Tuesday

Rocket has drawn a cloud on the lab whiteboard with five arrows pointing down.

"Rain, snow, sleet, freezing rain, hail," he says, tapping each arrow. "Five kinds, five different clouds. Right?"

Raven studies the drawing. "What do you notice about where the arrows start?"

"They all start in the same cloud," Rocket admits.

Nova projects a thin column of air under the cloud, colored blue at the top and red near the ground.

"The cloud is only the beginning," she says. "Think about what a drop meets on the way down."

Rocket grabs a marker and starts labeling the air warm and cold. "The journey matters," he says. "Not just the start."

In warmer clouds, drops grow by collision and coalescence. Drops bump into each other and join.

Eventually the drops become too large to stay suspended in the air, and they fall to the ground as rain.

In colder clouds, both ice crystals and water droplets are present. This is the ice crystal process.

Water vapor deposits directly onto the ice crystals, and the crystals grow until they are heavy enough to fall.

If the air near the surface is cold, snow reaches the ground. Otherwise the snowflakes melt into rain.

Sleet is rain that turns into ice pellets before it reaches the ground. The pellets are frozen raindrops, or snowflakes that mostly melted and then refroze.

Freezing rain stays liquid until it hits the ground. Then it freezes, creating a coating of ice on roads, walkways, trees and power lines.

Hail is different. It forms when updrafts in a thunderstorm carry raindrops upward into cold air.

Hailstones grow by colliding with supercooled water drops, which freeze onto them. Then the stones fall.

Hailstones are a quarter inch across or more. Hail one inch or larger is a sign of a severe thunderstorm.

Well done. Tomorrow is Station Lab: you build a rain gauge and learn to read it like a weather observer.