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• z.oa.a <br />related phenomena occurs at night when radiative cooling <br />of the valley walls and higher elevations cool the <br />adjacent air. This air, being more dense than the <br />surrounding air, slides down the slope draining into the <br />lowest portion of the valley or settles above colder <br />heavier air. <br />The shape and orientation of the North Fork Valley is <br />such that the drainage (or katabatic) flow is the <br />dominant flow for most hours of the day. This occurs <br />because the westerly draining orientation of the valley <br />width in its upper reaches, results in much of the upper <br />valley bottom being in shadow during the early and mid- <br />morning hours. The air adjacent to the south sides of <br />the mountains on the north side of the river is warmed <br />and rises directly up the aide of the mountains. Since <br />the lower valley is only sparsely vegetated with <br />generally dry soils, the greatest upslope flow occurs in <br />this area. The air from the upper reaches of the valley <br />thus continues to drain down the valley until nearly noon <br />• (particularly so in winter when the sun angle is low) and <br />up the valley (or anabatic) flow does not begin until <br />mid-day. Again, because of the narrowness of the upper <br />valley, the heating of the valley floor (say above <br />Somerset and beyond) is terminated by topographic shading <br />by mid-afternoon, and with the air still being warmed and <br />rising in the lower valley, the North Fork may be one of <br />the best examples of mountain-valley flow in the State of <br />Colorado. <br />Because of the narrowness of the upper valley, the cold <br />air layer becomes quite deep with the wind velocity at <br />the mouth of the upper valley usually reaching a maximum <br />in the early morning hours. Prevailing winds are from <br />the south southeast with wind speeds of ten miles per <br />hour occurring in June. Below Paonia the wind speed <br />appears to decrease sharply as the air spreads over the <br />lower valley. Because of this strong drainage wind, <br />frost pockets are not allowed to form, thus creating a <br />favorable microclimate for fruit tree farming. <br />• Y6PXIT APPLIGST09 G . oa - 4' - <br />