Monitoring changes of clouds

An analysis of the spatial and temporal scales of cloud variability and their coupling provided by the results from existing cloud observing systems allows us to reach the following conclusions about the necessary attributes of a cloud monitoring system. (1) Complete global coverage with uniform density is necessary to obtain an unbiased estimate of cloud change and an estimate of the reliability with which that change can be determined. (2) A spatial sampling interval of less than 50 km is required so that cloud cover distributions will generally be homogeneous, or statistically homogeneous, within a sample. (3) A sampling frequency of at least six times a day ensures not only that the diurnal and semi-diurnal cycles are not aliased into long term mean values, but also that changes in them can be monitored. (4) Since estimated climate changes are only evident on a decadal time-scale, unless cloud monitoring is continuous with a record length greater than 10 years and has very high precision ( 1%) instrument calibration with overlapping observations between each pair of instruments, it will not be possible either to detect or to diagnose the effects of cloud changes on the climate.