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State Reg. terrain effects of the Yampa River valley. Thus, the terrain effects tend to channel the prevailing synoptic west and southwest winds to parallel the east-west orientation of the Yampa River valley. As the winds enter the north end of the Seneca Mine Valley from the Yampa River valley they are forced to the south or up valley resulting in winds from the north-northwest. This explains how prevailing upper level west or south-west synoptic winds become northerly at the:surface in the Seneca Mine valley. Thus, the- winds on a typical day at the Seneca Mine site are controlled by two factors, the extent to which the cool air layer builds during the night and the intensity of the synoptic wind fields. As the night progresses, the drainage winds from the south develop unless the synoptic winds are strong enough to counteract the cooling effects. The valley drainage winds will presist until after sunrise when the valley surface begins to heat. In the summer, the drainage flow will break: down sooner than in the winter due to the more direct angle of the sunlight and greater amount. of surface heating. It is possible in the winter to never heat up enough to break the diurnal temperature inversion resulting in light variable winds throughout the day which would otherwise have been stronger and relatively constant as the valley winds linked up with the synoptic regime. In the summer, the synoptic weather patterns are much weaker than during the winter, allowing the surface of the valley and the valley air to heat up considerably. This hot air is bouyant and rises, frequenfly resulting in up valley flow from the north. This is analogous to the nighttime drainage flow but has more variability in the winds since the ~~`~ rising air may go up the valley sides rather than being 8-6