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Show The method of tower construction, and its height, depends on the maximum expected snow depth. The tower should preferably rise at least four or five feet above the deepest snow cover. It must be sturdy enough to withstand normal winter storm conditions and especially to survive the effects of snow settlement and of creep if the observation site is not perfectly level. In the most extreme case of very deep, heavy winter snow cover in a coastal climate, where maximum depths can run to 25- 30 feet, the precipitation gage has been mounted on a tall pole with a ladder up the side for access. ( Figure 11.) In less extreme conditions a tower is customary; but it must be so constructed that there are no diagonal guy wires and no cress- braces below the maximum snow level- These are susceptible to settlement pressure and eventually will be damaged. Only vertical members of the tower should penetrate through the snow cover. This means that these members must be stiff enough and sufficiently well anchored to provide the principal rigidity of the tower without external bracing except at" the top. Four telephone poles, properly creosoted and anchored in the ground, can form the basis for a suitable tower. A metal tower welded of six- inch steel pipe, like the one in Figure 12, has also proven suitable. The deck of this latter tower consists of steel landing mat, which can often be obtained from surplus sources. The exact details of construction are not critical, as long as the principles of good rigidity without any cross braces exposed to the snow are adhered to. The deck ought to measure at least six feet square. A solid hand rail is essential for safety. In continental climates with relatively shallow snow cover, a cross- braced wooden tower is acceptable, for there is much less problem from settlement forces. Figure 13 shows details of such a tower which has been satisfactory for use in the Colorado Front Range. The . sloping instead of vertical posts would not be acceptable in deep snow country, for they would be subjected to severe settlement loading. In locations such as this where there is not a wide range in snow depths during the winter, the weather instrument shelter can be fixed on top of the tower instead of in a position to be raised and lowered along the side. Regardless of the type of construction, instrument towers should be kept properly painted for preservation. Appropriate primer and finish paints should be used according to the material, wood or metal, making up the tower. If a weather instrument shelter is mounted on or alongside the tower, the tower should be painted white to minimize local heating from solar radiation. Otherwise a dark color is acceptable and in fact helps the tower to shed rime and snow when the sun comes out. 37 |
