The suns rays, heating down on the oceans, lakes, and rivers of the earth, cause the surface water to evaporate and enter the surrounding atmosphere. When the amount of vapor is great, we feel uncomfortable and say that it is humid. At any particular temperature the air can hold only a certain amount of water vapor. If the air is holding a maximum amount of vapor, no more water can evaporate. The meteorologist also says that the humidity at such a time is 100%.
Condensation. Fortunately, the process of evaporation can reverse itself. Otherwise, the humidity would always be high and precipitation would never take place. The process which is the reverse of evaporation is called condensation and it occurs when the temperature falls. As it becomes cooler, water molecules begin to move more slowly. Collisions take place and molecules hang together to form tiny droplets which are no longer invisible.
Cloud Formation. The condensation process explains how clouds are formed. Warm air, laden with moisture, rises into the atmosphere. As the air rises it tends to expand and this causes the temperature to drop. The falling temperature slows down the speed of molecular movement, and the water molecules gather together on dust particles floating around in the atmosphere. This condensation results in the formation of billions of tiny water droplets that make up a cloud. The droplets are so small and lightweight that they are hardly affected by gravity. Slight air movements keep them floating miles above the earth.
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