OCR Text |
Show uniform, plane area is available with dimensions at least twice the expected maximum snow depth. Ideally the snow stake area should be in the middle of a level plot with dimensions at least ten times the maximum snow depth, but practical constraints often will limit it to a smaller area. A snow stake area exposed to strong wind action must be avoided as much as possible. Either wind scour or the deposition of drifts can lead to highly erroneous measures of the actual snowfall and representative mean snow depth for an area. A sheltered glade in a heavy stand of timber is the best stake site, but is not likely to be found at areas located near or above timberline. Examination of terrain and prevailing storm wind directions will often suggest likely sites. The best site cannot always be determined in advance when an observation station is set up in a new area. A year or two of experience with the behavior of natural snowdrifts will often lead to relocation of the stakes at a better site. Even on a windswept, open mountainside there will usually be small pockets sheltered from the wind which permit reliable snow observations, but these can often be identified only by day to day observation during winter storms. Even within what appears to be a sheltered plot there will sometimes be local areas of drifting and others relatively drift- free. These, too, may be identifiable only after a year or two of observational experience. Almost all observation sites involve some sort of compromises with the four primary criteria. The advice of an experienced snow observer can help to achieve the optimum compromise and should be sought whenever possible. If, as is usually the case, a site accurately reflecting snow conditions at the avalanche fracture zone is the hardest criterion to meet, it will be useful to supplement the main observation site with additional snow stakes located higher on the mountain. The normal observation site is customarily equipped with an instrument tower. This structure supports the recording precipitation gage above the maximum expected level of the snow cover and can at the same time provide a support along which the weather instrument shelter ( A19) can be raised and lowered to keep it the proper distance above the snow surface. It is also a convenient place for storing above snow level such accessories as shovels, spare snow stakes, precipitation weighing scale, 8" precipitation gage can, or any special instruments which may be installed. The tower should be located in one corner of the observation plot where it will not interfere with snow deposition around the snow stakes. The elevation angle from top of the tower to the nearest obstruction, such as a tree, should ideally be about 30 degrees and should not exceed kS degrees to assure proper exposure of the precipitation gage. 36 |