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GLOSSARY <br />Advection: The process of transfer (of an air mass property) by virtue of motion. In <br />particular cases, advection may be confined to either the horizontal or vertical <br />components of the motion. However, the term is often used to signify horizontal transfer <br />only. <br />Air mass: Extensive body of air approximating horizontal homogeneity, identified as to <br />source region and subsequent modifications. <br />Barrier: A mountain range that partially blocks the flow of warm humid air from a <br />source of moisture to the basin under study. <br />Basin shape: The physical outline of the basin as determined from topographic charts or <br />field survey. <br />Cold front: Front where relatively colder air displaces warmer air. <br />Convective rain: Rainfall caused by the vertical motion of an ascending mass of air that <br />is warmer than the environment and typically forms a cumulonimbus cloud. The <br />horizontal dimension of such a mass of air is generally of the order of 12 miles or less. <br />Convective rain is typically of greater intensity than either of the other two main classes <br />of rainfall (cyclonic and orographic) and is often accompanied by thunder. The term is <br />more particularly used for those cases in which the precipitation covers a large area as a <br />result of the agglomeration of cumulonimbus masses. <br />Convergence: Horizontal shrinking and vertical stretching of a volume of air, <br />accompanied by net inflow horizontally and internal upward motion. <br />Cooperative station: The location of a weather observation site where an unpaid <br />observer maintains a climatological station for the National Weather Service. <br />Cyclone: A distribution of atmospheric pressure in which there is a low central pressure <br />relative to the surroundings. On large -scale weather charts, cyclones are characterized by <br />a system of closed constant pressure lines (isobars), generally approximately circular or <br />oval in form, enclosing a central low - pressure area. Cyclonic circulation is <br />counterclockwise in the northern hemisphere and clockwise in the southern. (That is, the <br />sense of rotation about the local vertical is the same as that of the earth's rotation.) <br />Dewpoint: The temperature to which a given parcel of air must be cooled at constant <br />pressure and constant water vapor content in order for saturation to occur. <br />Effective Barrier Height: The height of a barrier determined from elevation analysis <br />that reflects the effect of the barrier on the precipitation process for a storm event. The <br />actual barrier height may be either higher or lower than the effective barrier height. <br />8 <br />