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• foot levers at site C2 (Figure 2.1-1). Each stability class corresponds [o a <br />specific lapse rate range that has been empirically associated with a degree <br />of atmospheric mixing capacity (Environmental Research and Technology, 1978). <br />In mountainous regions without large metropolitan areas, such as Northwest <br />Colorado, four standard classes of atmospheric stability are expected. These <br />are stable, transitional, neutral, and unstable. All four occur in the Craig <br />vicinity and an additional two have been documented by meteorological station <br />(C2) data. Table 2.1-9 defines the six classes used for the Craig meteorolo- <br />gical studies and Table 2.1-10 shows the frequency of their occurrence from <br />1974-1976 (Stearns-Roger, 1974 and Environmental Research and Technology, <br />1978). <br />Unstable conditions (Classes 1 and 2) are dependent upon surface heating which <br />is a function of solar radiation, and are in direct cycle with the amount of <br />incoming sunlight. Rapid vertical mixing and rapid changes of wind direction <br />take place during these conditions. Instability usually occurs with a net <br />• input of solar radiation towards the earth and with low wind speeds (Environ- <br />mental Research and Technology, 1978). These conditions occurred most fre- <br />quently in the Craig vicinity in conjunction with westerly winds (day time <br />situations) and stronger upper air velocities. <br />Transitional conditions (Class 3) usually last less than an hour and are <br />expected to occur in the mornings or late afternoons. These conditions are <br />replaced by neutral conditions, and have been recorded on only a few occa- <br />sions. <br />Neutral conditions (Class 4) occur after thermal mixing eliminates stratifica- <br />tion of air layers and upper airstream patterns become dominant. There are no <br />major discontinuities in stability conditions from the ground surface upward. <br />There is very little exchange of heat by radiation, conduction or convection. <br />Vertical mixing is neither stimulated or inhibited (Environmental Research and <br />Technology, 1978). These conditions usually occur with cloudy skies or high <br />speed winds. <br />u <br />2-21 <br />