Diurnal variations of cloud from ISCCP data

Information on the diurnal cyle of low, mid and high level cloud amount, for the period December 1984 to November 1990, compiled by the International Cloud Climatology Project (ISCCP), is analyzed using complex empirical orthogonal functions applied to the complex envelope of seasonal variations in the diurnal cycle. It is found that previous results on the diurnal variation of cloud amount, obtained from satellite and station data for more restricted periods and regions than that used here, are verified by the ISCCP data. The early afternoon maximum in low level cloud amount over all the worldʹs continental land masses implies that diurnal variability enhances this cloud typeʹs albedo effect. Conversely the nightime and early morning maxima in mid and high level cloud amount, over much of the globe, enhance their greenhouse effect. Any secular change in the diurnal variability of these cloud types could therefore have a significant effect on the net radiation at the Earthʹs surface.