Attenuation of Erythemal Effective Irradiance by Cloudiness at Low and High Altitude in the Alpine Region

The transmittance of cloudiness was examined for the daily totals of global erythemal effective irradiance and global total irradiance in Innsbruck (577 m above sea level [a.s.l.], Western Austria) for the periods 1981‐1988 and 1993‐1994 and at Jungfraujoch (3576 m a.s.l., Switzerland) for the period 1981‐1990. The influence of varying cloudiness on the daily totals of global erythemal effective irradiance is considerably greater than the influence of varying ozone. The lowest transmittance for daily totals of global erythemal effective irradiance is 9.4% in Innsbruck (577 m a.s.l.) and 23.5% at Jungfraujoch (3576 m a.s.l.); the median and the 25‐75 percentile range at 10/10 cloudiness are 41.8% (28.9‐56.7%) and 76.8% (63.5‐86.1%), respectively. The greater transmittance of global erythemal effective irradiance at Jungfraujoch originates from smaller thickness of the cloud layer in high mountains than in valleys. Similar transmittances were obtained for the daily totals of global total irradiance (300‐3000 nm) as a function of cloudiness.