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<br />Section 3.02. Snowfall <br /> <br />Snowfall is that part of precipitation that forms and reaches the <br /> <br /> <br />ground as ice crystals. If air temperatures near the surface are warmer <br /> <br />than those aloft, snow crystals may melt as they descend and occur as <br /> <br /> <br />rainfall at the surface. In mountainous watersheds, snow may accumulate <br /> <br />in the higher elevations while rain occurs at lower elevations. Whether <br /> <br /> <br />precipitation falls as rain or snow can be estimated from records of air <br /> <br /> <br />temperatures at the surface, assuming that snowfall occurs wherever surface <br /> <br /> <br />temperatures are below 340F to 360F (lOC to 20C). In mountainous regions, <br /> <br /> <br />the elevation above which snowfall occurs at any particular time can be <br /> <br /> <br />determined by applying a lapse rate to surface air temperatures. This is <br /> <br /> <br />a cornwon practice since most air temperature stations are located in the <br /> <br /> <br />more accessible,lower elevation areas. <br /> <br />Section 3.03. Snowpack Measurement <br /> <br />Where spring snowmelt floods are of particular concern, it is usually <br /> <br /> <br />practical to evaluate the areal extent, water equivalent and depth of the <br /> <br />snowpack at specified times before and during the early melt period. The <br /> <br /> <br />measurements of water equivalent at selected points in the basin and <br /> <br /> <br />estimates of the areal extent of the snow covered area are of particular <br /> <br />importance. In a manner very similar to the isohyetal analysis of storm <br /> <br />rainfall, the amount of water in the snowpack and its areal extent can be <br /> <br /> <br />mapped. Such mapping must consider elevation and exposure in a mountainous <br /> <br />terrain. Where there is appreciable variation of snowpack within the basin, <br /> <br /> <br />the snowpack water equivalent should be measured at various elevations and <br /> <br /> <br />exposures. If direct measurement of the snowpack is not made, some approx- <br /> <br /> <br />imation of seasonal snow accumulation can be made from rainfall and temper- <br /> <br /> <br />ature measurements. <br /> <br />3-02 <br />