AWH 6-4

Chapter 6, Water Vapor 6-4 6.4 Relative Humidity Relative humidity is the ratio, usually expressed as a percentage, of water vapor actually in the air parcel compared to the amount of water vapor the air parcel could hold at a particular temperature and pressure. 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦 = 𝑊𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑊𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 While relative humidity is the most common method of describing atmospheric moisture, it is also the most misunderstood. Relative humidity can be confusing because it does not indicate the actual water vapor content of the air, but rather how close the air is to saturation. An air parcel with 100 percent relative humidity is saturated, while an air parcel with relative humidity less than 100 percent is unsaturated. An air parcel’s capacity to hold water vapor (at a constant pressure) is directly related to its temperature. It is possible to change an air parcel’s relative humidity without changing its water vapor content. Figure 6-2 illustrates this concept. An air parcel at sea level at a temperature of 30 °C has the capacity to hold 27 g of water vapor. If it actually held 8 g, its relative humidity would be 30 percent, and it would be unsaturated. However, if the air parcel’s temperature decreases to 20 °C, its water vapor storage capacity decreases to 15 g and its relative humidity rises to 53 percent. At 10 °C, the air parcel’s water vapor storage capacity decreases to equal the amount of water vapor it actually holds (8 g), its relative humidity increases to 100 percent, and it becomes saturated. During this cooling process, the air parcel’s actual water vapor content remained constant, but relative humidity increased with decreasing temperature. Figure 6-2. Temperature Effects on Relative Humidity 6.5 Dewpoint Dewpoint is the temperature an air parcel must be cooled at constant pressure and constant water vapor pressure to allow the water vapor in the parcel to condense into water (dew). When this temperature is below 0 °C (32 °F), it is sometimes called the frost point. Lowering an air parcel’s temperature reduces its capacity to hold water vapor. 6.6 Temperature-Dewpoint Spread (Dewpoint Depression) The difference between an air parcel’s temperature and its dewpoint is the dewpoint depression, or commonly referred to as the spread. Surface aviation weather reports (e.g., METARs/SPECIs) provide