OCR Text |
Show SECTION 1-- WIND A record of wind direction and velocity at the avalanche fracture zone is essential to snow safety decisions. This record is also expensive and technically the most difficult to obtain of all avalanche observations. Accurate, rugged instrumentation is indispensable. It is extremely difficult for an observer even fully exposed to the elements to estimate average wind velocity; his subjective impressions of peak velocities are also apt to be in error. Guessing velocities for a distant ridge during the midnight hours is impossible. F'revailing wind direction is usually much more obvious to a resident observer, either from direct observations or from later evidence of snowdrifts and cornices. But for wind direction as well, the obstacles of darkness and distance can only be overcome by instruments. Low- elevation wind regimes are often poorly representative of ridge- top conditions. The most severe instrumentation demands are made by those areas with a large difference in elevation, and consequently in storm and snow conditions, between ridge- top and valley floor. The anemometer and wind vane must be located high on the mountain at a place representative of the starting zones of the most frequent and troublesome avalanches. Where avalanches are present on several exposures, ridge crest locations are usually best. The output from the sensors must be transmitted to and recorded at the nearest convenient site accessible to the area administrator without use of the lift system. He must be able to determine current and recent wind conditions at night, before the lifts operate in the morning, and especially during those times when lift operation is halted by high winds or avalanche hazard. Data transmission by wire or radio telemetry is essential. It may be omitted where overnight residence of lift operators or ski patrolmen at a mountain- top facility is routine. In such instances, the recorder can be located near the wind sensors and the read- out verbally telephoned to the valley. VVi re lines for wind data transmission must provide reliable service in adverse mountain weather, including deep snow, icing, and high winds. Incorporation of extra circuits in lift control cables is satisfactory and the least expensive method. If this is not possible, separate wires must be installed to professional telephone standards. Improvised or patchwork cables are highly unsatisfactory. Contract installation by qualified professional telephone linemen is strongly recommended. Buried cable is the best but the cable cost is higher and the possibility of damage from bulldozer activity must be considered ( this is a frequent problem in developing ski areas). Overhead wires on poles are sometimes preferred, but wire strength, supports and pole spacing must all be designed for the severe winter environment. Cables laid on the ground will invariably be damaged by snow creep and |