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<br />3.0 Precipitation and Seasonality in Colorado <br />The distribution of precipitation in space and time in Colorado is strongly related to how <br />dry and wet periods occur. Three topics are interrelated to obtain a physical grasp of how <br />precipitation is controlled. These include seasonality, elevation and large events. Each of these <br />are discussed as they affect Colorado. A few comments about water in the atmosphere will help <br />to guide the discussion of precipitation. The amount of water vapor contained in a parcel of air at <br /> <br />a fixed relative humidity increases as the temperature increases. At 100% relative humidity the <br /> <br />mass of water per unit mass of air approximately doubles for each lOoC (180F) increase in air <br /> <br />temperature. Another feature of the atmosphere is that as air moves upward or downward <br /> <br />without condensation or evaporation, temperature decreases (up motion) or increases (down <br /> <br />motion) at a fixed rate which is about lOoC per 1,000 meters (60F per 1,000 feet) as the pressure <br /> <br />changes. Consequently, precipitation occurs when air moves vertically upward, cools by <br /> <br />expansion until near saturation, forms droplets or ice crystals which grow with continued rising <br /> <br /> <br />motion, and the particles then fall as they get large enough, The upward motion is provided in the <br /> <br /> <br />atmosphere or when air is forced to rise up by the topography. <br /> <br />3.1 Seasons <br />The primary factors that control the precipitation climate pertain to the mid-latitude, high <br /> <br />elevation, interior continental location of Colorado (370N to 41 oN). Figure 3 shows a depiction <br /> <br />of the seasonal cycle in Colorado. The annual cycle and associated types of storms control where <br /> <br />and in which season different parts of Colorado receive maximum precipitation. During the <br /> <br />winter months, the winds in the mid-troposphere (approximately 500 mb in pressure or 5,600 m <br /> <br />or 17,100 feet) in this latitude zone are relatively strong and blow primarily from west to east with <br /> <br />6 <br />