Immediately before and during the passage of a smoke cloud from forest fires, simultaneous measurements were made of total normal incidence solar radiation and that portion of the solar spectrum longer than 0.7μ. Calculations made of the relative amount of radiation that should be received for both the total and limited components checked closely with the ratios between measurements with a smoke-free atmosphere but showed variance with ratios obtained in the presence of smoke. The range between the maximum and minimum values of total radiation during a ten-minute period in the presence of smoke was 2.3 times as great as the range between the maximum and minimum values of infrared radiation; from which we conclude, as theory implies, that long-wave radiation passes much more freely through an atmosphere containing particles slightly less than one micron in diameter than does the shorter, or visible and ultraviolet radiation.

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