2. Clouds in nature

Water on the earth evaporates when it is heated by the sun. Some of the water molecules in oceans, rivers and lakes gain enough energy to break free of the water's surface and diffuse in the air. Some of these energetic water molecules then rise higher in the atmosphere, where the air is cooler. As the temperature decreases the molecules lose some of their energy, and condense onto particles in the atmosphere, forming clouds.

To try to imitate natural cloud formation, Aitken changed his experiment. He put a little water in the bottom of a large glass jar, and waited until the air in the jar became saturated with as many water molecules as possible held in the air.

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