Abstract
The fraction of diffuse sky radiation transmitted by forest or a crop each day is not much affected by the weather, and, for a uniform crop, shows little spatial variation. The fraction of direct solar radiation transmitted varies as solar altitude changes with time of year. This fraction also varies considerably from place to place and from day to day, because of local variation in canopy structure and cloud duration. The fraction of diffuse radiation is usually higher than that of direct, since transmission of the canopy increases towards the zenith. Therefore, the fraction of global radiation transmitted usually falls as totals in the open increase, and the proportion of direct in the global radiation in the open increases. Instrumental errors in measuring integrated daily fluxes under canopy are often large, particularly with instruments operating without a power supply, since the signal/noise ratio is much lower than in the open.
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