The spectral distribution of biologically active solar radiation at Miami, Florida, USA.

The spectral distribution of solar radiation was studied under different sky conditions during a 15-month period in Miami, Florida (USA), and over a latitudinal gradient at solar maximum. Spectroradiometric scans were characterized for total irradiance (300-3000 nm) and the relative energetic and photon contributions of the following wavelength regions: UV-B (300-320 nm); UV-A (320-400 nm); B (400-500 nm); PAR (400-700 nm); R (600-700 nm); and FR (728-732 nm). Notable results include: (i) significantly higher UV-A energy fluxes than currently in use for laboratory experiments involving the biological effects of this bandwidth (values ranged from 33.6 to 55.4 W/m2 in Miami over the year); (ii) marked diurnal shifts in B:R and R:FR, with elevated R:FR values in early morning: (iii) a strong correlation between R:FR and atmospheric water content; and (iv) unusually high PAR values under direct sunlight with cloudy skies (2484 mumol/2 per s).