CHAPTER 6 (Moran and Morgan, 1997) Planet Earth is often called the water planet because liquid water covers approximately 71 percent of the earth's surface. The climate of the earth is unique because the chemical water (H20) can exist in all three physical phases (liquid, solid and gas), as found within oceanic, terrestrial and atmospheric reservoirs. Since the total amount of water on the planet remains relatively constant, water can cycle between reservoirs through processes involving physical phase changes to include evaporation, transpiration, condensation, sublimation, deposition, and precipitation. These transfer processes occurring between reservoirs comprise the global hydrologic cycle. Since the processes involve the utilization of energy in the phase transformation process, the hydrologic cycle also helps redistribute heat energy, and ultimately contributes to maintaining a habitable climate on planet Earth. While the amount of water vapor in the earth's atmosphere is relatively small, comprising of less than 4 percent by volume, water vapor is an extremely important atmospheric constituent. Unfortunately, the amount of water vapor in the atmosphere, often called atmospheric humidity, is difficult to measure precisely. Therefore, meteorologists have developed various types of instruments to measure atmospheric humidity, a weather element, together with various ways of describing the concentration. STUDY NOTES CHAPTER 6 Inspect Table 6.1, noting the relative amounts of water stored in the various reservoirs on planet Earth. Rather than memorizing the specific percentage values, you should realize that most of the planet's water resides in the world's oceans. The second largest reservoir consists of a relatively small amount of water sequestered in the cryosphere (ice sheets and glaciers). At the other extreme, only a minuscule amount of water is found in the atmosphere, either as water vapor, or in clouds as liquid droplets or ice crystals. Spend a moment with Figure 6.1 reviewing the various pathways along which water is transported between the various reservoirs as part of the hydrologic cycle. Use this schematic along with Table 6.2, which accounts for the global and annual averages of the various flow rates between reservoirs. Since the entries in the table are very large, let us consider describing these flow rates in the same depth units, as we would report rainfall from a rain gauge. MAGNITUDE OF RECYCLING BETWEEN RESERVOIRS RECYCLING PROCESSES EQUIVALENT DEPTH [centimeters] [inches] PROCESS OVER OCEANS PRECIPITATION 89.5 35.2 EVAPORATION 94.4 37.2 NET LOSS from Oceans 9.9 3.9 PROCESS OVER LAND PRECIPITATION 66.3 26.1 EVAPORATION 41.9 16.5 NET GAIN on Land 24.4 9.6 GLOBAL BUDGET GLOBAL PRECIPITATION 82.8 32.6 GLOBAL EVAPORATION 82.8 32.6 NET GAIN 00.0 0.00 Before studying Tables 6.3 and 6.4, look at Figures 6.3 and 6.4, which are graphical representations of how the amount of water vapor in the atmosphere under saturation conditions changes with temperature. For the moment, consider only the variation in the saturation vapor pressure, since the variation in the saturation mixing ratio corresponds in similar fashion. The chart in Figure 6.3 is plotted with temperature on the horizontal axis and the saturation vapor pressure is on the vertical axis. You should momentarily ignore the blue insert that ranges from -15 degrees Celsius to 0 degrees Celsius. Note that the saturation vapor pressure changes in essentially an exponential fashion with temperature. In other words, this relationship corresponds with the rule of thumb described in the text, where the saturation vapor pressure doubles for each 11 Celsius degree increase in temperature. You should verify this statement, using the accompanying graph or table. The insert provides greater detail at temperatures below 0 degrees Celsius because in this temperature range the saturation vapor pressure over supercooled water is slightly greater than over ice at the same temperature. We will return to this concept of supercooled water and the differences in saturation vapor pressure between supercooled water and ice in our discussion of precipitation formation. In Figure 6.5, the air temperature is plotted in red with the scale on the left, while the relative humidity is plotted in green, using the scale on the right. Note that during daylight hours, from approximately 6:30 AM until 6 PM, the temperature increases until early afternoon, then starts to fall as cooling accompanies lower sun angles in the late afternoon. The relative humidity reaches a maximum near sunrise, then decreases to a minimum in early afternoon. Despite the large