THE EFFECT OF RADIATION SCREENS ON NORDIC TIME-SERIES OF MEAN TEMPERATURE

A short survey of the historical development of temperature radiation screens is given based upon research in the archives of the Nordic met eorological institutes. In the middle of the nineteenth century most t hermometer stands were open shelters, free-standing or fastened to a w indow or wall. Most of these were soon replaced by wall or window scre ens, i.e. small wooden or metal cages. Large free-standing screens wer e also introduced in the nineteenth century, but it took to the 1980s before they had replaced the wall screens completely in all Nordic cou ntries. During recent years, small cylindrical screens suitable for au tomatic weather stations have been introduced. At some stations they h ave replaced the ordinary free-standing screen as part of a gradual mo ve towards automation. The first free-standing screens used in the Nor dic countries were single louvred. They were later improved by double louvres. Compared with observations from ventilated thermometers the m onthly mean temperatures in the single louvred screens were 0.2-0.4 de grees C higher during May-August, whereas in the double louvred screen s the temperatures were unbiased. Unless the series are adjusted, this improvement may lead to inhomogeneities in long climatic time series. The change from wall screen to free-standing screen also involved a r elocation from the microclimatic influence of a house to a location fr ee from obstacles. Tests to evaluate the effect of relocation by paral lel measurements yielded variable results. However, the bulk of the te sts showed no effect of the relocation in winter, whereas in summer th e wall screen tended to be slightly warmer (0.0-0.3 degrees C) than th e double louvred screen. At two Norwegian sites situated on steep vall ey slopes, the wall screen was ca. 0.5 degrees C colder in midwinter. The free-standing Swedish shelter, which was used at some stations up to 1960, seems to have been overheated in spring and summer (maximum o verheating of about 0.4 degrees C in early summer). The new screen for automatic sensors appears to be unbiased compared with the ordinary f ree-standing screen concerning monthly mean temperature. (C) 1997 Roya l Meteorological Society.