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Laboratory tests can be controlled fairly well. However, outdoor exposure tests are at the mercy of the weather. The temperature, relative humidity, sunshine, and rain vary not only from one geographical location to another but also at the same location from month to month and from year to year. These variations are of sufficient magnitude so that actual outdoor exposure tests made in one particular year and in only one or two localities cannot be considered as a standard for rating the performance of laboratory simulated (artificial) weathering tests. Outdoor exposure tests intended for evaluating the reliability of artificial weathering tests should be made over several years in several localities.
Chief, National Bureau of Standards, Washington, D. C.