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A change of state (or change of phase) of a substance describes the change of a substance from a solid to a liquid, liquid to a vapor (or gas), vapor to a liquid, liquid to a solid, solid to vapor, or vapor to a solid. In meteorology you are con-cerned primarily with the change of state of water in the air. Water is present in the atmosphere in any or all of the three states (solid, liquid, and vapor) and changes back and forth from one state to another. The mere presence of water is impor-tant, but the change of state of that water in the air is significant because it directly affects the weather. The solid state of water is in the form of ice or ice crystals. The liquid state of water is in the form of raindrops, clouds, and fogs. The vapor state of water is in the form of unseen gases (water vapor) in the air.

Heat Energy

Energy is involved in the various changes of state that occur in the atmosphere. This energy is primarily in the form of heat. Each of the changes of state processes either uses heat from the atmosphere or releases heat into the at-mosphere. The heat used by a substance in chang-ing its state is referred to as the latent heat and is usually stated in calories. The calorie is a unit of heat energy. It is the amount of heat required to raise the temperature of 1 gram of water 1°C. A closer look at some of the major changes of state of the atmosphere helps to clarify latent heat. Refer to figure 2-2-1 during the following discussions.

Figure 2-2-1.—Thermal history of 1 gram of ice during changes of state.

Liquid To Solid and Vice Versa

Fusion is the change of state from a solid to a liquid at the same temperature. The number of gram calories of heat necessary to change 1 gram of a substance from the solid to the liquid state is known as the latent heat of fusion. To change 1 gram of ice to 1 gram of water at a constant temperature and pressure requires roughly 80 calories of heat. This is called the latent heat of fusion. Fusion uses heat. The source of this heat is the surrounding air.

The opposite of fusion is freezing—a liquid changes into a solid. Since it requires 80 calories to change 1 gram of ice to 1 gram of water, this same amount of heat is released into the air when 1 gram of water is changed to ice.

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