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The migration of a frontal system from one area and type of underlying surface to another often has a great modifying effect. It may cause the front to be regenerated in some instances or to dissipate in others. This transition affects cyclones, air masses, and fronts.

Movement Over Land Surfaces

So far, we have established that frontal systems generally weaken when moving from water to land surfaces. Once these systems are over land, further modification can be expected. A front that has just crossed the mountains and has weakened remains weak or dissipates unless something occurs to strengthen the contrast between the air masses. If a cold front has just moved onshore in winter and encounters ice and snow cover over the western half of the United States, the maritime air behind the front quickly takes on colder continental properties. The cold underlying surface may totally destroy the cold front, especially if the associated air mass is moving slowly. On the other hand, if the front is moving quickly enough that it is not totally destroyed or modified by the colder surface, it may quickly regenerate as it approaches a warmer underlying surface and air mass. These normally exist over the eastern half of the United States. In this particular situation, the air behind the front is much colder than when it started. As the front arrives at the edge of the snow field, it probably will encounter warmer moist air from the gulf or the ocean. This situation quickly results in fron-togenesis because of a sharp air mass contrast.

Strong lifting by the wedge of approaching cold air results in severe thunderstorms and abundant precipitation along the frontal surface. If the ice and snow field does not exist over the western half of the United States, then the weakened front gradually strengthens as it ap-proaches the warmer eastern United States. The weather will not be as intense; however, the cold front will have a much wider band of clouds and precipitation. With this situation, air mass con-trast is not strong. If the air masses behind and ahead of the front are weak, the front becomes stationary over the extreme southeast United States. The frontal systems are usually oriented in a northeast-southwest direction and occur mostly during the summer and autumn months. Frequently, stable waves develop and travel along this frontal system, causing unfavorable weather conditions. When these waves move out to sea and warmer moist air is brought into them, they become unstable waves and are regenerated as they move across the ocean.

As the cold fronts cross the Appalachian Mountains, they normally weaken once again because warm moist air is cut off. After passage over the mountains, warm Gulf Stream waters quickly resupply the frontal surface with the moisture and warm air needed for the front to strengthen.

Land to Water Migration

Once a cold front moves offshore, most forecasters and analysts forget about them and concentrate on the next approaching weather. When a front moves into the Atlantic, the weather generally becomes more intense, especially dur-ing fall and winter. While your station may be relaxing to some degree and enjoying the clear skies after frontal passage, Bermuda and ships at sea are most likely bracing for gale force wind and severe thunderstorm activity, In middle latitudes, ocean currents carry warm water away from the equator along the eastern coasts of continents and carry cold water toward the equator along the western coasts of continents. The most active frontal zones of the winter season are found where cold continental air moves over warm water off eastern coasts. This situation is noticeable off the east coast of the United States over the Atlantic Ocean. As a cold front moves off the coast and over the Gulf Stream, it inten-sifies, and frequently wave development occurs near the Cape Hatteras area. This gives the east coast of the United States much cloudiness and precipitation. This system and its newly intensified front eventually reaches Bermuda. A similar situa-tion occurs off the east coast of Japan. That area in the Pacific generates more cyclones than any other area in the world.

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