variation in the relative humidity, the amount of water vapor in the air, as specified by the dewpoint (not shown) would remain relatively constant throughout the day in this particular example. Inspect Table 6.5, making sure that you can reach the same results as described in the text. Note that you will have to use the Fahrenheit temperature values in conjunction with this table. Note Figure 6.6, that in the picture of a sling psychrometer, two mercury thermometers are employed. The thermometer on the left has the white muslin sock around its bulb, and is called the wet-bulb thermometer. This instrument is rotated (or slung) rapidly to aerate the wet-bulb using the crank handle on top of the instrument. Figure 6.7 -- A hygrograph, which typically uses a hair hygrometer (found behind the lever mechanism on the right hand side of the instrument) for recording changes in relative humidity over time. Spend some time studying the schematic appearing in Figure 6.8. This schematic shows how an air parcel that remains unsaturated will respond to changes in atmospheric pressure when it moves vertically in the atmosphere. ALTITUDE PRESSURE TEMPERATURE VOLUME [meter] [mb] [degrees C] [cubic meters] 4000 600 -20 1.44 3000 700 -10 1.28 2000 800 0 1.16 1000 900 +10 1.07 0 1000 +20 1.00 Figure 6.9 -- This schematic shows that as an unsaturated air parcel is lifted, it cools by the dry adiabatic lapse rate (10 Celsius degrees per kilometer). Above the LCL, parcel will cool at the saturation adiabatic lapse rate (approximately 6 Celsius degrees per kilometer). In this case, an air parcel is lifted from the surface, starting at a temperature of 30 degrees Celsius to the LCL at 2000 m altitude, with the air parcel cooling by 10 Celsius degrees. Lifting the saturated air parcel to an altitude of 4000 m would cool parcel by 12 Celsius degrees. Study Figures 6.10 and 6.11, representing diagrams for two distinctly different stability cases. In both cases, air temperature is plotted on the horizontal axis and altitude is plotted on the vertical axis. The actual air temperature profile for the layer is given by the blue line, which we can call the "environment" for simplicity. The red dashed line in each panel represents the dry adiabatic lapse rate. Use the so-called "parcel method" to convince yourself of the stability of the layer in each case. In either case, we will consider an air parcel that is initially at an altitude of approximately 1.5 km and has the same temperature and pressure characteristics of the environment. Figure 6.10 shows a temperature inversion. Try lifting the selected air parcel from the base of the inversion, causing the parcel to cool by the dry adiabatic lapse rate, which is equivalent to moving upward and to the left along the solid red arrow. You should note that upon displacement upward the parcel would become colder and denser than the environment at any new level. As a result of the greater density, the parcel will sink downward like a "lead balloon" to the original point. During this descent, the parcel will warm by 10 Celsius degrees per kilometer. In fact, the parcel may overshoot and continue downward along the dry adiabat below the original point, where the parcel will now become warmer and less dense than its environment. An upward return of the air parcel would be required in this case. Hence, the inversion case appearing in Figure 6.10 is one that is considered to be stable. On the other hand, using this same parcel method in the example appearing in Figure 6.11, we would find an unstable situation. In this case, the layer has an environmental temperature profile that is greater than the dry adiabatic lapse rate. (Inspection of the layer's lapse rate in this case leads to an estimate of approximately 18 Celsius degrees per 1000 meters.) Specifically, move the air parcel upward along the dry adiabat, noting that the parcel will remain warmer and hence less dense than its environment. As a result, the parcel would be more buoyant than its surroundings and continue to move upward as if it were a "hot air balloon". Convince yourself that this parcel, when moving downward from the original point, would continue to sink because it would be colder than its environment. Since the temperature profile in this case results in displaced air parcels continuing away from the origin, we will classify the layer in this case as being unstable. The two panels in Figure 6.12 show how the stability of the air near the earth's surface can change from dawn (Panel A) to noon (Panel B). You should think about how the heating of the earth's surface after sunrise could erode the surface radiation