View angle dependence of cloudiness and the trend in ISCCP cloudiness

The International Satellite Cloud Climatology Project cloud analysis has a small view angle dependence in its cloud estimate. There is about 10% more cloudiness reported at 60 degree view angle compared to normal views of the same scene. Over the twenty year ISCCP record, more geosynchronous satellites have been added to the analysis and the mean view angle over the globe has become more vertical. This systematic change in view point convolved with the view angle dependence in cloudiness produces much of the decreasing trend in ISCCP cloud amount, both regionally and globally. An empirical correction can be made to the ISCCP cloudiness time series which makes the data more useful for climate studies. This angular dependence is a real physical phenomena much like bi-directional reflectance for reflected sunlight from different surfaces. It should be incorporated in the conversion of instantaneous cloudiness at a particular view angle to averages over time and over view angle. Finally we discuss the impact on optical depth and radiative transfer in cloud fields.