Abstract
We report the most extreme overirradiance event of 1.6kW/m2 measured so far in Grimstad, southern Norway (latitude 58°20′), at altitude of only about 60m a.s.l. Images of the sky conditions taken with a wide-dynamic-range camera suggest that this is not the ultimate value for our location, but that extremes as high as 1.7kW/m2 may be possible. The phenomenon was caused by strong forward scattering of sunlight within 3° around the solar disk in thin, broken altocumulus clouds. The normalized spectrum of sunlight during such extreme events is very similar to the normalized clear-sky spectrum measured at Air Mass 1.4 and irradiance of 1.0kW/m2. We suggest that the values exceeding 1.8kW/m2 reported in the literature for a few equatorial regions do not represent the theoretical global maximum, and extremes of at least 2.0kW/m2 (two suns) may be possible at latitudes within±30° around the Equator, even at sea level.
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