Water undergoes the process of evaporation when changing from the liquid to a gaseous state. According to the molecular theory of matter, all matter consists of molecules in motion. The molecules in a bottled liquid are restricted in their motion by the walls of the container. However, on a free surface exposed to the atmosphere, the motion of the molecules in the liquid is restricted by the weight of the atmosphere or, more pre-cisely, by the atmospheric pressure. If the speed of the liquid molecules is sufficiently high, they escape from the surface of the liquid into the atmosphere. As the temperature of the liquid is increased, the speed of the molecules is increased, and the rate at which the molecules escape from the surface also increases. Evaporation takes place only from the free or exposed surface of a substance.

During the process of evaporation, heat is released. This heat is absorbed by the water that has vaporized. The amount absorbed is approxi-mately 539 calories per gram of water at a temperature of 100C. On the other hand, the amount is 597.3 calories, if the evaporation takes place at a water temperature of 0C. This energy is required to keep the molecules in the vapor state and is called the latent heat of vaporization. Since the water needs to absorb heat in order to vaporize, heat must be supplied or else evapora-tion cannot take place. The air provides this heat.

For this reason, evaporation is said to be a cooling process, because by supplying the heat for vaporization, the temperature of the surrounding air is lowered.

Condensation is the opposite of evaporation because water vapor undergoes a change in state from gas back to liquid. However, a condition of saturation must exist before condensation can occur. That is, the air must contain all the water vapor it can hold (100 percent relative humidity) before any of it can condense from the atmosphere.

In the process of condensation, the heat that was absorbed in evaporation by the water vapor is released from the water vapor into the air and is called the latent heat of condensation. As you might expect, condensation warms the surround-ing air.

Solid To Gas and Vice Versa 

Sublimation is the change of state from a solid directly to a vapor or vice versa at the same temperature. In physics and chemistry, sublima-tion is regarded as the change of state from solid to vapor only, but meteorologists do not make this distinction. The heat of sublimation equals the heat of fusion plus the heat of vaporization for a substance. The calories required for water to sublime are: 80 + 597.3 = 677.3, if the vapor has a temperature of 0C.

In the sublimation process of vapor passing directly into the solid form without going through the liquid phase, the calories released are the same as those for the sublimation of a solid to a gas. Sublimation of water vapor to ice frequently takes place in the atmosphere when supercooled water vapor crystallizes directly into ice crystals and forms cirriform clouds.

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