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Monthly means at Hayden range from 17.3 degrees F in January 50 65.9 <br />• degrees F in Juiy. Mean daily maximums range from 31.0 degrees F in <br />January to 85.3 degrees F in July. Extreme temperatures of 100 degrees <br />F and -45 degrees F were recorded at Hayden in 39 years of observations, <br />freezing temperatures can occur during any month of the year in Hayden. <br />Wind <br />The east-west orientation of the Yampa Valley is conducive to easterly <br />downslope breezes during nighttime hours and stronger westerly upslope <br />winds during the day. Local terrain has the effect of shielding the <br />valley with respect to surface air motion, and, consequently, calm or <br />near-calm conditions are common, especially at night and in the early <br />morning. <br />Preliminary information on atmospheric stability and associated air <br />motion has been collected near Craig and may be used as an indication of <br />• air movement patterns farther up the Yampa River Valley. four stability <br />categories are defined: stable, unstable, neutral, and transition <br />conditions. Stable conditions occur when the air layers close to the <br />ground undergo cooling with associated low wind speeds; neutral <br />conditions occur with cloudy skies and/or high wind speeds; transition <br />conditions occur in the short period of time following the breakup of <br />stable conditions and prior to the onset of unstable or neutral <br />conditions. Stable layers, at ground level, occurred approximately 60 <br />percent of the time. Such layers are typically as deep as 1,100 feet <br />and can persist for several days during winter and spring. Under stable <br />conditions, surface winds are light and variable. Local influences <br />control surface winds about 40 percent of the time. <br />Ordinarily, local influences are caused by the valleys of tributaries to <br />the Yampa River. However, the drainage air layers which flow down these <br />smaller valleys are typically only 100 to 200 feet deep. The existence <br />of stable layers is most common from 7 p.m. to 11 a.m., followed by <br />upslope, or westerly winds through about 6 p.m. The gradient westerly <br />winds have higher velocities than the drainage winds and are associated <br />2.04.8-10 <br />