A middle-latitude cyclonic storm spins counterclockwise over the eastern Atlantic. Fig. 8-CO, p.200

Table 8-1, p.203

FIGURE 8.2 Air mass source regions and their paths. Fig. 8-2, p.203

FIGURE 8.1 Here, a large, extremely cold winter air mass is dominating the weather over much of the United States. At almost all cities, the air is cold and dry. Upper number is air temperature(°F); bottom number is dew point (°F). Fig. 8-1, p.202

FIGURE 8.3 Average upper-level wind flow (heavy arrows) and surface position of anticyclones (H) associated with two extremely cold outbreaks of arctic air during December. Numbers on the map represent minimum temperatures (°F)measured during each cold snap. Fig. 8-3, p.205

FIGURE 8.4 Visible satellite image showing the modification of cP air as it moves over the warmer Gulf of Mexico and the Atlantic Ocean. Fig. 8-4, p.206

z T p.204a IR out to space = cooling from above The formation of lake-effect snows. Cold, dry air crossing the lake gains moisture and warmth from the water. The more buoyant air now rises, forming clouds that deposit large quantities of snow on the lake’s leeward shores. General “recipe” for destabilizing any air: warm from below and/or cool from above Moist adiabat z T stable unstable Environ. lapse rate p.204a

Areas shaded purple show regions that experience heavy lake-effect snows. p.204b

Surface weather map for 7 A. M. , EST, December 24, 1983 Surface weather map for 7 A.M., EST, December 24, 1983. Solid lines are isobars. Areas shaded green represent precipitation. An extremely cold arctic air mass covers nearly 90 percent of the United States. (Weather symbols for the surface map are given in Appendix B.) p.207

FIGURE 8.5 A winter upper-air pattern that brings mP air into the west coast of North America. The large arrow represents the upper-level flow. Note the trough of low pressure along the coast. The small arrows show the trajectory of the mP air at the surface. Regions that normally experience precipitation under these conditions are also shown on the map. Showers are most prevalent along the coastal mountains and in the Sierra Nevada. Fig. 8-5, p.208

“Herman’s Rule”: Rain in TUS requires colder than -21C at 500 mb over San Diego FIGURE 8.6 After crossing several mountain ranges, cool moist mP air from off the Pacific ocean descends the eastern side of the Rockies as modified, relatively dry Pacific air. Moist adiabat 500 z (mb) stable unstable -21C 15 C T Fig. 8-6, p.208 Fixed by Pacific ocean

FIGURE 8.7 Winter and early spring surface weather patterns that usually prevail during the invasion of cold, moist mP air into the mid-Atlantic and New England states. (Green-shaded area represents light rain and drizzle; pink-shaded region represents freezing rain and sleet; white-shaded area is experiencing snow.) Fig. 8-7, p.209

FIGURE 8.8 An infrared satellite image that shows maritime tropical air (heavy red arrow) moving into northern California on January 1, 1997. The warm, humid airflow (sometimes called “the pineapple connection”) produced heavy rain and extensive flooding in northern and central California. Fig. 8-8, p.209

FIGURE 8.9 Weather conditions during an unseasonably hot spell in the eastern portion of the United States that occurred between the 15th and20th of April, 1976. The surface low-pressure area and fronts are shown for April 17. Numbers to the east of the surface low (in red) are maximum temperatures recorded during the hot spell, while those to the west of the low (in blue) are minimums reached during the same time period. The heavy arrow is the average upper level flow during the period. The faint L and H show average positions of the upper-level trough and ridge. Fig. 8-9, p.210

FIGURE 8.10 During June 29 and 30, 1990,continental tropical air covered a large area of the central and western United States. Numbers on the map represent maximum temperatures (°F) during this period. The large H with the isobar shows the upper-level position of the subtropical high. Sinking air associated with the high contributed to the hot weather. Winds aloft were weak, with the main flow shown by the heavy arrow. Fig. 8-10, p.211

FIGURE 8.11 A weather map showing surface-pressure systems, air masses, fronts, and isobars (in millibars) as solid gray lines. Large arrows in color show air flow. (Green-shaded area represents precipitation.) Fig. 8-11, p.213

FIGURE 8.12 A closer look at the surface weather associated with the cold front situated in the southeastern United States in Fig. 8.11. (Gray lines are isobars. Dark green-shaded area represents rain; white-shaded area represents snow.) Fig. 8-12, p.213

FIGURE 8.13 A vertical view of the weather across the cold front in Fig. 8.12along the line X–X’. Fig. 8-13, p.214

FIGURE 8.14 A “back door” cold front moving into New England during the spring. Notice that, behind the front, the weather is cold and damp with drizzle, while to the south, ahead of the front, the weather is partly cloudy and warm. Fig. 8-14, p.215

