The influence of altitude on rainfall composition at Great Dun Fell.

The influence of altitude on rainfall composition and wet deposition has been investigated at Great Dun Fell in northern England. Measurements of rainfall at eight altitudes between 250 and 850 m on the western slopes of the hill show marked changes in both amount and composition when orographic cloud is present and a west or southwest wind is blowing. On average (20 precipitation events from autumn 1984 and spring 1985), the rainfall at the summit (847 m) exceeded that at 250 m by a factor of 2, and concentrations of SO2−4, NO−3, Cl−, NH+4 and H+ were larger at the summit by factors of between 2.2 and 3.1. Thus, wet deposition at the summit was larger than at 250 m by about a factor of 5. The concentrations of major ions in orographie cloud at 847 m exceeded concentrations in rain by a factor of between 2.0 and 3.9. A large change occurred in the concentrations of major ions in orographie cloud with altitude, decreasing with increasing altitude from cloud base. Such changes could generally be explained by the expected dilution as cloud liquid water content increased adiabatically. When the wind was from the east or with blocked flow no increase in concentration or rainfall amount was observed.