Abstract: In order to solve the problem that most of the equipments cannot obtain the whole sky cloud base height independently, based on the grey value and the cloud base height data measured by the visible / infrared sky imager and the millimeter wave cloud radar measured at the China Meteorological Administration Atmosphere Detection Base in June, 2019, the relationship between the downward infrared radiation of ground in 8-14 μm band iand the zenith angle is calculated using Santa Barbara DISORT Atmospheric Radiative Transfer mode, and then according to the linear relationship between the gray valueof the zenith cloud base measured in the infrared band and the cosine gray value of the zenith angle at the non-zenith position, the cloud base height of the whole sky is retrieved. The results show that, (1) compared with that in cloudless day, the zenith gray value is higher when there is cloud all day at the same time during a day. (2) When the cloud base height is more than 2000 meters and the cloud base is thick, the correlation between the cloud base height and the gray value measured by the infrared module is significantly better than that of the low clouds with thin cloud layer. (3) The case verification on August 12, 2019 shows that the correlation coefficient is 0.956 between the inversion cloud height and the actual cloud height measured by cloud radar, while the correlation coefficient between the inversion of cloud height and the cloud height obtained by taking the zenith angle of about 30° near the zenith is 0.9508.

Key words: all-sky imager, cloud radar, grey value, cloud base height