CHAPTER 8 (Moran and Morgan, 1997) Precipitation represents liquid or solid water that is falling. It is one of the weather elements and it is important to many interests. Few cloud types produce significant precipitation because the liquid droplets or ice crystals in most clouds are often too small to fall through the updraft and down to ground. Several theories have been devised to explain precipitation formation. Precipitation reaches the ground in several forms depending upon the temperature structure of the atmosphere between cloud base and the earth's surface. These precipitation types include liquid (rain and drizzle), frozen (snow and ice pellets) and freezing (freezing rain and freezing drizzle). Precipitation is measured either by rain gauges or estimated by Doppler radar units. Precipitation stimulation and enhancement are used as an intentional weather modification project. Atmospheric optics is another means of providing visual cues as to atmospheric processes. STUDY NOTES CHAPTER 8 Figure 8.1 -- Take a moment to familiarize yourself with this diagram. The scale on the bottom of the diagram portrays the approximate size of various objects in the atmosphere, while the corresponding scale on the top of the diagram identifies the terminal velocity of these objects. Note, the scales are not linear (a one to one correspondence between intervals on the graph and the size or speed intervals), but are drawn in manner (called a logarithmic scale) so as to cover nine orders of magnitude in size. The major point to be drawn from this diagram is that the larger sized hailstones, raindrops and drizzle have a higher terminal velocity than the small sized cloud droplets that remain suspended in the cloud-forming updrafts. Figure 8.2 -- Study the schematic used to describe the collision-coalescence process. With the aid of the legend and the text, visualize this precipitation formation process. Figure 8.3 -- Study the schematic used to describe the ice crystal process. The ice crystal grows at the expense of the supercooled liquid droplet because of the difference in saturation vapor pressure between the droplet and the crystal as noted in the text. Compare this process with that of the collision-coalescence process shown in Figure 8.2. Figure 8.4 -- Look at this photograph that contains virga. At times this feature may be mistaken for a tornado or funnel cloud. If you do not see rotation in such a feature, then it is probably virga and not a tornado. Figure 8.6 -- Spend a moment inspecting the four varieties of snowflake forms. You should realize that these forms depend primarily upon a given temperature range at which the snowflake grows within the cloud. Figure 8.7 -- Note the size and the structure of hailstones. Figure 8.8 and Figure 8.9 -- Take a moment to note the variety of rain gauges used by weather observers. The specifics as to their operation are provided in the text. Figure 8.11 -- Inspect the schematic that shows the effect of refraction. Study the description of this process appearing in the figure legend. Figures 8.12 and 8.14 show the path of sunlight through an ice crystal and how refraction of the light produces a 22-degree or 46 degree halo around either the sun or the moon. Note that in the photograph in Figure 8.13, a 22-degree halo appears as a bright circle that is 22 degrees away from the sun. While the description of this 22-degree angle in the accompanying footnote is correct, you can estimate the described angle by extending your arm to full length and spreading your fingers. The angle between an imaginary line extending from your eye to your thumb and a similar line from your eye to your small finger when held in this outstretched position would approximately equal 22 degrees. You could use your thumb to shield the sun from your eyes and allow your small outstretched finger to reach for the halo. NOTE: Never look directly at the sun without using approved safety filters. Have you ever seen this optical phenomenon? Figure 8.15 -- Identify the location of the sun dogs in this picture. They appear as faintly illuminated patches near the edges of the picture, at the same vertical distance of the sun above the horizon. Such an optical phenomenon can be used to identify ice crystals and cirrus-types of clouds. Have you ever seen this optical phenomenon? Figure 8.16 -- In this figure of a primary rainbow, trace the ray from the sun to the raindrop and then back to the observer. Note that the path of the sun's rays passing into the raindrop is bent by refraction, then reflected from the back of the drop with one final ray bending upon leaving the droplet. Because the refraction of light passing into water depends upon wavelength, the white light is also dispersed into its component colors (not shown in this diagram). Figure 8.17 -- Inspect this schematic of the overall geometry for observing a primary rainbow. You should trace the ray from the sun to the raindrops and back to the viewer. You should also note the color sequence of the primary bow, with blue toward the center of the bow and red on the outside. Together with this diagram and the description in the text, you should try to locate a rainbow when the conditions are appropriate, namely, sunlight shining upon a veil of falling rain. Figure 8.18 -- Note that in this picture, the primary bow is the most brilliant in color and that the color sequence has blue on the bottom of the bow. A fainter secondary bow appears above the tree on the left-hand side of page. This secondary bow has a reverse color sequence compared with the primary bow. Figure 8.19 -- Note that in the schematic showing the mechanism for a double rainbow, a solar ray experiences two internal reflections within the raindrop before being directed toward the observer. Figure 8.20 and 8.21 -- The photograph in Figure 8.20 shows how a glory would appear around the airplane's shadow to someone riding in the airplane. In Figure 8.21, note the optical processes involved with the production of the glory. Read the Weather Fact (Identical Snowflakes?) on Page 183. Read the Special Topic (When is it Too Cold or Too Warm to Snow) on page 185 -- This topic answers a basic question and dispels some commonly held beliefs. Read the Special Topic (Mirages) on pages 194 and 195 -- While the details may be somewhat involved, the main point to remember is that the rapid changes in temperature with height above the earth's surface produce air density differences that bend light rays. As a result of this refraction, several types of mirages can be produced, depending upon whether the ground is colder or warmer than the overlying air. You are encouraged to look for such phenomena. CHAPTER 8 (Moran and Morgan, 1997) PRECIPITATION, WEATHER MODIFICATION, AND ATMOSPHERIC OPTICS We have examined the various saturation and cloud forming processes operating within the atmosphere. Our treatment of water in the atmosphere concludes with a discussion of precipitation. A major point here is that most clouds do not yield precipitation because a special set of circumstances is required in order for cloud droplets or ice crystals to grow large enough to precipitate. After describing the collision-coalescence and Bergeron processes, we distinguish among the various forms of precipitation and discuss how precipitation is measured. Intentional weather modification is an attempt to stimulate natural precipitation processes. In our discussion of rainmaking, a special emphasis is on the problem of verification of precipitation enhancement. This chapter closes with a section on atmospheric optical phenomena most of which are caused by sunlight interacting with clouds or precipitation. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: explain the significance of terminal velocity in the formation of precipitation. distinguish between warm clouds and cold clouds in both composition and appearance. describe the collision-coalescence process. describe the Bergeron process. distinguish among the various solid and liquid forms of precipitation. describe how rainfall and snowfall are measured. explain the objectives of cloud seeding. comment on the effectiveness of precipitation enhancement efforts. explain how halos and rainbows form. describe the optics of coronae and glories. 8 Precipitation, Weather Modification, and Atmospheric Optics 178 Precipitation Processes 179 Forms of Precipitation 182 Precipitation Measurement 186 Weather Modification 187 Atmospheric Optics 190 Conclusions 197 Special Topic: When Is It Too Cold or Too Warm to Snow? 185 Weather Fact: Identical Snowflakes? 188 Special Topic: Mirages 194 Key Terms 197 Summary Statements 197 Review Questions 198 Quantitative Questions 198 Questions for Critical Thinking 199 Selected Readings 198 3 2