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<br />CLOUD DEVELOPMENT <br /> <br />In order for a cloud to form and grow, two basic elements <br />must exist: <br /> <br />1. A supply of moist air. <br />2. A mechanism to cool the moist air. <br /> <br />All air is moist to some degree. It is the degree of <br />moisture which is of great significance - that property <br />which the weatherman refers to as relative humidity. <br /> <br />Let us consider this property in more detail. <br /> <br />The warmer the air, the more water vapor (moisture) it can <br />accommodate. For instance, an air parcel at 800F may halre <br />a relative humidity of 50% (it contains half .the water vapor <br />it could hold at that temperature). As the air cools, the <br />relative humidity increases. At 600F the relative humidi.ty <br />reaches 100% and the air is said to be saturated. Any further <br />cooling results in the formation of cloud droplets throu~~h con- <br />densation. <br /> <br />But how is the air parcel cooled? <br /> <br />As the air rises into regions of lesser air pressure, it <br />expands. The expansion is accompanied by a cooling of the <br />air parcel. An analogy is the cooling of an aerosol spray <br />as it leaves the pressurized can. <br /> <br />What causes the air to rise? <br /> <br />Orographic (terrain) lift <br /> <br />When air moving near the ground encounters mountains (suc:h <br />as the Black Hills) it is forced to rise. <br /> <br /> <br />Frontal lift <br /> <br />A similar effect occurs when warm and cold air masses c()llide <br />(the boundary is called a "front"). As the cold air mass <br />moves along the surface it acts as a dense "mountain" of air, <br />forcing the less dense, warm air to rise over it. <br /> <br />29 <br />