CHAPTER 4 (Moran and Morgan, 1997) From the previous discussion, we have found that when we view the entire planet for a span of several years, Planet Earth is essentially in radiative equilibrium. In other words, the radiational heating rate resulting from the absorption of solar radiation by the earth-atmosphere system is balanced by the radiational cooling rate associated with the emission of terrestrial radiation from the top of the earth's atmosphere. Certain segments of this system may not be in radiative equilibrium, thereby requiring other energy transport mechanisms to maintain the energy balance of the entire system. Specifically, the earth's surface absorbs more radiational energy while the atmosphere looses more radiation than it receives. To alleviate the imbalance between the earth's surface and atmosphere, heat is transferred into the atmosphere primarily by convection. In addition, the spherical shape of the earth causes an imbalance, where the more vertical rays of the incoming solar radiation causes an energy surplus in the tropics, while an energy deficit in subpolar and polar latitudes where the rays are at a more oblique angle. To reduce this north-south energy imbalance, convection currents in both the atmosphere and oceans transport sensible and latent heat poleward from the tropics. The temperature of a locale can be influenced not only by the local radiation but also by air mass advection. STUDY NOTES Table 4.1 -- Look at this table as the annual energy budget for the entire earth-atmosphere system scaled as a percentage of the incoming solar radiation at the top of the earth's atmosphere. In this table positive (+) numbers would represent the gain (or input) of energy into planetary system and the negative (-) numbers correspond to the loss (or output) from the system. Compare this table with the graphical version appearing as Figure 4.1. You should use this figure to trace how incoming solar radiation available to the planet is utilized within the earth-atmosphere system and how the absorbed energy leaves the system. Red arrows describe the disposition of solar energy entering the planetary system, while blue arrows portray how long-wave radiation is emitted and absorbed in the earth-atmosphere system. The green arrows are the non-radiative energy transport mechanisms from the surface into the atmosphere. Note that as a result, net heating would equal net cooling. Figure 4.2 -- Review this schematic that shows the three physical phases of water and the phase transformation processes, symbolized with blue arrows that depict water (H2O) molecules changing phase. Red arrows show the gain or loss of latent heat by the water molecules during these phase changes. Since ice on the left side of the diagram is at the lowest energy state, any phase change to the liquid phase by melting or to the vapor through sublimation requires the external addition of energy to the ice molecules. The added energy resides as latent heat within the molecules that are either in the liquid or vapor phase. Similarly, vaporization from liquid to the vapor requires addition of heat to the molecules from the environment. Since these processes are completely reversible, energy from the molecules in the high-energy vapor phase is released into the atmosphere as latent heat when condensation or deposition occurs. Figure 4.3 -- This schematic not only compares the magnitude of the latent heats of melting and vaporization with the sensible heat but also demonstrates the distinction between latent and sensible heating. Note the relatively large value of latent heat involved with melting and with vaporization as compared with that of sensible heat. When energy is added to water and a physical phase change takes place with no change of temperature, latent heat is involved. An increase in the water temperature indicates the addition of sensible heat energy. Figure 4.4 -- This schematic with the accompanying photograph shows why puffy "cumulus type" clouds develop. Such types of clouds provide a visual clue that the convection process shown in Figure 3.7 is taking place within the lower troposphere. (Quickly scan the entries for the Bowen ratio for various geographic locales in Table 4.2) Figure 4.6 -- Inspect the three ways that excess energy is transferred from the earth's surface into the atmosphere. The units appearing in this figure are the same as those dimensionless units appearing in Table 4.1/Figure 4.1, and the values in parentheses represent the percentages of each component to the total heat flow from the surface into the atmosphere. You should note that the latent heat transport associated with the evaporation of water from the earth's surface (component A) is by far the largest energy transport process from the surface, representing approximately 52% of the energy transport. This component is followed in magnitude by the net infrared radiation (B), representing the difference between the upwelling and downwelling infrared radiation components, with slightly less than 33% of the total energy. Finally, the smallest component is the sensible heat flux (C), with approximately 15%. Figure 4.7 -- Take some time to inspect this figure, which displays the north-south (or latitudinal) distribution of radiation measured by satellites for several years. The horizontal axis depicts the latitude ranging from the North Pole on the left edge to the South Pole on the right edge. The vertical axis has a scale expressed in appropriate radiant flux units (watts