Monday

A morning mist hangs over Station Rooftop, and Rocket walks straight into it. "I am inside a cloud!" he calls.

Raven follows, her sleeves already damp. "What do you notice? You can feel it, but you cannot grab it."

"It is made of water," Rocket says, "but it is not raining. How does water float?"

Nova hovers at the edge of the mist, her light making a glowing ball in the fog. "Remember the cold can," she says. "What did the air need before its vapor turned to drops?"

"It had to cool down to the dew point," Raven says.

"And something to condense onto," Rocket adds, remembering the can's cold side. "But up in the sky there is no can."

Nova dims her light. "There is always something," she says. "Even if it is far too small to see."

The National Weather Service defines a cloud as a visible collection of minute particles of water or ice. They form when water vapor condenses in the atmosphere.

Water vapor itself is invisible. A cloud appears when that vapor condenses into droplets or freezes into ice crystals big enough to scatter light.

The droplets are tiny. The U.S. Geological Survey notes that cloud droplets can be as small as 10 millionths of a meter.

Clouds can be carried along by strong winds or stay in one place while the wind blows through them. They form and dissolve constantly.

Water molecules on their own are too small to join together into a droplet. They need a surface to condense onto.

Those surfaces are called cloud condensation nuclei: tiny specks of smoke, ocean salt or wind-blown soil floating in the air.

The specks attract water molecules. Every cloud droplet has a speck of dust, salt or smoke at its core, though the droplet is still almost pure water.

The specks alone are not enough. The air must also cool to its dew point before vapor condenses onto them.

Good start. Tomorrow you will learn why rising air makes clouds, and the four Latin words that name them.