AWH 6-5

Chapter 6, Water Vapor 6-5 observations of both temperature and dewpoint. The temperature greatly affects the air parcel’s ability to hold water vapor, while the dewpoint indicates the actual quantity of water vapor in the parcel. As the spread decreases, relative humidity increases. When the spread decreases to zero, relative humidity is 100 percent, and the air parcel is saturated. Figure 6-3 illustrates the relationship between temperature- dewpoint spread and relative humidity. Surface temperature-dewpoint spread is important in anticipating fog but has little bearing on precipitation. To support precipitation, air must be saturated through thick layers aloft. Figure 6-3. Temperature-Dewpoint Spread Effect on Relative Humidity Relative humidity depends on the temperature-dewpoint spread. In Figure 6-3, dewpoint is constant, but temperature decreases from left to right. On the left panel relative humidity is 50 percent, indicating that the air parcel could hold twice as much water vapor as is actually present. As the air parcel cools, the temperature-dewpoint spread decreases while relative humidity increases. When the air parcel’s temperature cools to equal its dewpoint (11 °C), its capacity to hold water vapor is reduced to the amount actually present. The temperature-dewpoint spread is zero, relative humidity is 100 percent, and the air parcel is now saturated. 6.7 Change of Phase Water changes from one state of matter, e.g., solid, liquid, or vapor, to another at the temperatures and pressures experienced near the surface of the Earth. Interestingly, water is the only substance on Earth that exists naturally in all three phases: as water droplets, ice crystals (visible as clouds), and water vapor. Water has some unique thermal properties, which make it a powerful heat transport mechanism. It has the highest specific heat capacity of any naturally occurring substance (see Table 5-4). This means that water has a much higher capacity for storing heat energy (with little resulting temperature change) than other substances. These properties make water an ideal heat transport mechanism and have important implications on weather and climate.