High-fl ying Clouds Even though clouds form in similar ways, they don’t all look alike. T at’s because altitude, location, and other factors can aff ect how a cloud looks. One kind of cloud forms so close to the
ground, you can walk through it. It’s fog! It covers the ground like a thick blanket, hiding everyday objects—trees, houses, cars, or even roads. As you walk through fog, you can feel the cloud’s tiny water droplets on your skin. Other clouds form high above Earth. You’d
have to fly up 6 km to get close to cirrus clouds, the wispy clouds that look like horsetails. It’s so cold in this part of the atmosphere
that water droplets freeze. So these clouds are made of ice crystals. It’s not just cold high above the ground, it’s
windy, too. Winds race more than 300 kph. T ey whoosh through the ice crystal clouds, tearing them into thin, white wisps.
Clouds that Glow Cirrus clouds aren’t the highest clouds in Earth’s atmosphere, though. Some clouds form at the edge of space. T ese clouds are called night-shining clouds.
T ey look like bright blue ripples stretching across a dark sky. T ese clouds shine because they’re so high
up—about 80 km above Earth’s surface. Up there, light from the sun shines on clouds even in the middle of the night. Everything else about night-shining clouds
is a mystery. T at part of the atmosphere is drier than a desert, so no one knows how vapor or ice crystals can exist there. No one knows where the dust comes
from that the ice or water clings to, either. Some scientists think it may come space, or from meteors burning up as they enter our atmosphere.
20 NATIONAL GEOGRAPHIC EXPLORER
Saucer-shaped Clouds Altitude is just one factor that aff ects how clouds form and what they look like. Winds also play a big role as they blow through the atmosphere. Winds push clouds across the sky. T ey tear clouds apart. T ey swirl clouds into diff erent shapes. Imagine, for instance, clouds in the shape
of alien spaceships. T ey really exist. I know because I saw one as a boy. I was high up on a snowy mountain in
Austria, learning to ski. A bitter wind was blowing. When I stopped shivering long enough to look up, I saw a giant shape hovering over my head. It was the wildest cloud I’d ever seen. It had a smooth surface and a round shape.
In fact, it looked like discs stacked on top of one another. It made me think of a flying saucer. Even though the wind blasted, this cloud looked like it was hovering.
How the Cloud Formed I later learned that the wind and the mountain helped shape the cloud. First, the wind slammed into the mountain, which is part of Earth’s geosphere. T e collision forced the wind upward. When the wind whipped over the
mountaintop, nothing blocked its path. T en the wind sank. T is sudden change in motion made the wind wobble up and down like a wave, instead of blowing in a straight line. T e waves of wind made water vapor in the
air bounce up and down, too. At the top of the wave, the air was cool enough to condense, or turn into water droplets. As a result, a cloud that looks like a smooth disc formed. If the air is in layers of moist air, with dry
layers in between, these clouds stack up on top of each other. T at’s when people start to notice, and the UFO sightings start to roll in!
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