Randy Cerveny is a climatologist at Arizona State University. He is a President’s Professor in the School of Geographical Sciences. That means he is a very smart guy. Cerveny knows all about clouds and lots of other interesting weather stuff. That’s his job. He studies weather and climate and then teaches others about what he learns. Students always have lots of questions for the professor. Want to learn something about clouds? Keep reading:

QUESTION: Lots of people say that they can see shapes formed in the clouds. But exactly how do clouds form into shapes?

PROFESSOR CERVENY: The process of cloud formation is called “cloud physics.” It’s a pretty complex subject. But think about it this way. Clouds form as bubbles of moist air. Those bubble rise and then cool to the point that the water condenses out. In other words, the water changes from being a vapor, like steam, and becomes a liquid — water. Dry air sinks around the bubbles. That creates irregularly shaped bubbles that form different shapes. But those shapes are just accidental. They don’t have any meaning other than being pretty to look at. A cloud is really just a big bubble of rain that hasn’t fallen to the ground yet.

QUESTION: I know that clouds are formed by molecules of water that get all smooshed together. But why do they form different shapes?

PROFESSOR CERVENY: The answer is air. Air currents, to be exact. Air currents are constantly pushing clouds up and down. Different layers of the air have different amounts of water in them. Cloud shapes shift and change as the air pushes them in different directions.

QUESTION: But wouldn’t all the molecules squeeze together? Then all the clouds would be one shape, and only one shape. Why don’t clouds all look like a circle or an oval?

PROFESSOR CERVENY: If you were to form a cloud in a nice sealed chamber where you keep constant conditions (winds, temperature and humidity) inside, you would be right. Every cloud would form into the same shape. But our world is not a sealed chamber. Nature contains lots of chaos. Conditions are always changing. That is why clouds can form lots of different shapes.

QUESTION: I notice that some clouds are thinner than others. That’s because some clouds contain lots more water than others. But why don’t the clouds break apart and become smaller?

PROFESSOR CERVENY: The key is how high up you push the clouds. All air contains water. But if you push some of that air up high enough (to where it is colder), that water condenses out from vapor (steam) to liquid (water). We see the liquid as a cloud. How high up a cloud gets pushed and how much water vapor it contains are the important factors that determine how thick a cloud will be.

QUESTION: How do clouds stick together? What holds the water molecules together?

PROFESSOR CERVENY: Sometimes they don’t. When the water droplets get heavy enough, they fall out of the cloud. We call that rain.

QUESTION: How fast do clouds really move?

PROFESSOR CERVENY: That depends on how fast the winds are blowing around and above the clouds. On nice calm days, the puffy cumulus clouds we see are not moving at all. However, some storm clouds can move at 60 or 70 miles per hour.

QUESTION: What would our weather be like if the Earth was shaped like a square instead of a round sphere?

PROFESSOR CERVENY: A square-shaped planet with an atmosphere would not exist for very long as a square. It would quickly become round-shaped. The power of weather plus time is very strong. It might take millions of years, but the power of wind, water, and heat would cause the “edges” of the cube to be rounded. Just like a square pebble or rock in a river gradually becomes rounded. Or think about mountains as an example. Mountains are built quickly (geologically-speaking). But over time, the power of weather erodes the even the tallest, sharpest mountains into smooth, round hills. Mother Nature prefers a nice smooth, rounded surface to blow across.