OCR Text |
Show maximum accuracy over a range of 20° or 30° C below the freezing point, but an easily procurable model with a range of - 100° to + k0° C ( A3) will give satisfactory service. These thermometers should be handled carefully to prevent bending of the stem. A protective case is necessary for transport in the field. A lightweight one can be fashioned from a block of styrofoam in an aluminum can. Holes punched in the styrofoam to fit the thermometer stems give good protection. A minimum of two dial thermometers should be used for pit observations. Three are convenient for speedier work, and offer the advantage of easy comparison if one develops a calibration error. All thermometers, including the metallic dial type, should be checked periodically for calibration by insertion in an ice bath ( slush of snow and water). If they do not read exactly at the freezing point ( 0° C, 32° F), mark the correction on the dial and use it with subsequent readings. Liquid-in- glass thermometers may be used for snow temperature measurements at the pit wall, but they break too easily in hard snow and thus are not recommended. A snow cutting plate ( A7) is useful for digging out and trimming density samples, and for smoothing or cutting sections of the pit wall. Bevel one end by grinding to provide a sharper cutting edge. A mason's trowel also makes a good cutting plate and has the added advantage of a handle. The snow modelling brush ( A6) is used after the pit wall has been smoothed to bring out details in the stratigraphy ( brushing the snow emphasizes differences in hardness). Snow crystals from the various layers in the pit are checked for size with the aid of a millimeter reticle. Crystal size and type examination requires a hand magnifier that will also be used for observing new snow crystal type during snow storms. The larger ( around 1 inch diameter) double magnifier ( A9) is convenient for this work. A smaller pocket lens, such as a 10X Hastings, is more easily carried at all times but harder to use because of the restricted field of vision. Pit observations or fracture line inspection on slopes require a clinometer to check slope angle. This instrument is also generally useful to the snow ranger for recording the steepness of avalanche paths, ski runs, and for rough- checking artillery targets. The common Abney level is satisfactory for most purposes. A compact pocket level ( A12) is also available. The centimeter scale on the ram penetrometer normally provides a reference scale for locating levels in a snowpit, but on many occasions a supplementary scale is required, for instance to measure fracture line heights or determine new snow depths in the field. A pocket tape ( A8a) or folding rule ( A8b) which can be easily carried is preferred. They should be graduated in both metric and English scales, for added convenience. 28 |