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There is a limit to the amount of water vapor that air, at a given temperature, can hold. When this limit is reached, the air is said to be saturated. The higher the air temperature, the more water vapor the air can hold before saturation is reached and condensation occurs. (See fig. 1-5-1.) For approximately every 20°F (11°C) increase in temperature between 0°F and 100°F (–18°C and 38°C), the capacity of a volume of air to hold water vapor is about doubled. Unsaturated air, containing a given amount of water vapor, becomes saturated if its temperature decreases sufficiently; further cooling forces some of the water vapor to condense as fog, clouds, or precipitation.

Figure 1-5-1.—Saturation of air depends on its temperature.

The quantity of water vapor needed to produce saturation does not depend on the pressure of other atmospheric gases. At a given temperature, the same amount of water vapor saturates a given volume of air. This is true whether it be on the ground at a pressure of 1000 mb or at an altitude of 17,000 ft (5,100 meters) with only 500 mb pressure, if the temperature is the same. Since density decreases with altitude, a given volume of air contains less mass (grams) at 5,100 meters than at the surface. In a saturated volume, there would be more water vapor per gram of air at this altitude than at the surface.


Although the quantity of water vapor in a saturated volume of atmosphere is independent of the air pressure, IT DOES DEPEND ON THE TEMPERATURE. The higher the temperature, the greater the tendency for liquid water to turn into vapor. At a higher temperature, therefore, more vapor must be injected into a given volume before the saturated state is reached and dew or fog forms. On the other hand, cooling a saturated volume of air forces some of the vapor to condense and the quantity of vapor in the volume to diminish.


Condensation occurs if moisture is added to the air after saturation has been reached, or if cooling of the air reduces the temperature below the saturation point. As shown in figure 1-5-2, the most frequent cause of condensation is cooling of the air and often results when; (a) air moves over a colder surface, (b) air is lifted (cooled by expansion), or when (c) air near the ground is cooled at night as a result of radiational cooling.

Pressure (Dalton’s Law)

The laws relative to the pressure of a mixture of gases were formulated by the English physicist, John Dalton. One of the laws states that the partial pressures of two or more mixed gases (or vapors) are the same as if each filled the space alone. The other law states that the total pressure is the sum of all the partial pressures of gases and vapors present in an enclosure.

For instance, water vapor in the atmosphere is independent of the presence of other gases. The vapor pressure is independent of the pressure of the dry gases in the atmosphere and vice versa. However, the total atmospheric pressure is found by adding all the pressures—those of the dry air and the water vapor.


The actual amount of water vapor contained in the air is usually less than the saturation amount. The amount of water vapor in the air is expressed in several different methods. Some of these principal methods are described in the following portion of this section.

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