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,'.t,- Ofl1259 -35- 1 4. Even in the absence of notably wet layers, cold snow in combination with warm air, or the reverse, may cause core to bridge in the tube so that part of the snowpack i,s pushed ahead or to ,one, side of the tube. Analogous difficulty may result from leaves, twigs, or other debris embedded in the snowpack; also, from a rough or improperly waxed tube. 5. If in cutting a sample the tube is, "pumped'r up ,and down or rotated excessively, snow may be forced through the slots. Generally the snow moves into the tube so that the determination of water equivalent is too large, 6. Part of the snow core may be lost from the lower end, of the sampling ,tube as it is withdrawn from the pack, especially if the tube is jarred. A part' also may be lost from the upper end of the tube as it is brought to a horizontal position for weighing. (Whenever water equivalent from a snow sample appears to be unduly small in comparison with that from other points or courses, inspections of the original notes to check core length against snow depth may disclose a potentially unreliable sample, Any core length as much as 20 percent less than snow depth should be questioned, because good field, practice requires that occurrence of a short core be noted and that a new sample be taken.) 7. Weight of the sample and tube maybe misread from the, weighing scale. Little, if anything, can, be done to correct this sort of error unless it is detected in the field. Occasionally, if such error seems obvious, a possible explanation is that ,for snow depths greater than 12.5 feet the conventional weighing scale is graduated in 2-inch increments rather than I-inch increments. A related source of error is to add or remove a driving wrench, without noting that action under "empty weight" of the sampler. 8. In some jurisdictions, it is customary to run the sampling tube well into soil beneath the snowpack, then to pick out the soil before the sample is weighed. If all soil particles are not so removed (position of land surface in ,the core may not show clearly), both weight and length of snow core may be recorded in error. For snow courses having ten sample points, values for average water equivalent are simply calculated and usually are reliable. However, for courses that comprise more or less than ten sample points, errors may be made in computing the average; these can be corrected by recomputing from the original notes. Because an occasional sample can be non-representa- tive, the average of water-equivalent values along a course probably is more subject to error than is the median value; by convention, however, median values generally are used only in research and special-purpose studies. ,k