Cold condenses the vapor in the air into drops of moisture. Steam is at ?rst invisible. It becomes visible when the cold air begins to condense it. All vapor is at ?rst invisible.
Lesson 14
Clouds, Rain, Dew
"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.
