Abstract:In 2000, the Automatic Surface Observation System (ASOS) was set up by China Meteorological Administration (CMA), and several surface meteorological parameters started to be automatically measured. Because of the difference between manual and automatic observations, the 2 min wind data from 2195 meteorological observation stations from 2000 to 2010 are compared to analyze the difference between the two observation methods. The discrepancy of frequency of calm condition between manual and automatic observations is calculated, and its reason is studied. Meanwhile, the application method of annual frequency of calm condition is explored. The results indicate that the national annual mean frequency of calm is 26.3% by manual observation and 10.0% by automatic observation. The greater the average wind speed, the lower the frequency of calm, the smaller the difference between the two kinds of observations. During the two year comparative observation period in Hubei, the annual mean frequency of calm by manual observation in the 1st and 2nd years is 29.1% and 28.8% separately. However, the frequency of calm by automatic observation in 1st and 2nd years is 18.1% and 12.4% lower than manual observation. The difference of instruments and observation methods both create the deviation. Manual observation takes the rounded to integer way, and only when the wind speed is less than 0.5 m·s-1, the manual observation records calm. But because of the automatic equipment has higher precision, calm condition would be recorded when the wind speed is less than or equal to 0.2 m·s-1. The calm difference between automatic and manual observation in the 2nd year in Hubei is assessed. According to the artificial observation rules, and by reprocessing the automatic observation data to a manual dataset, the revised annual mean frequency of calm is 22.8%, and its deviation from the average value by manual observation on the 2nd year is -6.0%, obviously smaller than the average data of original automatic observation.