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<br /> <br />;.., '. C. <br /> <br />-34- <br /> <br />"If the activity on the snow course persists, an alternate snow <br />course is probably in order--so--be prolific in your writing on,the <br />snow notes; they are our best clue to winter conditions on the snow <br />oourses you take. Snow notes are reviewed for consistency in the <br />office to determine to what degree such conditions are advElrsely <br />affecting the measurements ycu:take. tour messages on the snow <br />notes eliminate arry sUppositio~ by the office." <br /> <br />Sources :of error <br /> <br />Errors of observation or technique <br /> <br />The majority of observational errors in snow~course surveys are <br />related to adverse sampling conditions; most of but not all the errors <br />cause the determinations of water e~uivalent to be too s~all. Inclement <br />weather is at times a principal coni!ributing cause in that it may subject <br />the snow surveyor to considerable physical stress in gaining access to the <br />course site, and so may dull his perception. The more (1ommon sources of <br />error include: <br /> <br />1. Samples are taken commonly at stakes or at specified <br />distances from end markers of the course. After the sampling tube <br />is withdrawn, the hole tends to fill or close, but usually with snow <br />at less-than-normal density. ~hen" in a subsequent round of samples, <br />if precisely the same point isre-occupied, the subsequent sample <br />is deficient in water equivalent. To obviate this possibility, one <br />experience snoW surveyor systematically samples at, station' ,," <br />1 + 1 foot, station 1 + 2 feet, ,and so on. <br /> <br />2. In snow depths greater than 6 feet, densities may exceed <br />30 percent in the lower foot ottwo of snow, in part because the <br />older snow compacts under weight of the overlying younger snow. <br />Ice layers within a snowpack may have been generated by periodic <br />condensation, rain on the snow 'surface, or melting. Under either <br />of these conditions it may be difficult to obtain a core of snow <br />(inside the snow tube) representing total water equivalent of the <br />snowpack. A thick compact layer may not be penetrated completely. <br />A thin compact layer may break and be pushed ahead of the cutter, <br />so that some of the snow is thrust aside. <br /> <br />3. When liquid water or moist snow is present in the snowpack, <br />and the air and sampling tubes are below freezing temperature, snow <br />may stick in or on the tubes in such amount that accurate measure- <br />ment is impossible. Snow also may stick in the tube iI' the pack is <br />wet at the top and cold at depth. The tube even may freeze firmly <br />in. To avoid such difficUlties. samples may be taken only early or <br />late in the day, when there is no surface melt. <br />