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<br />Terrestial Radiation <br /> <br />Snow is considered to be an almost perfect "black body" with respect <br /> <br /> <br />to terrestial (or longwave) radiation. Therefore, snow absorbs all such <br /> <br />radiation incident upon it and emits the maximum possible radiation. For <br /> <br /> <br />clear skies, the heat gain from longwave radiation from the atmosphere <br /> <br /> <br />and forest cover is generally less than the heat loss. The presence of <br /> <br /> <br />cloud cover has a marked effect on the longwave energy exchange at the <br /> <br />snow surface. Since a cloud is also eonsidered to be a "black body" with <br /> <br /> <br />respect to longwave radiation, temperature differences between a cloud <br /> <br /> <br />base and the snow surface govern the energy exchange. Evaluation of <br /> <br /> <br />longwave radiation exchange with the snowpack is usually simplified by <br /> <br /> <br />assuming a linear relationship between longwave radiation and air temp- <br /> <br />erature. This assumption is valid for the limited range of temperatures <br /> <br /> <br />that are normally experienced in snowmelt computations. <br /> <br />Convection-Condensation <br /> <br />The vertical and horizontal fluxes of heat energy between the air <br /> <br /> <br />mass and snowpack are usually of secondary importance to radiation. How- <br /> <br />ever, significant amounts of sensible heat energy can be transferred from <br /> <br /> <br />an overlying air mass to a colder snowpack. Similarly, if vapor pressure <br /> <br /> <br />gradients are such that condensation occurs on the snow surface, the latent <br /> <br /> <br />heat of vaporization (approximately 600 calories per gram of water) is <br /> <br /> <br />released to the snowpack. The importance of these sources of energy <br /> <br /> <br />depends upon not only the atmospheric or storm conditions, but also on the <br /> <br /> <br />basin exposure to wind and the related topographic and forest character- <br /> <br /> <br />istics. <br /> <br />Minor Sources of Heat EnerRY <br /> <br />In addition to the previously discussed sources of heat energy for <br /> <br />snowmelt, the conduction of heat from the ground and the heat content <br /> <br /> <br />of rainfall provide, in most instances, rather minor amounts of heat <br /> <br />3-04 <br />