Pressure (Dalton’s Law)

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. Temperature 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 Condensation occurs if moisture is added to the air after it is saturated, or if cooling of the air reduces the temperature below the saturation point. As shown in figure 1-11, the most frequent cause of condensation is cooling of the air from the following results: (a) air moves over a colder surface, (b) air is lifted (cooled by expansion), or (c) air near the ground is cooled at night as a result of radiation cooling. Pressure (Dalton’s Law) The English physicist, John Dalton, formulated the laws relative to the pressure of a mixture of gases. 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. 1-19 SATURATED CONDENSING HEAT COOL 60 F o 80 F o 60 F o AGf0110 Figure 1-10.—Saturation of air depends on its temperature. WARM COLDER COLDER AIR MOVES IN OVER COLDER SURFACE. COOLED BY EXPANSION. RADIATION COOLING LIFTING A B C AGf0111 Figure 1-11.—Causes of condensation.