forming a roll. If the wave continues, a series of roll clouds, called a street, can form.

Image: Above: Flickr/jonnyr1. Below: Flickr/tlindenbaum.

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