HOMOGENIZING LONG NORWEGIAN PRECIPITATION SERIES

The standard normal homogeneity test has been applied to 165 Norwegian precipitation series of 75 years or more. Of these series, 50 were fo und to be homogeneous, while 79 became homogeneous after being adjuste d for a single inhomogeneity. Almost 50% of the inhomogeneities were c aused by relocation of the precipitation gauge. Other reasons for inho mogeneities were changes in the immediate environment (trees, building s) and changes in instruments (windshield, new gauge type). About 80% of the inhomogeneities could be traced to information in the station h istory files. Most of the inhomogeneities caused by changes in instrum entation led to increased gauge catch. There was a similar tendency fo r inhomogeneities caused by changes in environment. Consequently, tren d studies based on groups of untested series may give dubious trends. Homogeneity testing and station history archives are essential tools f or finding the real trends and fluctuations in precipitation series.