Evaporation and Vapor Pressure

Evaporation

Some molecules momentarily acquire a very high speed from the impacts of other molecules. If this collision occurs in liquid water near the surface, and the high speed is in an outward direction, the molecules may escape into the air. This is evaporation, the process by which a liquid water molecule becomes a water-vapor molecule. Since molecules with the highest energy content escape, leaving behind in the liquid those with a lower energy content, the average level of energy of this liquid is decreased. The decrease in energy level results in a decrease in temperature of the liquid. Therefore, evaporation is a cooling process. Each molecule escaping into the air by a change of state takes with it nearly 1,000 times the energy needed to raise the temperature of a water molecule 1° F.

Vapor pressure

The pressure at the water-air boundary resulting from molecular motion in the direction of escape from the liquid is called the vapor pressure of water. This pressure varies only with the temperature of the water and determines the rate at which water molecules escape to the air and become vapor molecules. The water-vapor molecules which escape to the air displace air molecules and contribute their proportionate share to the total atmospheric pressure. This portion is called the partial pressure due to water vapor, or for simplicity, the vapor pressure.

Vapor pressure depends on the actual water vapor in the air, and it may vary from near zero in cold, dry air to about 2 inches of mercury in warm, moist air. High values can occur only in the warm, lower layers of the troposphere. The pressure produced by the vapor causes some water-vapor molecules to re-enter water surfaces by condensation. The same amount of heat energy that was needed for evaporation is liberated to warm the condensation surface.

At the water-air boundary, molecules are exchanged in both directions continuously, but the exchange is usually greater in one direction or the other. Evaporation occurs when more molecules leave the water surface than enter it, and condensation occurs when the opposite takes place. Actually, both condensation and evaporation occur at the same time. As noted earlier, a similar exchange of molecules takes place between water vapor and ice in the process of sublimation. The vapor pressure of ice is somewhat less than that of water at the same temperature. Hence, at low temperatures sublimation on ice is accomplished more readily than condensation on a water surface.

When the vapor pressure in the atmosphere is in equilibrium with the vapor pressure of a water or ice surface, there is no net exchange of water molecules in either direction, and the atmosphere is said to be saturated. A saturated volume of air contains all the vapor that it can hold. The vapor pressure at saturation is called the saturation vapor pressure. The saturation vapor pressure varies with the temperature of the air and is identical to the vapor pressure of water at that temperature. The higher the temperature, the more water vapor a volume of air can hold, and the higher the saturation vapor pressure. Conversely, the lower the temperature, the lower the saturation vapor pressure. Table: Saturation Vapor Pressure illustrates how the saturation vapor pressure varies with temperature. In the common range of temperatures in the lower atmosphere, the saturation vapor pressure just about doubles for each 20^o F increase in temperature. With this understanding of evaporation, condensation, and vapor pressure, we can now define several terms used to indicate the amount of moisture in the atmosphere.

The air near the surface is usually not saturated; therefore, the actual vapor pressure is usually less than the saturation vapor pressure. The actual vapor pressure can be raised to saturation vapor pressure by evaporating more moisture into the air, or, since saturation vapor pressure varies with temperature, the air can be cooled until the saturation vapor pressure is equal to the actual vapor pressure. Evaporation alone does not ordinarily saturate the air except very close to the evaporating surface. Normal circulation usually carries evaporated moisture away from the evaporating surface.