Cloud cover as derived from surface observations, sunshine duration, and satellite observations

The proportion of the sky visibly covered by low and middle level clouds increases towards the horizon, due to their vertical extent, and this was considered in relation to its effect on direct solar radiation. The increase was found as a function of elevation above the horizon and of cloud amount, and it was shown to follow from its functional form that photographs from polar-orbiting satellites would give lower estimates of cloud cover than ground observations, and so would sunshine duration measurements in low latitudes. In high latitudes sunshine duration would yield higher estimates than ground observations. Other investigators had found that both the other methods gave lower estimates than surface observations at all latitudes, and the discrepancies were larger than those in this study. The differing results could be seen as due at least in part to differing treatments of high cloud.