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 equal numerically to specific humidity, but it is always slightly greater. The mixing ratio has the same characteristic properties as the specific humidity. It is conservative (values do not change) for atmospheric processes involving a change in temperature. It is nonconservative for changes involving a gain or loss of water vapor.

Previously it was learned that air at any given temperature can hold only a certain amount of water vapor before it is saturated. The total amount of vapor that air can hold at any given temperature, by weight relationship, is referred to as the saturation mixing ratio. It is useful to note that the following relationship exists between mixing ratio and relative humidity. Relative humidity is equal to the mixing ratio divided by the saturation mixing ratio, multiplied by 100. If any two of the three components in this relation-ship are known, the third may be determined by simple mathematics.

The dewpoint is the temperature that air must be cooled, at constant pressure and constant water vapor content, in order for saturation to occur. The dewpoint is a conservative and very useful element. When atmospheric pressure stays con-stant, the dewpoint reflects increases and de-creases in moisture in the air. It also shows at a glance, under the same conditions, how much cooling of the air is required to condense moisture from the air.

Aerographers Mate 3 and 2, NAVEDTRA 10363-E1, Naval Education and Training Program Development Center, Pensacola, FL, 1976. Byers, Horace Robert, Glossary Of Meteorology, American Meteoro-logical Society, Boston, MA, 1959.

Dynamical and Physical Meteorology, NAV-AIR 50-1B-533, McGraw-Hill Book Com-pany, NY, 1957.

Meteorology For Army Aviators, United States Army Aviation Center, Fort Rucker, AL, 1981. Riley, Denis and Spolton, Lewis, Trewartha, Glenn T. and Horn, Lyle H., Introduction To Climate, McGraw-Hill Book Company, NY, 1980.

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