"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.
