“They are,” said Uncle George. “In fact, the clouds in the sky and the cloud behind the engine are just the same kind of thing. They are both made up of tiny particles of water.
“We have learnt that the streak of cloud left by the steam-engine melts away quickly on a hot day, also that the puff is farther from the funnel on a hot day. This shows us that the warmer the air is, the more water can it take up and hold. We have also learnt that warm air is light and rises up.[2]
“What happens when warm air, which holds much water-gas, rises up to the higher and colder parts of the sky?”
“It gets cooled,” answered Frank.
“Yes, and its water-gas gets cooled too. Then we can see it as great masses of water-dust. These masses we call clouds. If these masses of cloud get further cooled, the tiny water particles run together to form great drops—as they did on the cold knife. They are now too large and heavy to float in the air, so they fall to the earth as rain.”
A Showery Day.
Questions and Exercises.
1. If you place a saucer full of water outside on a hot day in the morning, and go back to it in the evening, you will find the saucer dry. Where has the water gone? 2. Fill a tin can with a mixture of salt and snow (or chopped ice). Place it in a warm room. Frost comes on the outside of the tin. Place a glass jug of water in the same room. Dew is formed on the outside of the jug. Can you explain this? 3. Let the steam puff of the kettle strike against a cold sheet of glass or metal, or a slab of stone. What is formed?