Some observations of insolation transmitted through pine crowns in mountains of the western United States have been analyzed with respect to crown closure, cloudiness and sun angle. The relation was extended over a wider range by use of published studies in which stand parameters could be interpreted in terms of crown closure, and shows strong interception of radiation in even sparse stands, caused by absorption in depth. Incomplete expression of crown depth by closure results in departures from the relation, and a better relation is found when stem density is employed.
Radiation transmission and absorption are important, on both a regional and a local scale, in climatological analyses of forest as a heat source in a snow-covered region, and in hydrologic analyses of forest effects on snow melting and evapotranspiration. Illustrations show how the presence of forest stands affects snow melting over an entire region, and how melting rates differ in forested and open sites. Finally, some problems and suggestions for field work are offered, looking toward more complete reporting of physical dimensions of forest stands, and radiation observations that cover the solar spectrum as well as the great variations within a day and within a stand. Such procedures would make available more material for long-range research, as well as for the immediate questions of botany, ecology, and hydrology.