CHAPTER 7 (Moran and Morgan, 1997) Meteorologists routinely report the cloud amounts and cloud types since clouds are sign-posts in the atmosphere. Specifically, cloud observation can be used to determine how close the atmosphere is to saturation with respect to water vapor; the stability of the atmosphere to vertical air movement; the air temperature; and the wind speed and direction at various atmospheric levels. Condensation or deposition of water vapor onto airborne particles forms clouds. Clouds are formed usually by lifting an air parcel to saturation. Fog results from low-level saturation. Other low-level saturation processes include dew and frost formation. To assist the meteorologist in describing clouds, an international cloud classification scheme has been devised to categorize clouds according to appearance and altitude. STUDY NOTES CHAPTER 7 Take a look at Figures 7.1 through 7.3, noting the delicate forms of condensation and deposition products that form upon objects near the earth's surface. Note the dew (in Figure 7.1) that forms by condensation and how it differs from the types of hoarfrost (in Figures 7.2 and 7.3) that are the result of deposition. Compare the fog types appearing in Figures 7.4, 7.5, 7.7 and 7.8, making note of what physical processes were responsible for the fog formation. Consider the schematic in Figure 7.6. Follow how the warm, humid air crossing a cold water surface could create a fog layer, then once the air moves across a warmer land surface, the fog dissipates. Figure 7.9 -- Look at this diagram that shows the temperature range where a variety of deposition nuclei become active. You should note that many apparently common materials could serve as deposition nuclei. In addition, many of these substances do not become active until temperatures fall to between -5 degrees Celsius and -25 degrees Celsius. Memorization of the exact activation temperature values for each substance is not necessary. You should look at the various cloud pictures (Figures 7.10 through 7.23) in the text and attempt to visualize those basic features that would permit classification of the featured cloud in the way identified. In other words, in Figure 7.10, the thin wispy cloud feature would make it a cirrus cloud. You should also attempt to compare these photographs with clouds that you have seen yourself. Figure 7.24 -- A schematic that depicts how a wave cloud would form downwind of a mountain barrier (appearing to the right-hand side of the diagram). Table 7.1 -- Cloud classification scheme. You should study this table, so that you can identify the ten fundamental cloud types in terms of their overall shape and appearance as well as their relative height (e.g., high, low or vertically developed). Table 7.2 -- This tabulation of the names of some of the cloud species is provided primarily for reference purposes. Note a cloud species is a subdivision of a cloud genus based upon a particular shape or appearance. Table 7.3 -- Inspect this summary table, noting the cloud composition and the relative height of the cloud. Skim Special Topic (Freeze Prevention) on page 156 -- Note the various strategies used by farmers and gardeners when frost or freeze conditions can damage crops. If you have a garden and live in a region where frost could be a problem, you should have an appreciation for what techniques are involved to save tender vegetation or crops. Skim Special Topic (Jack Frost and Autumn Color) on pages 162 and 163. Note the factors that contribute to the annual march of fall colors from the northern states in mid September to the southern Appalachians and southern Rockies by late November. Read the Weather Fact (Foggiest Places in the United States and Canada) on page 163 for general background information. CHAPTER 7 (Moran and Morgan, 1997) DEW, FROST, FOG AND CLOUDS In this second of three chapters on moisture in the atmosphere, we consider low-level saturation processes, cloud formation, and cloud classification. Low-level saturation is brought about by radiational or advective cooling of a humid air mass or when vaporization of water at the Earth's surface increases the mixing ratio of overlying air. The products of low-level saturation include dew, frost, and fog. Fog is a cloud in contact with the Earth's surface and forms as a consequence of radiational cooling, advective cooling, vaporization of water, or ascent of humid air along a mountain slope. Clouds develop as a consequence of condensation or deposition of water vapor on nuclei, tiny solid and liquid particles that are suspended in the atmosphere. Clouds are classified by appearance and by altitude (high, middle, low, and vertical). CHAPTER OBJECTIVES After reading this chapter, the student should be able to: explain how dew and frost form. distinguish between the dewpoint and the wet-bulb temperature. identify the various fog-forming processes. list the conditions required for extreme nocturnal radiational cooling. explain the role of nuclei in cloud development. list the sources and types of cloud nuclei. explain the significance of hygroscopic nuclei. describe how clouds are classified. distinguish among the various cloud types. 7 Dew, Frost, Fog, and Clouds 152 Low-Level Saturation Processes 153 Cloud Development 161 Classification of Clouds 164 Unusual Clouds 172 Conclusions 175 Special Topic: Freeze Prevention 156 Special Topic: Jack Frost and Autumn Color 162 Weather Fact: Foggiest Places in the United States and Canada 163 Key Terms 175 Summary Statements 176 Review Questions 176 Quantitative Questions 176 Questions for Critical Thinking 177 Selected Readings 177