Maximum and minimum temperatures: a backward and a forward look

Abstract

The effects of changing exposure of thermometers at land stations are reviewed, and the documented influences of instrumental changes in the late ninteenth and early twentieth century are presented, as well as some more recent cases. In general, the effect of improved thermometer exposure has been an apparent reduction in the diurnal temperature range, of several tenths °C. Careful intercomparisons, lasting several years, are therefore needed between present and proposed future instrumentation, if further artificial changes are to be avoided. Past data should be carefully compensated for changes of instrumentation and observing practices. The analysis of monthly mean maximum and minimum temperatures is an insufficient basis for an understanding of current changes in diurnal range. Concomitant changes of cloudiness and wind strength should be analysed. Ideally, daily data should be stratified by cloud amount and wind strength so that radiative and advective influences can be assessed. Changes in diurnal range in calm, cloudless conditions may indicate changes in radiative balance owing to greenhouse gases and aerosols.