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001469 IV.2 The expensive recording equipment for radiation also tends t.o favor stations iil cities. But even while atmospheric pollution is beooming region~wid~ in some parts of the United states, too many cities are distinguished from surrounding rural areas by the domes of polluted air over them.. Comparison of city .and rural radiation stations in several parts of this country, as well as in Europe, indicates that city influence reduces radiation by a fifth or more, especially in the season of low sun (Landsberg, p. 318). Radiation stations usually are sited to avoid obstructions on the horizon thU would shorten the length of day and reduce the area of the sky as a sQurce of diffuse radiation. This matter should be che.cked before reoOrds are analyzed. HoriZon. obstrAAtion is likely to exist in the places 'to which radiation data are being applied~-for example, in studies of. $nOW melting. There are geometrical means of dealing with obstruotiona, provided it is knOwn how much of the shortwave radiation comes direct from the sun and how II1U.ch is scattered in traversing the atmosphere and arrives in diffuse form. At a mountain station radiation may be both diminished by loss of shortwave radiation at the ends of the day, and iner.eased by reflection from slopes that face the inst1"Ulllent, especially if those slopes are snow-covered. Reflections ~ clouds cause short, abrupt rises in the radiation record, which might not entirely cl\Incel out with time if clouds fom regularly in the same place. Aside from measurements of the solar beam at nomal incidel10e for research pUl"ppses, Illost measurements of dOwnward radiation, both short- wave and albrave, are made on horizontal surfaces. In hydrologic studies of snow melting or transpiration on slopes, observations on horiJlontal surfaces are inappropriate. Most methods for converting them for slopes of various degrees of aspect and steepness are valid only for the direct solar beam. Conversion of diffuse shortwave radiation and of lo~ve from the sky has to be done separately. A characteristic of radiation stations that might be overlooked is the fact that they are under the open sky. Thus, radiation beneath a forest canopy, necessar.y in some studies, has been measured only experimentally, and in types of forest that are described too poorly for the measurements to be transferred easily to other fore.st types. .