Air stability About clouds Precipitation A mass of moist, stable air gliding up and over these mountains condenses into lenticular clouds. Fig. 5-CO, p.110 air in unstable equilibrium will move--up/down Fig. 5-1, p.112 Adiabatic = w/ no exchange of heat from outside! Dry adiabatic rate = no heat added from condensation as air rises. moist adiabatic rate = less because latent heat added during condensation of rising air The dry adiabatic rate. As long as the air parcel remains unsaturated, it expands and cools by 10 C per 1000 m; the sinking parcel compresses and warms by 10 C per 1000 m. Fig. 5-2, p.113 FIGURE 5.3 A stable atmosphere. An absolutely stable atmosphere exists when a rising air parcel is colder and heavier (i.e., more dense) than the air surrounding it. If given the chance (i.e., released), the air parcel in both situations would return to its original position, the surface. Fig. 5-3, p.113 Inversion acts as lid Absolutely unstable air! = if rising air is warmer and less dense than air around it, once parcels start upward, they will continue! Air becomes more unstable as environmental lapse rate steepens (air is colder w/ ht) Generally, as surface air warms during the day, the air becomes more and more unstable FIGURE 5.4 Cold surface air, on this morning, produces a stable atmosphere that inhibits vertical air motions and allows the fog and haze to linger close to the ground. Fig. 5-4, p.114 Fig. 5-5, p.115
FIGURE 5.6 Unstable air. The warmth from the forest fire heats the air, causing instability near the surface. Warm, less-dense air (and smoke)bubbles upward, expanding and cooling as it rises. Eventually the rising air cools to its dew point, condensation begins, and a cumulus cloud forms. Condensation level = level above sfc where cloud first forms Fig. 5-6, p.115 IOW: if unsaturated, unstable air is somehow lifted to a level where it becomes saturated, instability may result FIGURE 5.7Conditionally unstable air. The atmosphere is conditionally unstable when unsaturated, stable air is lifted to a level where it becomes saturated and warmer than the air surrounding it. If the atmosphere remains unstable, vertical developing cumulus clouds can build to great heights. Fig. 5-7, p.116 surface heating and convection FIGURE 5.8 The primary ways clouds form: (a) surface heating and convection; (b) forced lifting along topographic barriers;(c) convergence of surface air; (d) forced lifting along weather fronts. Fig. 5-8, p.118 Fig. 5-8a, p.118 forced lifting along topographic barriers Fig. 5-8b, p.118 Fig. 5-8c, p.118
Cumulus clouds form as hot, invisible air bubbles detach themselves from the surface, then rise and cool to the condensation level. Below and within the cumulus clouds, the air is rising. Around the cloud, the air is sinking. Fig. 5-8d, p.118 Fig. 5-9, p.118 Cumulus clouds building on a warm summer afternoon. Each cloud represents a region where thermals are rising from the surface. The clear areas between the clouds are regions where the air is sinking. Cumulus clouds developing into thunderstorms in a conditionally unstable atmosphere over the Great Plains. Notice that, in the distance, the cumulonimbus with the anvil top has reached the stable part of the atmosphere. <= tropopause Fig. 5-10, p.119 Fig. 5-11, p.119 Orographic uplift, cloud development, and the formation of a rain shadow. Fig. 5-12, p.120 The formation of lenticular clouds. Fig. 5-13, p.120
Relative sizes of raindrops, cloud droplets, and condensation nuclei. Lenticular clouds (mountain wave clouds) forming over Mt. Rainier, Washington. Fig. 5-14, p.121 Fig. 5-15, p.121 Collision and coalescence. (a) In a warm cloud composed only of small cloud droplets of uniform size, the droplets are less likely to collide as they all fall very slowly at about the same speed. Those droplets that do collide, frequently do not coalesce because of the strong surface tension that holds together each tiny droplet. Fig. 5-16, p.122 Fig. 5-16a, p.122 Collision and coalescence. (b) In a cloud composed of different size droplets, larger droplets fall faster than smaller droplets. Although some tiny droplets are swept aside, some collect on the larger droplet s forward edge, while others (captured in the wake of the larger droplet) coalesce on the droplet s backside. Fig. 5-16b, p.122 A cloud droplet rising then falling through a warm cumulus cloud can grow by collision and coalescence and emerge from the cloud as a large raindrop. Fig. 5-17, p.123
In a saturated environment, the water droplet and the ice crystal are in equilibrium, as the number of molecules leaving the surface of each droplet and ice crystal equals the number returning. The greater number of vapor molecules above the liquid indicates, however, that the The distribution of ice and water in a cumulonimbus cloud.. Fig. 5-18, p.123 Fig. 5-19, p.124 saturation vapor pressure over water is greater than it is over ice. Ice particles in clouds. The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water Fig. 5-20, p.125 Fig. 5-21, p.125 vapor and grow larger, while the water droplets grow smaller. Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain. Fig. 5-22, p.127 The streaks of falling precipitation that evaporate before reaching the ground are called virga. Fig. 5-23, p.128
Which of the three drops drawn here represents the real shape of a falling raindrop? p.129 The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks. The bending of the streaks is due to the changing wind speed with height. Fig. 5-24, p.129 Table 5-1, p.130 Computer color enhancedimage of dendrite snowflakes. Fig. 5-25, p.130 Sleet forms when a partially melted snowflake or a cold raindrop freezes into a pellet of ice before reaching the ground. Fig. 5-26, p.131 An accumulation of rime forms on tree branches as supercooled fog droplets freeze on contact in the below-freezing air. Fig. 5-27, p.131
A heavy coating of freezing rain during this ice storm caused tree limbs The accumulation of small hail after a thunderstorm. The hail formed as super cooled cloud droplets collected on ice particles called graupel inside a cumulonimbus cloud. to break and power lines to sag. Fig. 5-28, p.131 Fig. 5-29, p.133 This giant hailstone the largest ever reported in the United States with a diameter of 17.8 cm (7 in.) fell on Aurora, Nebraska, during June, Fig. 5-30, p.133 2003. 5.31Hailstones begin as embryos (usually ice particles) that remain suspended in the cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact. The ice particles eventually grow large enough and heavy enough to fall Fig. 5-31, p.133 toward the ground as hailstones. Components of the standard rain gauge. Fig. 5-32, p.134 The tipping bucket rain gauge. Each time the bucket fills with onehundredth of an inch of rain, it tips, sending an electric signal to the remote recorder. Fig. 5-33, p.135
(a) Doppler radar display showing precipitation intensity over Oklahoma for April 24, 1999. The numbers under the letters DBZ represent the logarithmic scale for measuring the size and volume of precipitation particles. Fig. 5-34a, p.136 (b) Doppler radar display showing1-hour rainfall amounts over Oklahoma for April 24, 1999. Fig. 5-34b, p.136