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Show PART III CLIMATE Temperature Temperatures over the area cover a wide range due to seasonal effects, differences in elevation and diurnal fluctuations. Tempera- ture extremes in the region have varied from -60°F. at Taylor Park, Colorado, to 115°F. at Lees Ferry, Arizona. At most climatological stations mean monthly temperatures are lowest in January and highest in July and generally show about a 50°F. seasonal difference. Average annual temperatures vary from below freezing at elevations around 10,000 feet to about 50°F. for river valleys below 5,000 feet. Thus a decrease in temperature of about 3 or 4 degrees for each 1,000 foot increase in elevation occurs. Diurnal temperature fluctuations are a minimum in winter with variations of maximum and minimum daily temper- atures of 20-25°F. and reach a maximum during the summer with a daily variation of 40°F. Variations in temperature and elevation produce large differences in the frost-free period. Freezing temperatures may occur at any time of the year in the higher mountain areas. The average number of days between the last 32°F. occurrence in the spring and the first occur- rence in the spring and the first occurrence in the fall average 20 days or less at elevations about 8,500 feet to more than 180 days at elevations below 5,000 feet. Wind and Evaporation A wide range of surface wind conditions occurs in various portions of the region as the general west to east wind movement over the area is modified by topographical influences. Average wind velocities are rather high in mountains and ridges with westerly exposure, while the average wind speeds are relatively low in the more protected valley areas and assume direction according to local terrain. During much of the year high pressure systems and fair weather dominate the inter- mountain area, and these stagnant conditions result in light wind movement in the valley floor. Typically surface winds move up the valley slopes during the day and down the slopes at night. Cool nights are a characteristic of the valley floor. These conditions often prevail for a considerable period and create consistently high average wind movement at stations located in or near upland slopes or canyon areas. Average annual wind velocity for the region varies from about 8 to 14 miles per hour. Over the northern portion wind movement is greater during the winter and spring months, averaging about 15 mph and ranging down to summertime velocities of about 7 to 10 mph with occasional gusts reaching up to about 40 mph. In the central and southern portion wind movement is highest during the summer months. 11 |