Periodic characteristics of cloud at Guanghan airport in Sichuan province

Based on the hourly manual observational data from meteorological station at Guanghan airport from 1986 to 1995,the periodic characteristics of low cloud were analyzed by band-pass filtering method,power spectrum analysis method and wavelet analysis method.The results indicate that Guanghan airport is influenced by stratocumulus(Sc) and cumulus(Cu),and the daily variation characteristics of the two clounds are contrary.Other clouds such as stratus fractus(Fs),nimbostratus(Ns) and fracto-nimbus(Fn) seldom appear at Guanghan airport.Variations of the cloud cover show an obvious quasi-seasonal cycle of 60-120 day,an intra-seasonal cycle of 30-60 day and a quasi-biweekly oscillation cycle of 10-20 day.