Extreme overirradiance events in Norway: 1.6 suns measured close to 60 degrees N

摘要

We report the most extreme overirradiance event of 1.6 kW/m(2) measured so far in Grimstad, southern Norway (latitude 58 degrees 20'), at altitude of only about 60 m 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 1.7 kW/m(2) 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 1.0 kW/m(2). We suggest that the values exceeding 1.8 kW/m(2) reported in the literature for a few equatorial regions do not represent the theoretical global maximum, and extremes of at least 2.0 kW/m(2) (two suns) may be possible at latitudes within 30 around the Equator, even at sea level. (C) 2015 Elsevier Ltd. All rights reserved.