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Show Anemometer Mast Proper location and secure support for the anemometer and wind vane are essential for obtaining an accurate and reliable wind record. Assuming that a ridge crest or summit location has been selected which will give an accurate representation of prevailing winds affecting avalanche release zones, careful attention must be given to positioning the instrument tower in respect to local obstructions. Ideally the mast should be on a prominence which elevates it well above any local influences on wind flow. If large rock outcrops, trees, or other obstructions exist in the vicinity of the mast site, every effort should be made to keep the mast well away from them or else extend its height above the tops of the obstructions. If a single obstruction to wind flow exists, say a large tree, which cannot be entirely avoided, then the mast ought to be located so that it is downwind from the tree only for the least common wind direction. Placing the anemometer and vane downwind for prevailing storm winds from prominent obstructions should be avoided. It may not always be possible to locate the anemometer and vane on a fully exposed promontory where they will measure the unimpeded flow of the wind. Locations on lower ridges or saddles may sometimes be necessary. Such sites will often experience modifications in both direction and velocity of the prevailing wind flow. Usually the degree of this modification can be determined only by measurements over a considerable length of time. Passes or saddles on a ridge are particularly poor sites because they tend to funnel the wind. If one of these less favorable sites must be chosen, caution will have to be used in interpreting the wind data until some experience has been built up about its relation to the actual prevailing wind. From the standpoint of the avalanche forecaster working in a given area, such qualified wind data may still be very useful, once experience has been built up in interpreting it. Wind data from such sites, though, are much less useful in analyzing the relationship of wind to avalanche formation in the larger view. The recommended anemometer mast ( A37, see also Appendix C) should be guyed at center and top. Some types of available TV towers are self- supporting ( no guys) even in 100 mph wind if properly anchored in a good base. But these are not designed for such use when they are heavily loaded with rime; even a self- supporting tower should be guyed when it is installed on a mountain top. The guy wires should be 3/ 16" stranded steel cable, with properly fitting clamps, thimbles and turnbuckles. The guy wires themselves may have to withstand high wind pressures when they are thickly coated with rime, so there should be no compromise with strength. The anchor points of the guy wires should not be located where they k2 |
