Overirradiance (Cloud Enhancement) Events at High Latitudes

Contrary to intuition, solar irradiance peaks at partially cloudy conditions. Clouds can boost sunlight by over 1.5 times, even at high latitudes. Depending on cloud velocity, the bursts last from seconds to minutes. Measuring irradiance on a tilted surface with 10-ms resolution allows for a detailed study of such events in Southern Norway, almost at sea level. All monthly maxima from April through September 2011 exceeded 1300 W/m
²
. The slow sensor registered an annual maximum of 1413 W/m
²
, while the fast sensor's range was found insufficient. A burst reaching 1528 W/m
²
was registered in June 2012. Near the Equator, bursts exceeding 1800 W/m
²
have been observed. These numbers are striking since the extraterrestrial solar irradiance peaks in January at about 1400 W/m
²
. The phenomenon is attributed mainly to forward scattering of light in optically thin clouds (adjacent to the sun), which is much stronger for angles within 5° around the solar disk.