Absolute humidity (AH) is a measure of the physical amount of water vapor in the air and is often expressed in grams per kilogram (g/kg). Relative humidity (RH) is a ratio obtained by dividing the air's absolute humidity (AH) by the air's vapor capacity (C). Relative humidity is calculated as a percent using the formula below. Table 1 x 100 Table 1 illustrates that air's vapor capacity is temperature-dependent. C RH -40 -30 -20 -10 0 5 10 15 20 25 30 35 40 AH C - The capacity of air of specific temperatures (g/kg) -or- How many grams of water vapor it takes to saturate 1 kg of air 0.1 0.3 0.75 2 3.5 5 7 10 14 20 26.5 35 47 Assume that a 1-kilogram parcel of air has an absolute humidity (AH) of 7 grams. Assume that this same parcel of air has a temperature of 20 C. Select the formula needed to calculate the air's relative humidity. RH = x 100 14 20 RH RH RH = = = 35 5 7 14 x 100 x 100 x 100