"I little thought," said Willie, "when we started our chats about vapor, how far they would lead us. Why, that tumbler of cold water has told us all about the dew-drops we see on the grass in the morning; the steam from the spout of the kettle explains the clouds that we see in the sky, and-""You forget, Will," said Norah, joining in, "youhave not yet explained all this to me. I want to know, please.""Well, then," said Will, "suppose I try to explain it to you now. You know that evaporation is always going on around us from the surface of the earth, from seas and rivers, and from plants and the bodies of animals.
"We do not see the vapor in the usual way. But sometimes the air, instead of being transparent and allowing us to see through it, is thick and opaque. The vapor in the air has been condensed, because the air around is cold. This condensed vapor is called fog.
"When the air all round is not cold, the vapor rises and floats away over our heads, and we do not see it till it meets with the cold air above. This cold air condenses the vapor and forms a cloud.""Then a fog and a cloud must be very much the same thing," said Norah, "except that the cloud is high up in the sky and the fog low down near the ground.""Yes, they are," said Willie. "People who travel over mountains often find themselves in the midst of a dense cloud, and they walk through it just as we walk through a fog.""I know now what clouds are," said Norah. "Often when we look up at the sky we can see clouds, but I can"t quite understand yet why the clouds send us rain.""I think I can explain that," said Willie. "The clouds consist of vapor condensed into little particles which are just able to float in the air. If a cloud passes through still colder air, the vapor will condense still more, and the little particles will rush together, and form actual drops of water too heavy to float in the air. Then they must fall to the earth as rain.
"Sometimes the drops are very small indeed, although too heavy for the air to hold. They fall as extremely fine rain, which we call mist.""You have not yet explained the dew on the grass," said Norah, "although you said the tumbler of cold water told you all about it.""This is the way teacher explains that to us," said Willie. "He says that often after a warm day the air near the earth becomes suddenly cold at night. The evaporation is still going on rapidly, but as the vapor is formed it is condensed on the cold surface of the ground, the grass, and the leaves of plants, just as the vapor in the room condensed on the cold tumbler. This forms drops of dew."SUMMARY
Invisible vapor is always rising from the earth. It becomes visible when it is condensed. If the air around us is cold the vapor condenses into a fog. It usually rises and floats away before it condenses, and then forms a cloud. When the cloud condenses into actual water, it drops as rain. Vapor condenses on the cold earth and forms dew.
Lesson 15