By means of the hour-to-hour MM5-predicted basic element fields and physical-quantity diagnostic fields,as well as the recording humidity data of 25 stations over Ningxia Autonomous Region from May to September 2003,predictions are made of the 48-hour hour-to-hour relative humidity of 25 stations in Ningxia from May to September using the multiple linear and successive regression MOS methods.Verification of the forecasts is conducted for summer(June to August) 2004.The results indicate that to make the 48-hour hour-to-hour relative humidity forecast by means of the two MOS methods is feasible,and the results are of reference significance.The forecasts resulted from the two methods are very close;the one from successive regression is slightly better than that from multiple linear.The forecast error is obviously lower at 08:00 than that at 20:00.The MOS-predicted results are stable while the weather situation is stable with a mean absolute error of about 10%;the forecast error fluctuates greatly while there are obvious weather changes such as temperature change.