Ground-level Clouds Even though all clouds form in about the same way, they don’t all look alike. T at’s because altitude, location, and other factors change how a cloud looks. Let’s start at ground level. Fog is the only
kind of cloud that forms so close to the ground that you can walk through it. It blankets the ground, hiding everyday objects, such as trees, houses, cars, and skyscrapers. You can feel the tiny water droplets that make up fog when you walk through it. Other clouds form much higher. You’d
have to fl y up 6 kilometers 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 up to 300 kilometers per hour. T ey whoosh through the ice-crystal clouds, tearing them into thin white wisps.
Glowing Clouds 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 form in a very cold and dry
part of our atmosphere—about 80 kilometers above Earth’s surface. T is area is drier than a desert, so no one knows how water vapor or ice crystals form there. No one knows where the dust comes from that the ice or water clings to, either. Some scientists think it may come from meteors. T ese rocks from space leave lots of dust behind as they burn up in the atmosphere. Night-shining clouds are so high up that
sunlight shining on them causes them to glow. T ey even glow in the middle of the night.
20 NATIONAL GEOGRAPHIC EXTREME EXPLORER NATIONAL GEOGRAPHIC EXPLORER
Saucer-shaped Clouds Altitude is just one factor that aff ects how a cloud forms and what it looks like. Winds also play a big role as they blow through the atmosphere. Winds push, shred, and swirl clouds. T ey cause clouds to move and change shape, sometimes forming strange and unusual shapes you’ve not seen before. Long ago, when I was a boy, I was on a
mountain in Austria, learning to ski. A bitter cold wind was blowing, giving me the shivers. 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 fl ying saucer. T e cloud looked like it was hovering right above me, hanging in the sky waiting to beam me up.
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, over the mountain. Once the wind rose above the mountain, it sank quickly. T is sudden change in altitude changed the direction in which the wind moved. Instead of moving in a straight line, it bobbed up and down. T e wind was acting like a wave. T e waves of wind caused water vapor in the
air to 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. T ese clouds don’t always form alone. If
the air has layers of moist and drier air, these clouds stack on top of one another. T at’s when people notice them, and the UFO sightings start to roll in!
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24