Homogeneity of the long-term urban data records

The factors causing apparent trends or discontinuities in long-term climato logical observations are studied using urban records as examples. In additi on to instrumental and methodological inhomogeneities urban records also in clude various progressive changes in the surroundings of the observation st ation. The effects of site changes on a record are also found alongside urb anisation effects. The apparent urban temperature trend is shown to be arou nd 0.1 degrees C per decade. Precipitation is generally greater at urban th an at adjacent rural sites, but the differences are mostly due to the openn ess of neighbouring sites. Urbanisation tends to lower humidity and wind sp eed, but any effect on cloudiness are contradictory. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.