Homogeneity of the long-term urban data records

The factors causing apparent trends or discontinuities in long-term climatological observations are studied using urban records as examples. In addition to instrumental and methodological inhomogeneities urban records also include various progressive changes in the surroundings of the observation station. The effects of site changes on a record are also found alongside urbanisation effects. The apparent urban temperature trend is shown to be around 0.1°C per decade. Precipitation is generally greater at urban than at adjacent rural sites, but the differences are mostly due to the openness of neighbouring sites. Urbanisation tends to lower humidity and wind speed, but any effect on cloudiness are contradictory.