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Show plane at 11,000 feet, and no doubt this new form of recreational travel will become increasingly important in desert regions and elsewhere. The lowering of temperature with increasing altitude helps to explain the permanent coolness of mountaintops, and why they constitute islands with a northern climate rising above the surrounding ocean of warmer air. On these island summits cling assemblages of cold- climate plants and animals that have been stranded there since the last Ice Age, when more than one- half of the continent was buried under a vast, unmelting crust of ice and snow. In southern regions these mountaintops usually are choice recreation areas. LIFE ZONES AS MODIFIED BY DIRECTION OF SLOPE AND OTHER LOCAL INFLUENCES Although latitude and altitude are the major factors influencing the climate of the Colorado River Basin, many interesting and striking modifications of climate by minor influences are visible even to the casual traveler and recreationist, and a knowledge of some of their causes heightens interest in the region. Direction of slope.- South- facing mountainsides are exposed to the warming and drying effects of sunshine to a much greater degree than are north- facing slopes. On steep slopes, the difference often is so extreme that a given climate or life zone, say the Upper Sonoran oak brush, may extend on the south side all the way to the summit, at 7,500 feet, in spite of the low temperature of the surrounding air at this altitude. On the shady, moist, north side of this mountain, on the other hand, the Upper Sonoran Zone may give way near the base of the mountain, at 6,000 feet, to a cool forest of pon- derosa pine. One may see just such a contrast on the opposite sides of innumerable hills, buttes, and low mountains throughout northern Arizona and New Mexico and southern Utah. The warming effect on south- facing slopes is still further increased by the circumstance that an inclined surface facing the sun at a right angle may receive almost one and one- half times as much heat and light as a level surface. The inclined north- facing slopes on the other hand, receive less sunlight than a level surface. Additional results of this difference are that snowbanks on north- facing slopes linger longer, and soils warm up and dry out more slowly or not at all. Spring flowers come later and birds may commence nesting 2 weeks later than their neighbors on the other side of the mountain. On many mountain peaks in the southern part of the basin, it is only on the shady north slopes that sufficient moisture remains throughout the dry season to permit the existence of a Boreal Zone fir forest. This situation occurs on Sitgreaves, Bill Williams, O'Leary, and the San Francisco Peaks, and in the Santa Catalina Mountains, as well as numerous other places in Arizona. 7 In general, mountain peaks intercept a greater precipitation than lowlands. The degree of interception is approximately proportional to the altitude of the peaks above the lowlands, the precipitation increasing about 1 inch per 160- to 200- foot rise. 8 9 In some places, however, the atmospheric currents often have been so robbed of their moisture by the time they reach the lee side of the mountains that the latter have a dry climate that is out of proportion to their altitude. Blacks Fork in the lee of the Wasatch Range is an example of this. Though at an elevation of 9,500 feet, it has an annual average precipitation of only 21.6 inches. Similarly, in Utah, Monticello receives much more winter and spring precipitation than La Sal at the same altitude, because the former is on the windward side of high mountains, whereas La Sal is on the lee side. 10 Moisture.- In general it has been established that for an area such as the Colorado River Basin, the northward distribution ( or on a mountain, the upward distribution) of plants is determined by the lowering temperatures, while the southward ( or downward) distribution is limited by the progressive decrease of moisture. In many parts of the basin there are striking il- 7 Shreve, 1915, pp. 12, 33. 8 Woodbury, 1941, p. 255. 9 Precipitation does not increase with altitude at a regular rate in all regions. In southeastern Utah, for example, to take some extreme cases, it averages 1 inch for each 112 feet of rise between St. George and Andersons Ranch ( near Leeds); 1 inch per 328 feet between Castle Dale and Hiawatha; 1 inch per 730 feet between Green River and Castle Dale; 1 inch per 403 feet between Green River and Watson; and 1 inch per 515 feet between Duchesne and Fruitland. Martin and Corbin, 1930, p. 3. 10 Martin and Corbin, 1930, p. 3. 12 |