Abstract: Clouds play an important role in the earth radiation budget and climate change. Their shape, size, distribution and movement indicate the condition of the atmosphere.Nowadays, cloud amount and cloud height are collected by means of both satellites and ground-based instruments. Satellite cloud images provide global coverage, and these data are used widely in weather forecast. Ground-based cloud images are very local ones which contain more details of clouds.Cloud classification using satellite images has been done for many years, while the study of ground-based cloud classification is still underway. A method using fuzzy uncer tainty texture spectrum and essential information in cloud images is proposed to classify five sky conditions (stratus, cumulus, altocum ulus, cirrus and clear sky) autom atically based on cloud images obtained from the whole sky infrared cloud measuring system (WSICMS).The WSICMS is a ground-based passive sensor that uses an uncooled microbolometer detector array to measure downwelling atmospheric radiance in the 8—14μm wavelength band of the electromagnetic spectrum. It provides a way to identify clouds, obtain clouds distributions and calculate clouds amounts continuously with no difference in sensitivity during day and night. The primary WSICMS components are optical detector, environmental parameter sensors, controller and power component. The optical detector is an uncooled microbometer array containing 320×240 pixels. It obtains nine images at zenith and at each eight orientations under the control of the scan servo system. A whole sky image is accomplished after spelling nine images, water vapor correction and zenith angles correction.The WSICMS locating at Nanjing, China has been working since August 2006. The 200 cloud images according to human observations are selected randomly from these sample sets. Before cloud classification, an appropriate FUTS filter window (7×7) is chosen. Analyses of FUTS of five different sky conditions and same sky condition (cumulus) show that FUTS can serve as a good discriminating tool in cloud classification. Based on above analysis, a supervised classification with minimum distance rule is used to classify sky conditions. The classification accuracy rates of stratus, cumulus, altocumulus, cirrus and clear sky compared with human observations increase sharply after adding essential information in cloud images. Importance of the cloud characteristic is shown in cloud classification. The final classification result are 100%, 100%, 90%, 100% and 100% respectively, the average accuracy rate is 98%. Altostratus, cumulostratus and complex sky conditions are not discussed here. Future work on this project will focus on this. In addition, more particular sample sets should be built up to improve the accuracy of both training and test data.