Relative Humidity
Saturation of surface air is a condition of favorable fire weather; that is, conducive to low fire danger. Less favorable are conditions of unsaturation, which permit evaporation from forest fuels, increasing their flammability and the fire danger. Therefore, a very useful measure of atmospheric moisture is the relative humidity. It is the ratio, in percent, of the amount of moisture in a volume of air to the total amount which that volume can hold at the given temperature and atmospheric pressure. Relative humidity is also the ratio of actual vapor pressure to saturation vapor pressure, times 100. It ranges from 100 percent at saturation to near zero for very dry air. Relative humidity depends on the actual moisture content of the air, the temperature, and the pressure.
The dependence of relative humidity on temperature must be kept in mind. Suppose that we have air at 80°F. and 24 percent relative humidity. Using Table:Dew Point, Vapor Pressure, and Absolute Humidity, we find that the saturation vapor pressure for 80° is 1.032 inches of mercury. We can compute the actual vapor pressure by multiplying 1.082 by 0.24. The actual vapor pressure is 0.248 rounded off. The dew point for this vapor pressure is 40°. We now know that if the air was cooled from 80°F. to 40°, with no other change, the humidity would increase from 24 percent to 100 percent and the air would be saturated. At that temperature the actual vapor pressure would equal the saturation vapor pressure. The absolute humidity in the table could be used in a similar manner. Thus; the relative humidity may change considerably with no addition of moisture, just by cooling alone.
Factors influencing variations in relative humidity
Relative humidity is much more variable than absolute humidity. It often changes rapidly and in significant amounts from one hour to the next and from place to place. Relative humidity is much more variable because it depends not only on absolute humidity but also on air temperature. It varies directly with moisture content and inversely with temperature. Because of these relationships, it is often not possible to make general statements about relative humidity variations, particularly vertical variations within short distances above the ground.
Encyclopedia ID: p435
