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• surface air motion within this vicinity. Figure 2.1-2 displays the topograph- <br />ical features and their elevational ranges. <br />Localized smaller terrain features also influence surface wind patterns and <br />speed. The higher terrain, approximately two miles west of Colorado Highway <br />13 adjacent to the Craig Generating Station, rises some 450-500 feet above the <br />Yampa River floodplain and tends to block the downslope flow moving generally <br />from the east to the west along the Yampa Valley. This barrier also initiates <br />a surface air pattern flowing from the south to [he north from the Williams <br />Fork River drainage system. Eventually this air movement curves westward into <br />[he main airstream flowing down the Yampa River Valley Stearns-Roger, 1974). <br />2.1.3.2 Atmospheric Stability Classifications <br />The third factor influencing surface air flow movements, after regional air <br />patterns and topographic features, is the presence and duration of various <br />atmospheric stability conditions. Stability is a measure of the thermal <br />structure of the atmosphere and indicates the potential of air [o diffuse <br />emissions both laterally and vertically. The more unstable an air mass, the <br />more rapidly emissions can be mixed and diluted within i[. A method of defin- <br />ing stability is based on the vertical change in air temperature, known as the <br />atmospheric lapse rate. The lapse rate describes the net solar radiation ~in- <br />coming or outgoing) by measuring whether or not the earth's surface is warm or <br />cold with respect to the atmosphere. If [he ground is warmer than the air <br />above it, heat is transferred rapidly into the lowest layers of the atmos- <br />phere. Thus, vertical mixing is increased and the atmosphere is termed unsta- <br />ble. If the ground is cooler than the air above it, heat is transferred from <br />the air to the ground. The temperature decreases more slowly or actually <br />increases with height and vertical mixing is inhibited. This situation typi- <br />fies stable air. The lapse rate is quantified by taking [he difference <br />between [he hourly temperatures at different levels on a meteorological tower. <br />The lapse rate for the Craig area was determined between the 20 foot and 200 <br />U <br />2-19 <br />