NARRATOR: All clouds share the same basic composition. But just by looking outside on two different days—or even at different parts of the sky on the same day—you know that the atmosphere produces a huge variety of cloud shapes and sizes.
Why do some clouds form entertaining shapes, while others look gloomy, or even ominous? What's going on inside a cloud that gives it a distinct appearance? And could identifying the different types of clouds help us predict the weather or even the course of climate change?
We can’t really begin to analyze clouds without first categorizing their many variations. Most experts classify clouds into 10 or so separate types according to two main characteristics: their height in the atmosphere and their shape.
There are three height categories—low, middle, and high—and four shape categories: There’s nimbus, meaning, well, "rain cloud,"—these are low-hanging, flat, gray clouds that can drop rain for many hours, or more than a day if you’re unlucky; Stratus, meaning "layer"—which refers to their characteristic stacked appearance; Cumulus, meaning “heap” or "pile" but "puffy" might be a better way to remember them; and Cirrus, meaning "hair", because they kind of look like fine, wispy hairs.
Combinations of these height and shape characteristics give each cloud type its name, and they also tell us something about how they form and behave.
The height at which a cloud forms has a huge influence on its composition. In general, the portion of the atmosphere where clouds form gets colder the higher you go. That means low clouds are almost always made of water droplets, while high clouds are made up of ice crystals, and middle clouds are often a mix of liquid and ice.
These differences can often be seen in a cloud’s overall appearance, with clouds made up mostly of water having more clearly defined edges than icy clouds do.
A cloud's shape can also tell us about winds in the atmosphere. For example, wispy cirrus clouds take on their characteristic shape because of strong winds high in the atmosphere, which pull at the clouds' frayed edges.
In some cases, cloud types and their characteristics can be used to forecast the weather. For example, cirrus clouds are often the first signs of an approaching warm front and can foretell long periods of steady rain or snow in the days to come. Mid-level cumulus clouds called altocumulus forming on a summer morning suggest that there is significant moisture and heat in the atmosphere and that powerful afternoon thunderstorms might be in store. And if you see layered rain clouds called nimbostratus outside your window, you might as well settle in with a good book—it could be raining for a while.
Scientists are also studying clouds for clues about their possible role in climate change. For example, they’ve discovered that certain types of clouds can cause the planet to warm, while others have a cooling effect. What we don’t know yet, however, is which of these clouds types will become more or less common as the planet warms. Time will tell, and as it does, researchers will be watching.