Table 1 The variability in *N*_{d} explained by each predictor.*R*^{2} is given in percent for both unweighted calculations and a robust calculation using bi-square weights, using a bias-corrected *N*_{d} data set. Spatial *R*^{2} values are calculated by applying the same regression procedure and using bi-square weights, but using time-averaged values of bias-corrected *N*_{d} for the period September through April. Additional notes following asterisks give the *R*^{2} values calculated using a 9-year monthly climatology of MODIS *N*_{d} including retrievals at all SZAs. Uncertainties are the SDs of estimates of *R*^{2} from 10-fold cross-validation, repeated 10 times.