The response and impact of clouds remains one of the largest outstanding questions in GCMs. Clouds are not homogeneous, though they are treated as such in the models. When averaged over areas typically used as numerical grid elements by GCMs, observations suggest that there are some clouds at all relative humidities.
Relative humidity is the best single indicator of cloud coverage.
However, if there is a relationship between cloud coverage and relative humidity, our current models and observations are inadequate to reveal exactly what that relationship is. Climate change predictions are extremely sensitive to modeled cloud cover estimates. There are numerous methods for predicting cloud cover from relative humidity, yet none of them are firmly grounded in observation. The intent of this research is to study the relationship between cloud cover and relative humidity using available observations. This produces a scatter diagram of cloud cover versus relative humidity, the gross features of which are summarized in Figure 1.