FRONTS (Chapter 11, pp. 306–319) Figure 11.12 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.) (Chapter 11, pp. 306–319)

FIGURE 11.14 A surface 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 rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.) Fronts occur in the boundary zone between polar and tropical air masses.

World War I Aerial Photograph “No man’s land” trenches trenches

Ben Franklin’s Storm (Nov. 2, 1743) 500 mb L Boston L Philadelphia

Norwegian Cyclone Model polar/tropical air mass boundary waves form along fluid density boundaries cold front moves faster than warm front Page: 323 FIGURE 12.1 The idealized life cycle of a mid-latitude cyclone (a through f) in the Northern Hemisphere based on the polar front theory. As the life cycle progresses, the system moves northeastward in a dynamic fashion. The small arrow next to each L shows the direction of storm movement. (pp. 322–324) cold front catches warm front warm air is pinched upward storm dies

Typical Cold Front Convective precipitation 50–100 mile wide band along front Page: 308 FIGURE 11.16 A closer look at the surface weather associated with the cold front situated in the southern United States in Fig. 11.15. (Solid gray lines are isobars. Green-shaded area represents rain; white-shaded area represents snow.)

forced upward along and ahead of the surface front. Cold Front Cross-Section Page: 309 FIGURE 11.18 A vertical view of a model representing the weather across the cold front in Fig. 11.16 along the line X–X´. Cold fronts are rapidly advancing cold (dense) air wedges. Warm (often moist) air is forced upward along and ahead of the surface front.

Typical Warm Front “Overrunning” Clouds and precipitation precede surface front Stratiform clouds starting about 750 miles from front Stratiform precipitation starting about 350 miles from front “Overrunning” Page: 312 FIGURE 11.21 Surface weather associated with a typical warm front in winter. A vertical view along the dashed line P-P´ is shown in Fig. 11.22. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.)

Warm Front Cross-Section Figure 11.19 Vertical view of clouds, precipitation, and winds across the warm front in Fig. 11.18 along the line P–P’. Warm fronts are receding cold (dense) air wedges. Warm air gradually fills in the space vacated by the cold air.

Warm Front Cross-Section WARM AIR COLD AIR Figure 11.19 Vertical view of clouds, precipitation, and winds across the warm front in Fig. 11.18 along the line P–P’.

Page: 312 FIGURE 11.22 Vertical view of a model illustrating clouds, precipitation, and winds across the warm front in Fig. 11.21 along the line P–P’.

Occluded Front FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

OCCLUDED FRONTS Heavy precipitation along front FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.) OCCLUDED FRONTS Heavy precipitation along front Convective or stratiform Short duration

FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

Occluded Front Cross-Section (“Cold Occlusion”) FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

“Warm Occlusion” FIGURE 11. 23 (at left) 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.

Figure 11.21 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.

Occluded Front Cross-Section (“Warm Occlusion”) Figure 11.21 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.

“Warm Occlusion” FIGURE 11. 23 (at left) 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.

Cold Occlusion Warm Occlusion coldest air is behind coldest air cold front coldest air is ahead of warm front FIGURE 11. 22 The formation of a cold-occluded front. The faster-moving cold front (a) catches up to the slower-moving warm front (b) and forces it to rise off the ground (c). (Green-shaded area in (d) represents precipitation.)

(QUASI-) STATIONARY FRONT Partly cloudy or cloudy with widespread stratiform precipitation

Cyclone Family Cold Air Warm Air (pp. 306–319)

Quick Summary—Fronts (Chapter 11): Fronts are boundary zones between polar and tropical air masses. Fronts are stable—the colder, denser air is close to the surface and warmer, less dense air is above it. Cold front weather: cumuliform clouds, showery, convective precipitation along the front. Warm front weather: stratiform clouds and precipitation in the cold air well ahead of the front. Occluded front weather: very heavy precipitation of short duration in the vicinity of the front.