Chapter 9Learning ObjectivesChapter OutlineAnswers to the Chapter ReviewChapter 9WEATHER PATTERNSLearning ObjectivesAfter reading, studying, and discussing this chapter, students should be able to do the following:1. Recognize midlatitude cyclones as the primary weather producers in the middle latitudes and describe their size, movement, duration, circulation, and fronts commonly associated with them.2. Compare and contrast warm fronts and cold fronts in terms of their structure and associated weather.3. Describe stationary fronts, occluded fronts, and drylines.4. Distinguish between cold-type and warm-type occluded fronts.5. Outline and explain the stages in the life cycle of a wave cyclone as described by the Polar Front Model.6. Describe the changes in wind direction, pressure tendency, cloud type and coverage, precipitation, and temperature when either a warm, a cold, or an occluded front passes.7. Relate the development and movement of cyclones to upper-level flow including the roles of speed divergence, directional divergence, and vorticity.8. List the areas where most cyclones that influence North America form and what types of air masses are associated with each.9. Describe what blocking highs are, when and where they tend to form, and the effects that they have on weather.10. Comment on the intensity of cyclonic storms during the spring and explain why they can be so strong.11. Define the terms listed in the vocabulary review.58Chapter Outline59I. Polar-Front Theory (Norwegian Cyclone Model)A. Midlatitude cycloneII. FrontsA. WarmB. ColdC. StationaryD. Occluded1. Cold type2. Warm typeE. DrylinesIII. Life Cycle of a Midlatitude CycloneA. Formation1.Clash of Air masses2. CyclogenesisB. Development of Cyclonic FlowC. Occlusion1.Beginning of the endD. Idealized Weather of a Midlatitude Cyclone1.Cyclone Formation2.Cyclonicand anticycloniccirculation3.D60ivergence and convergence alofta. Speeddivergenceb. Directionaldivergence4.Traveling cyclonesa.Patternsofmovementb.Airflow aloftand cyclone migration5. Anticyclonicweather an61d blocking highsIV. Case Study of a Midlatitude CycloneA. Violent spring weatherB. Weather in Peoria62Answers to the Chapter Review1. Early Norwegian meteorologists visualized boundary surfaces separating air masses of different densities as analogous to battle lines and called them fronts, like battlefronts. 2. The frontal surface would be about 2 km above the ground.3. The weather associated with warm fronts is generally much more mild than that associated with cold fronts. Warm fronts usually produce light to moderate precipitation over a large area and for an extended period. After a warm front passes, temperatures gradually rise. Cold front weather is usually more violent than warm front weather with more intense precipitation over a smaller area. Cold fronts often produce severe weather including thunderstorms and tornadoes. A marked temperaturedrop and wind shift also usually accompanies the passage of cold fronts. 4. Because cold fronts are steeper and move more rapidly than warm fronts, the vertical displacement of air is often rapid enough such that latent heat released by condensing water vapor adds appreciably to the air’s buoyancy. The common result is cumulonimbus clouds with their characteristic heavy showers and occasionally severe weather.5. The first line of the proverb pertains to warm fronts. It refers to the fact that these fronts are often preceded by a succession of clouds that foretell of the coming nimbostratus clouds that usually produce light to moderate precipitation of relatively long duration. The second line refers to a cold front. Since cold fronts usually do not have a series of clouds preceding them, there is little or no forewarning of the cumulonimbus clouds and showery precipitation that often accompany a cold front. 63Precipitation associated with cold fronts is “soon past” because cold fronts usually move relatively quickly.6. Gentle to moderate precipitation is likely along stationary fronts because overrunningusually occurs along them.7. A cold-type occluded front forms when the air behind a cold front is colder than the air underlying the warm front that it is overtaking. When the air behind the cold front is warmer than the air underlying the warm front, a warm-type occluded front is said toexist. The weather associated with cold-type occluded fronts often resembles the weather of a cold front, whereas conditions resembling warm front weather more commonly accompany warm-type occluded fronts.8. Cyclones form along fronts. Cyclonic formation (cyclogenesis) occurs when two air masses of different densities (temperatures) are moving roughly parallel to the front, but in opposite directions. 9. If conditions are suitable for cyclogenesis, the frontal surface that separates two contrasting air masses will take on a wave shape that is usually several kilometers long. Midlatitude cyclones that intensify develop waves that change in shape over time, much like a gentle ocean swell does as it moves into shallow water and becomesa tall breaking wave.10. During occlusion, the warm sector is forced aloft. When this process is complete, the cyclone has lost its primary source of energy, the sinking of cold air and the risingof warm. Hence, the storm comes to an end. 11. When a midlatitude cyclone passes to the north of an observer, a warm front passes first followed by a cold front. Different types of weather are associated with the passage of each front type.64a. As the warm front approaches, winds generally blow from the east or southeast, and as the warm front passes, winds shift to the south or southwest. When the cold front passes, a wind shift from the southwest to the northwest occurs.b. Pressure drops steadily until the warm front passes. Then, pressure tendency steadies. Pressure rises again after the cold front passes.c. As a warm front nears, cirrus clouds would be sighted first, followed by cirrostratus, altostratus, and finally nimbostratus. Skies clear after the warm front passes. Once the cold front arrives, cumulonimbus clouds usually appear. Skies again clear after the cold front passes. d. As a warm front moves in, cloud cover would get progressively greater, from a fewtenths coverage with cirrus to completely overcast with the coming of the nimbostratus. Skies clear after the front passes. Later, with the approach of the cold front, cumulonimbus clouds fill