Cyclones are important generators of precipitation inthe Tropics as well as in midlatitudes.Factors to be considered in arriving at an accurateforecast are listed below; these factors are not listed inany order of importance:The source region of the parent air mass.Nature of the underlying surface.The type and slope of the front(s).Wind and contour patterns aloft.Past speed and direction of movement of thelow or front(s).Familiarization with the normal weatherpatterns.As pointed out earlier, a thorough understanding ofthe physical processes by which precipitation developsand spreads is essential to an accurate forecast.FRONTAL AND OROGRAPHICCLOUDINESS AND PRECIPITATIONThere are unique cloud and precipitation featuresand characteristics associated with the cold and warmfronts, as well as orographic barriers. The following textdiscusses these features and characteristics.Cold FrontYou will find it helpful to use constant pressurecharts in conjunction with the surface synopticsituation in forecasting cold frontal cloudiness andprecipitation. When the contours at the 700-hPa levelare perpendicular to the surface cold front, the band ofweather associated with the front is narrow. Thissituation occurs with a fast-moving front. If the front isslow moving, the weather and precipitation will extendas far to the rear of the front as the winds at the 700-hPa level are parallel to the front. In both of the abovecases, the flow at 700 hPa also indicates the slope of thefront. Since the front at the 700-hPa level lies near thetrough line, it is apparent that when the flow at 700hPa is perpendicular to the surface front, the 700-hPatrough is very nearly above the surface trough; hence,the slope of the front is very steep. When the 700-hPaflow is parallel to the surface front, the 700-hPa troughlies to the rear of the surface front and beyond theregion in which the flow continues parallel to the front.Consequently, the frontal slope is more gradual, andlifting is continuing between the surface and the 700-hPa level at some distance to the rear of the surfacefront.Another factor that contributes to the distribution ofcloudiness and precipitation is the curvature of the flowaloft above the front. Cyclonic flow is associated withhorizontal convergence, and anticyclonic flow isassociated with horizontal divergence.Very little weather is associated with a cold front ifthe mean isotherms are perpendicular to the front.When the mean isotherms are parallel to the front,weather will occur with the front. This principle isassociated with the contrast of the two air masses;hence, with the effectiveness of lifting.Satellite imagery provides a representative pictureof the cloud structure of frontal systems. Active coldfronts appear as continuous, well-developed cloudbands composed of low, middle, and high clouds. This iscaused by the upper wind flow, which is parallel, ornearly parallel, to the frontal zone (fig. 4-5).Figure 4-5.—An active cold front.4-4
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