inversion, by initially heating the surface and then the lower levels of the stable layer from below. By noon, the entire layer shown here has become unstable. Figure 6.13 provides an example of a small inversion that can be situated between two unstable layers. Take a moment to study this situation. Use the graph in part A of Figure 6.14 in combination with the tabulation below in Part B as a means of summarizing the stability for five different temperature profiles (A through E). The dry and saturation adiabatic lapse rates (dashed red and dashed green lines, respectively) are also provided for comparison. You should first consider what the five soundings represent, and then compare them with the dry and moist adiabatic lapse rates. Use the parcel method as described previously to determine if each layer would be stable or unstable. In each case, you would have to perform the test twice, once if the air parcel that you lift would be unsaturated and once with the parcel being saturated. Figures 6.16 and 6.17 are vertical cross-sections through a warm front and cold front, respectively. These cross-sections would extend upward to an altitude of 10 km. Both diagrams provide examples of the lifting mechanisms involved to form clouds and where these clouds typically form along a warm front and cold front. Figure 6.18 provides a schematic of how clouds are produced by passage of moist air over a mountain barrier. Read Special Topic (Humidity and Human Comfort) on page 128-131. Learn how the National Weather Service often uses the Apparent Temperature (or Heat Index) during the summer when high air temperature and high humidity conditions warrant. You should inspect Table 1, and follow through the provided example. You should also be able to identify the Categories of Hazards for a given apparent temperature. Skim Special Topics (Clouds by Mixing) on page 145. Read Weather Fact (The Rainiest Place on Earth) page 146. Mathematical Note -- (Energy Conservation and the Dry Adiabatic Process) on pages 147 to 149. Skim this note that is intended for mathematically inclined. Briefly, the discussion centers upon the First Law of Thermodynamics, which simply stated describes the utilization of energy, with the constraint of conservation. CHAPTER 6 (Moran and Morgan, 1997) HUMIDITY AND STABILITY This is the first of three chapters on moisture in the atmosphere. In this chapter, we present some fundamental concepts: the global hydrologic cycle, ways of expressing the water vapor concentration of air, the nature of saturation, achieving saturation through expansional cooling, stability of air, and lifting processes. On Earth, water occurs in all three phases and is distributed among oceanic, terrestrial, and atmospheric reservoirs. Water is transferred between the atmosphere and the other reservoirs through evaporation, transpiration, condensation, sublimation, deposition, and precipitation. The water vapor concentration of air is described in terms of vapor pressure, mixing ratio, and relative humidity. Uplift and expansional cooling of air lead to saturation and cloud development. Stability of the air either enhances or suppresses uplift that is triggered by convection, fronts, topography, and converging surface winds. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: identify the principal reservoirs in the global hydrologic cycle. distinguish among the various phase change processes of water. describe how the water vapor concentration of air is quantified. compute the relative humidity from either the mixing ratio or the vapor pressure. explain why and how relative humidity is temperature dependent. distinguish between expansional cooling and compressional warming. describe how the adiabatic assumption applies to the atmosphere. distinguish between dry adiabatic lapse rate and moist adiabatic lapse rate. explain how atmospheric stability is determined. describe how atmospheric stability affects vertical air motion and cloud development. identify the various lifting processes that operate within the atmosphere. 6 Humidity and Stability 124 The Hydrologic Cycle 125 How Humid Is It? 127 The Saturation Concept 131 Relative Humidity 133 Humidification 134 Humidity Measurement 135 Achieving Saturation 136 Atmospheric Stability 139 Lifting Processes 142 Conclusions 146 Special Topic: Humidity and Human Comfort 128 Special Topic: Clouds by Mixing 145 Weather Fact: The Rainiest Place on Earth 146 Mathematical Note: Energy Conservation and the Dry Adiabatic Process 147 Key Terms 149 Summary Statements 149 Review Questions 150 Quantitative Questions 150 Questions for Critical Thinking 151 Selected Readings 151 4