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<br />Section 3.04. Sources of Heat Energy <br /> <br />Solar Radiation <br /> <br />Solar (or shortwave) radiation is the prime source of all energy at <br /> <br /> <br />the earth's surface. The amount of heat energy transferred to the snowpack <br /> <br /> <br />by solar radiation varies with latitude, season, time of day, atmospheric <br /> <br /> <br />conditions, forest cover, basin orientation, and reflectivity (albedo) of <br /> <br /> <br />the snOW. Cloud cover significantly affects the transmission of solar <br /> <br /> <br />radiation and is the cause of the largest variations in solar radiation <br /> <br /> <br />incident to the snow surface. The direct measurement of solar radiation <br /> <br /> <br />at the snow surface, therefore, reflects the influence of clouds. Inasmuch <br /> <br /> <br />as residual solar radiation measurements are not generally available, it <br /> <br /> <br />is usually necessary to estimate incoming radiation indirectly from <br /> <br /> <br />observations of duration of sunshine, observations of cloud conditions, or <br /> <br /> <br />in some cases, daily air temperature fluctuations. <br /> <br /> <br />Local conditions, such as basin slopes and orientation, and forest <br /> <br /> <br />type and cover, have a marked effect upon the amount of solar radiation <br /> <br /> <br />received on the snow surface. Solar radiation incident on south-facing <br /> <br />slopes in the northern hemisphere is much greater than on north-facing <br /> <br /> <br />slopes. Similarly, the amount of solar radiation incident on a snowpack <br /> <br />is greater in a nonforested area than in a dense forest. The percentage of <br /> <br /> <br />transmitted solar radiation is related to forest composition, density, and <br /> <br />condition of trees. Determination of the amount of sunshine transmitted <br /> <br /> <br />through the forest is, at best, an approximation. For basin application, <br /> <br /> <br />the transmission factor and forest canopy density can be combined into a <br /> <br /> <br />single factor, usually termed tl~ effective forest cover. <br /> <br /> <br />The albedo of a snowpack varies considerably with time, and'is, therefore, <br /> <br /> <br />important in estimating the amount of solar energy absorbed by the snowpack. <br /> <br /> <br />Albedo is expressed as the percen,t of reflected shortwave radiation incident <br /> <br /> <br />on the snow surface. Values range from more than 80 percent for new-fallen <br /> <br /> <br />snow to as little as 40 percent for an older, ripe snowpack. Since <br /> <br /> <br />reflectivity depends on the angle of incidence as well as on snow surface <br /> <br /> <br />characteristics, the albedo will vary with season, latitude, and basin slope <br /> <br />for the same surface characteristics. <br /> <br />3-03 <br />