• The primary weather producer in the middle latitudes (for our purposes, the region between southern Florida and Alaska, essentially the area of the westerlies) is the middle-latitude or midlatitude cyclone. Middle-latitude cyclones are large low pressure systems with diameters often exceeding 1000 kilometers (600 miles) that generally travel from west to east. They last a few days to more than a week, have a counterclockwise circulation pattern with a flow inward toward their centers, and have a cold front and frequently a warm front extending from the central area of low pressure. In the polar front theory, (also called the Norwegian cyclone model), wave cyclones develop in conjunction with the polar front.

  

• Fronts are boundary surfaces that separate air masses of different densities, one usually warmer and more moist than the other. As one air mass moves into another, the warmer, less dense air mass is forced aloft in a process referred to as overrunning. The five types of fronts are (1) warm front, which occurs when the surface (ground) position of a front moves so that warm air occupies territory formerly covered by cooler air, (2) cold front, where cold continental polar air actively advances into a region occupied by warm air, (3) stationary front, which occurs when the air flow on both sides of a front is neither toward the cold air mass nor toward the warm air mass, (4) occluded front, which develops when an active cold front overtakes a warm front and wedges the warm front upward, and (5) a dryline, a boundary between dry, dense air and less dense humid air often associated with severe thunderstorms during the spring and summer. The two types of occluded fronts are the cold-type occluded front, where the air behind the cold front is colder than the cool air it is overtaking, and the warm-type occluded front, where the air behind the advancing cold front is warmer than the cold air it overtakes.

  

• According to the polar front model, cyclones form along fronts and proceed through a generally predictable life cycle. Along the polar front, where two air masses of different densities are moving parallel to the front and in opposite directions, cyclogenesis (cyclone formation) occurs and the frontal surface takes on a wave shape that is usually several hundred kilometers long. Once a wave forms, warm air advances poleward invading the area formerly occupied by colder air. This change in the direction of the surface flow causes a readjustment in the pressure that results in almost circular isobars, with the low pressure centered at the apex of the wave. Usually, the position of the cold front advances faster than the warm front and gradually closes the warm sector and lifts the warm front. This process, known as occlusion, creates an occluded front. Eventually, all the warm sector is forced aloft, and cold air surrounds the cyclone at low levels. At this point, the cyclone has exhausted its source of energy, and the once highly organized counterclockwise flow ceases to exist.

  

• Guided by the westerlies aloft, cyclones generally move eastward across the United States. As an idealized midlatitude cyclone moves over a region, the passage of a warm front places the area under the influence of a maritime tropical air mass and its generally warm temperatures, southerly winds, and clear skies. The passage of a cold front is easily detected by a wind shift, the replacement of a south or southwesterly airflow with winds from the west or northwest. There is also a pronounced drop in temperature. A passing occluded front is often associated with cool, overcast conditions, and snow or glaze during the cool months.

  

• Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In a cyclone, divergence aloft does not involve the outward flow of air in all directions. Instead, the winds flow generally from west to east, along sweeping curves. Also, at high altitudes, speed variations within the jet stream cause air to converge in areas where the velocity slows, and to diverge where air is accelerating. In addition to speed divergence, directional divergence (the horizontal spreading of an air stream) and vorticity (the amount of rotation exhibited by a mass of moving air) also contribute to divergence (or convergence) aloft.

• During the colder months, when the temperature gradients are steepest, cyclonic storms advance at their greatest rate. Furthermore, the westerly airflow aloft tends to steer these developing pressure systems in a general west-to-east direction. Cyclones that influence western North America originate over the Pacific Ocean. Although most Pacific storms do not cross the Rockies intact, many redevelop on the lee (eastern) side of these mountains. Some cyclones that affect the United States form over the Great Plains and are associated with an influx of maritime tropical air from the Gulf of Mexico. Another area where cyclogenesis occurs is east of the southern Appalachians. These cyclones tend to migrate toward the northeast, impacting the eastern seaboard.

• Due to the gradual subsidence within them, anticyclones generally produce clear skies and calm conditions. One to three times each winter, large highs, called blocking highs, persist over the middle latitudes and deflect the nearly zonal west-to-east flow poleward. These stagnant anticyclones block the eastward migration of cyclones, keeping one section of the nation dry for a week or more while another region experiences one cyclonic storm after another. Also due to subsidence, large stagnant anticyclones can produce a temperature inversion that contributes to air pollution episodes.

  

• In the spring, Earth's pronounced north-south temperature gradient can generate intense cyclonic storms. At a midlatitude location, as a spring cyclone with its associated fronts passes, temperatures can change quickly from unseasonably warm to unseasonably cold, and thunderstorms with hail can be followed by snow showers.

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