per square meter). The solid curve represents the north-south variation in the radiant energy input into the planetary system (identified as solar radiation absorbed within the earth and atmosphere), while the dashed curve is the latitudinal distribution of the system's output (the infrared radiation emitted from the top of the atmosphere). An annual radiant energy surplus occurs in the tropical and subtropical region where the input curve exceeds the output curve. The regions in each hemisphere poleward of this tropical zone experience an annual radiant energy deficit since the outgoing radiation curve exceeds the incoming curve. Not by coincidence, the largest poleward transport of sensible and latent energy is found in the atmosphere and oceans in the midlatitude region near the boundary between the energy surplus and sink regions. Figure 4.8. Spend several minutes tracing the utilization of incoming solar energy within the earth-atmosphere system. From the previous discussion, some incident solar radiation is absorbed in the atmosphere (as noted on the right side of the figure), while a large fraction is absorbed by the earth's surface (see left side). As a consequence of the temperature of the object, energy is radiated toward space from both the atmosphere and the earth's surface. A portion of the absorbed energy is transferred from the surface into the atmosphere by radiative and non-radiative transfer processes (sensible and latent heat). In addition, a fraction of the energy is converted to kinetic energy associated with atmospheric motion (that is, the wind). This kinetic energy is dissipated through turbulence and friction to heat energy, which also warms the earth's surface and atmosphere. Compare the seasonal variations in the monthly air temperature for a tropical station (Clevelandia, Brazil) in Figure 4.9 with a subpolar station (Regina, Saskatchewan) in Figure 4.10. Two major points should be noted: One point is that a larger variation in the annual temperature cycle is found at the subpolar station than at the tropical station. The other point is that the average annual temperature for the tropical station is higher than that for the subpolar station. Figure 4.11 -- Inspect this map that provides a schematic representation of both warm air advection and cold air advection. The blue arrow (A) identifies cold air advection where northwesterly winds transport cold air from north of Lake Superior southeastward across the isotherms (lines of equal temperature drawn on this map) toward a warmer region in the Ohio Valley. On the other hand, the red arrow (B) in the Plains states represents warm air advection as the wind transports warm air from the southern Plains northward to colder regions near western Lake Superior. SPECIAL TOPIC (The Unique Thermal Properties of Water) on page 94 -- Read through this topic, noting that water (H2O) is a special substance because of its somewhat unique chemical and physical properties. These unusual properties, as compared with other chemical compounds of similar molecular size, are the result of the dipolar structure of the water molecule. Consider that without these particular properties our planet would probably be uninhabitable. SPECIAL TOPICS (Solar Power) on pages 100-102 -- Skim this Special Topic, noting that this example of a renewable resource requires a region where bright sunshine is abundant throughout much of the year. Read Weather Fact (Why Mountaintops are cold) on pg. 106. CHAPTER 4 (Moran and Morgan, 1997) HEAT BALANCES AND WEATHER In this chapter, we are concerned with differences in rates of radiational heating and radiational cooling within the Earth-atmosphere system. Absorption of solar radiation causes warming whereas emission of infrared radiation causes cooling. Imbalances in rates of radiational heating and cooling occur between the Earth's surface and the troposphere and between the tropics and higher latitudes. In response, the atmosphere circulates so that heat is redistributed from locations of net radiational heating to locations of net radiational cooling. Circulation of the atmosphere involves convection currents, movement of air masses from place to place, and storm systems. With this as a background, it becomes evident that air temperature is governed by local radiational conditions plus air mass advection. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the Imbalances in radiational heating and cooling within the Earth-atmosphere system. distinguish between sensible heating and latent heating. explain why latent heating is more important than sensible heating on a global scale. identify the significance of the Bowen ratio. describe the various processes involved in poleward heat transport within the Earth-atmosphere system. list the principal energy conversion processes operating within the Earth-atmosphere system. distinguish between radiational controls and air mass controls of air temperature. explain how properties of the Earth's surface influence air temperature. 4 Heat Imbalances and Weather 90 Heat Imbalance: Atmosphere Versus Earth's Surface 92 Heat Imbalance: Variation by Latitude 97 Weather: Response to Heat Imbalances 98 Variation of Air Temperature 102 Conclusions 106 Special Topic: The Unique Thermal Properties of Water 94 Special Topic: Solar Power 100 Weather Fact: Why Mountain tops Are Cold 106 Key Terms 106 Summary Statements 106 Review Questions 107 Quantitative Questions 107 Questions for Critical Thinking 107 Selected Readings 107 3 4