Abstract: In this paper, the spatiotemporal distribution and change trend of relative humidity were systematically analyzed based on the mean relative humidity data observed by 5 meteorological stations in Wuwei City during the period from 1960 to 2009. The result shows that relative humidity was low in the north but high in the south. Annual and decadal relative humidity was holistically in a decrease trend, and there was a 6-8 year periodic change. Relative humidity was the highest in summer, than in autumn, it was the lowest in spring, holistically in an increase trend  in winter, and in a decrease trend in other seasons. Relative humidity was the highest in August and September and the lowest in April, and it was high in the morning and evening but low around noon. There was a counter correlation between the diurnalvariations of relative humidity and temperature. The daily ECMWF numerical forecast grid field data at 20:00 during the period from 2003 to 2007 were used, the forecast factor was initially elected with Press criterion, the forecast factor was chosen with stepwise regression forecast method, and the relative humidity forecast equations were developed with optimal subset regression. The overall situation and the most superior significance forecast equations of mean monthly relative humidity were determined with the CSC double grading criterion, and the prognostic equations were at a confidence level of  α=0.01. Back substitution fitting rate of the prognostic equations varied in a range of 70%-77%, and the forecast accuracy rate in a range of 65%-72%. Objective and effective instruction products could be provided for forecasting relative humidity.

Key words: relative humidity, spatiotemporal distribution, precipitation, temperature, numerical forecast, Wuwei City