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.
