Like most cats, I don’t love wet fur. I check a weather app every morning to see if I need an umbrella. But how rain happens was a mystery to me.

So, I talked about rain with my friend Nathan Santo Domingo. He’s a field meteorologist with AgWeatherNet of Washington State University. That’s a weather tool for farmers, gardeners and other people in Washington.

“The first thing to remember is that Earth’s surface is 71% water,” Santo Domingo said. “We also have a giant orb in the sky—the sun—that’s feeding energy into the atmosphere and reaching down to Earth’s surface.”

The sun’s energy changes the water in the oceans, rivers and lakes. The water changes from a liquid to a gas called water vapor. That water vapor floats up into the bubble of gas that surrounds Earth—called the atmosphere.

The higher the water vapor floats, the colder the air is. That changes the water vapor back into liquid water. Those drops of liquid water way up high in the atmosphere are incredibly tiny. They’re so light they float. A bunch of tiny drops all floating together is a cloud.

Sometimes a cloud floats into a place with low air pressure. Or it bumps into a mountain. The tiny water drops move up and down. They bonk into each other. When two water drops bump together, they merge into a bigger water drop.

“Eventually, a water droplet becomes so heavy the air can’t support it,” Santo Domingo said. “It starts to fall to the ground. It hits your head, jacket or umbrella in the form of a raindrop. Or a snowflake if it’s cold.”

That rain flows back into the oceans, rivers and lakes. Someday, the sun’s energy will turn it back into water vapor. The journey a water drop makes from Earth’s surface up into the atmosphere and back is called the water cycle.

Credit: NOAA

Sometimes you can tell it’s going to rain by looking at the sky. But weather forecasts can tell us if it’s going to rain much farther out than our eyes can.

Back in the day, weather scientists used tools like thermometers and barometers to predict rain. Thermometers measure changes in how hot it is. Barometers measure how much pressure there is. That’s how much air is above you, pushing down due to gravity.

Weather scientists still use those tools. Now they also use supercomputers to track temperature and air pressure. They measure all the way up and down the atmosphere. They also use math equations about water, air, sunlight, plants and ocean temperatures to make predictions.

That’s how weather scientists make accurate forecasts today. That way we can check a weather app and know if we need an umbrella to keep our fur dry.


Dr. Universe