visibility is low. Icing conditions, generally quitesevere, are present in the clouds. After this mP air hasbeen over land for several days, it has stabilized andweather conditions improve significantly.TRAJECTORY PATHS B AND C(CYCLONIC).—Trajectory paths B and C air with alonger overwater trajectory dominate the west coast ofthe United States during winter months. When there israpid west-to-east motion and small north-to-southmotion of pressure systems, mP air may influence theweather over most of the United States. Because of alonger overwater trajectory, this mP air is heated togreater heights, and convective instability is present upto about 10,000 feet.This air has typical k characteristics—turbulentgusty winds, steep lapse rate, good visibility at groundexcept 0 to 3 miles in precipitation, as well as cumulusandcumulonimbuscloudswithshowers.Theseshowers are not as intense as those produced in theshorter trajectory mP air, but the total amount ofprecipitation is greater.TRAJECTORY PATH D (ANTI-CYCLONIC).—This trajectory usually is over waterlongenoughtopermitmodificationstoreachequilibrium at all levels. When the air reaches the coast,it is very stable with one or two subsidence inversions.Stratus or stratocumulus clouds are frequently found.Ceilings are usually 500 to 1,500 feet and the tops ofclouds are generally less than 4,000 feet. Visibility isfair except during the early morning hours when hazeand smoke reduce the visibility to less than 1 mile. Thistype of air is found over the entire Pacific coast. It isincorrectly referred to as mT air, since it follows thenorthern boundary of the Pacific anticyclone. However,mT air does on rare occasions move into Californiaalong this path.GraduallymPairdriftseastwardwiththeprevailing west-east circulation. In crossing the coastalranges and the Rocky Mountains, much of the moisturein the lower layers is condensed out; the heat ofcondensation liberated is absorbed by the intermediatelayers of air. On the eastern slopes of the mountains, theair is warmed as it descends dry-adiabatically. As itflows over the cold and often snow-covered landsurface east of the mountains, the warm mP airbecomes stable in the lower layers.The flying conditions in mP air east of the RockyMountains are in general the best that are experiencedin winter. Relatively large diurnal temperature rangesare observed. Turbulence is almost absent and visibilityis good, except for the smoke and haze in industrialareas. Ceilings are generally unlimited, since either noclouds or only a few high clouds are present. This typeof mild winter weather occasionally spreads eastwardto the Atlantic coast. When mP air crosses the RockyMountains and encounters a deep, dense dome of cP air,it is forced to overrun it and results in storm conditionsthat produce blizzards over the plains states.Maritime Tropical (mT) Air Pacific in WinterMaritime tropical (mT) air is observed onlyinfrequently on the Pacific coast, particularly near thesurface. Air flowing around the northern boundary ofthe Pacific anticyclone is at times mT air but is usuallymP air. This air has the weather characteristics (as wellas the low temperature) of mP air, having had a longtrajectory over the water. (See fig. 4-9.)Occasionally the eastern cell of the Pacificanticyclone splits, and one portion moves southwardoff the coast of southern California. This portion of theanticyclone is then able to produce an influx of mT air.4-10AG5f0408Figure 4-8.—Trajectories of mP air over the Pacific Coast inwinter.
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