Chapter 12 — THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



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. 



Water vapor, in the atmosphere, for 

 instance, is independent of the presence of 

 other gases; the vapor pressure is indepen- 

 dent 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. 



TERMS 



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 manners. Some 

 of the principal methods are described in the 

 following portion of this section. 



Relative Humidity 



Although the major portion of the atmos- 

 phere is not saturated, for weather analysis 

 it is desirable to be able to say how near it 

 is to being saturated. This relationship is 

 expressed as relative humidity. The relative 

 humidity of a volume of air is the ratio (in 

 percent) between the water vapor actually 

 present and the water vapor necessary for 

 saturation at a given temperature. 



Assume, for instance, that the temperature 

 is 25° C. The amount of water vapor needed to 

 saturate a cubic meter of air at this tempera- 

 ture is 23.05 grams. If observation indicates 

 only 11.525 grams of vapor in a cubic meter, 

 the sample volume is half saturated, or its 

 relative humidity is 50 percent. 



Relative humidity is also defined as the 

 ratio (expressed in percent) of the observed 

 vapor pressure to that required for saturation 

 at the same temperature and pressure. 



Relative humidity shows the degree of 

 saturation, but it gives no clue to the actual 

 amount of water vapor in the air. Thus, other 

 expressions of humidity are useful. 



Absolute Humidity 



The mass of water vapor present per unit 

 volume of space, usually expressed in grams 

 per cubic meter, is known as absolute humidity. 

 It may be thought of as the density of the water 

 vapor. 



Specific Humidity 



Humidity may be expressed as the mass 

 of water vapor contained in a unit mass of 

 air (dry air plus the water vapor), or as the 

 ratio of the density of the water vapor to the 

 density of the air (MIXTURE OF DRY AIR 

 AND WATER VAPOR). This is called the 

 specific humidity and is expressed in grams 

 per gram or in grams per kilogram. Since this 

 value depends upon the measurement of mass, 

 and mass does not change with temperature 

 and pressure, the specific humidity of a parcel 

 of air remains constant unless water vapor 

 is added to or taken from the parcel. For this 

 reason, air which is unsaturated may move from 

 place to place or from level to level, and its 

 specific humidity remains the same as long 

 as no water vapor is added or removed. However, 

 if the air is saturated and cooled, some of the 

 water vapor must condense; consequently, the 

 specific humidity (which reflects only the water 

 vapor) decreases. If saturated air is heated, 

 its specific humidity remains unchanged unless 

 water vapor is added to it, in which case the 

 specific humidity increases. The maximum 

 specific humidity which a parcel can have 

 occurs at saturation and depends upon both 

 the temperature and the pressure. Since warm 

 air can hold more water vapor than cold air 

 at constant pressure, the saturation specific 

 humidity at high temperatures is greater than 

 at low temperatures. Also, since moist air is 

 less dense than dry air at constant tempera- 

 ture, a parcel of air has a greater specific 

 humidity at saturation, if the pressure is low, 

 than when the pressure is high. 



Mixing Ratio 



The mixing ratio is defined as the ratio 

 of the mass of water vapor to the mass of 

 DRY AIR and is expressed in grams per gram 

 or in grams per kilogram. It differs from 

 specific humidity only in that it is related 

 to the mass of dry air instead of to the total 

 dry air plus water vapor. It is very nearly 



271 



