Although the major portion of the atmosphere 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

Figure 1-5-2.Causes of condensation.

saturation at a given temperature. When the air contains all of the water vapor possible for it to hold at its temperature, the relative humidity is 100 percent. (See fig. 1-5-3.) A relative humidity of 50 percent indicates that the air contains half of the water vapor that it is capable of holding at its temperature.

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.

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.

Humidity may be expressed as the mass of water vapor contained in a unit mass of air (dry air plus the water vapor). It can also be expressed 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. 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 that is

Figure 1-5-3.Relative humidity and dewpoint.

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 this case the specific humidity increases. The maximum specific humidity that 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 con-stant pressure, the saturation specific humidity at high temperatures is greater than at low tempera-tures. Also, since moist air is less dense than dry air at constant temperature, a parcel of air has a greater specific humidity at saturation if the pressure is low than when the pressure is high.

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