Table 8-2, p.215

FIGURE 8. 15 Surface weather associated with a typical warm front FIGURE 8.15 Surface weather associated with a typical warm front. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.) Fig. 8-15, p.216

FIGURE 8.16 Vertical view of clouds, precipitation, and winds across the warm front in Fig. 8.15 along the line P–P’. Fig. 8-16, p.216

Table 8-3, p.217

FIGURE 8. 17 The formation of a cold occluded front FIGURE 8.17 The formation of a cold occluded front. The faster moving cold front in (a) catches up to the slower-moving warm front in (b) and forces it to rise off the ground (c). (Green-shaded area represents precipitation.) Fig. 8-17, p.218

FIGURE 8. 17 The formation of a cold occluded front FIGURE 8.17 The formation of a cold occluded front. The faster moving cold front in (a) catches up to the slower-moving warm front in (b)and forces it to rise off the ground (c). (Green-shaded area represents precipitation.) Fig. 8-17d, p.218

FIGURE 8. 18 The formation of a warm-type occluded front FIGURE 8.18 The formation of a warm-type occluded front. The faster-moving cold front in (a) overtakes the slower-moving warm front in (b).The lighter air behind the cold front rises up and over the denser air ahead of the warm front. Diagram (c) shows a surface map of the situation. Fig. 8-18, p.219

FIGURE 8. 18 The formation of a warm-type occluded front FIGURE 8.18 The formation of a warm-type occluded front. The faster-moving cold front in (a) overtakes the slower-moving warm front in (b).The lighter air behind the cold front rises up and over the denser air ahead of the warm front. Diagram (c) shows a surface map of the situation. Fig. 8-18c, p.219

Table 8-4, p.219

Fig. 8-19, p.220 Cyclonic flow (counter clockwise in NH) Low pressure trough FIGURE 8.19 The idealized life cycle of a wave cyclone (a through f) in the Northern Hemisphere based on the polar front theory. As the life cycle progresses, the system moves eastward in a dynamic fashion. The small arrow next to each L shows the direction of storm movement. Fig. 8-19, p.220

FIGURE 8.20 A series of wave cyclones (a “family” of cyclones) forming along the polar front. Fig. 8-20, p.221

FIGURE 8. 21 (a) Typical paths of winter mid-latitude cyclones FIGURE 8.21 (a) Typical paths of winter mid-latitude cyclones. The lows are named after the region where they form. (b) Typical paths of winter anticyclones. Fig. 8-21, p.222

The surface weather map for 7:00 A. M The surface weather map for 7:00 A.M. (EST) December 11, 1992, shows an intense low pressure area (central pressure 988 mb, or 29.18 in.), which is generating strong northeasterly winds and heavy precipitation (area shaded green) from the mid-Atlantic states into New England. This northeaster devastated a wide area of the eastern seaboard, causing damage in the hundreds of millions of dollars. p.223

FIGURE 8.22 If lows and highs aloft were always directly above lows and highs at the surface, the surface systems would quickly dissipate. Fig. 8-22, p.224

FIGURE 8.23 Convergence, divergence, and vertical motions associated with surface pressure systems. Notice that for the surface storm to intensify, the upper trough of low pressure must be located to the left (or west) of the surface low. Fig. 8-23, p.224

The formation of convergence (CON) and divergence (DIV) of air with a constant wind speed (indicated by flags) in the upper troposphere. Circles represent air parcels that are moving parallel to the contour lines on a constant pressure chart. Below the area of convergence the air is sinking, and we find the surface high (H). Below the area of divergence the air is rising, and we find the surface low (L). p.225

(a) Upper-air chart showing a long wave with three short waves (heavy dashed lines) embedded in the flow.(b) Twenty-four hours later the short waves have moved rapidly around the long wave. Notice that the short waves labeled 1 and 3 tend to deepen the long wave trough, while shortwave 2 has weakened as it moves into a ridge. p.226

FIGURE 8.24 (a) As the polar jet stream and its area of maximum winds (the jet streak, or MAX), swings over a developing mid latitude cyclone, an area of divergence (D) draws warm surface air upward, and an area of convergence (C) allows cold air to sink. The jet stream removes air above the surface storm, which causes surface pressures to drop and the storm to intensify. (b) When the surface storm moves northeastward and occludes, it no longer has the upper-level support of diverging air, and the surface storm gradually dies out. Fig. 8-24, p.227

FIGURE 8.25 Summary of clouds, weather, vertical motions, and upper-air support associated with a developing mid-latitude cyclone. Fig. 8-25, p.227