Abstract:The data in the second year of relative humidity from Sichuan 135 stations observed by automatic and manual in parallel, are used to analyze the difference between the relative humidity information by automatic and manual observations, and the reasons for the differences. The parallel comparison observations indicated that the average daily relative humidity observed by automatic way was 2.727% less than that by manual way, the average monthly relative humidity observed by automatic way was 2.797% less than that by manual way, and the average annual relative humidity observed by automatic way was 2.7472% less than that by manual way. There were 56.35% stations whose difference values in relative humidity respectively observed in the two ways were less than 5%, the difference values were below 10% at 86.66% stations, and the difference values were above 20% at 2.61% stations. The relative humidity D value between automatic and manual observations has no significant regional difference. Some factors may affect the results, even make the significant difference between manual and automated observations, such as the irregular use of wet bulb gauze, uncleanness of gauze, the error that people read dry bulb thermometer and wet bulb thermometer, the state of ventilation dry bulb thermometer and wet bulb thermometer, inconsistently observation time, the automated observation nonlinear in high humidity and so on.