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-8- Cloudiness regimes also may differ in other geographic situations, which are less distinctive topographically than mountain and lowland. Coastal and inland regions may have 'different amounts of cloud cover, and their regimes of shortwave and longwave radiation are different. Thus, caution should be taken in extrapolating radiation data over any long distance. The expensive recording equipment for radiation also tends to favor stations in cities. But even while atmospheric pollution is becoming region-wide 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 influ~ ence 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 that would shorten the length of day and reduce the area of the sky as a source of diffuse radiation. This aspect shOUld be checked before records are analyzed. Horizon obstruction is likely to exist in the places to which radiation data are being applied--for example, in studies of snow melting. There are geometrical means of dealing with obstructions, provided it is known how much of the shortwave radiation comes direct from the sun and how much 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 end of the day, and increased by reflection from slopes that face the instrument, especially if those slopes are snow-covered. Reflections from clouds cause short, abrupt rises in the radiation record, which might not entirely cancel out with time if clouds form regularly at the same place. Aside from measurements of the solar beam at normal incidence for research purposes, most measurements of downward radiation, both short- wave and allwave, are made on horizontal surfaces. In hydrologic studies of snow melting or transpiration on slopes, observation on horizontal 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 longwave 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, necessary 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 forest types.