Phoenix, Arizona is a vast urban area situated within the boundaries of the Sonoran Desert. The rural region surrounding the Phoenix urban core consists of primarily irrigated agriculture. While the rural region remains predominantly more humid than the urban region, at certain times in the annual and diurnal cycle the Phoenix urban region approximates or surpasses rural moisture values.

Moisture gradients (TD) between urban and rural (U-R) locations demonstrate the interaction between surface cover and overlying atmospheric conditions. To ascertain the vagaries in the overall synoptic flow during extreme urban-rural moisture gradient responses, synoptic-scale atmospheric composites were analyzed. The results show that maximum moisture gradients are enhanced from synoptic flow that directs mid-latitude cyclone passage through the region (winter), and strengthens monsoon surges into the region (summer). Minimum gradients are established from atmospheric flow that suppresses the Arizona/Mexico Monsoon (summer), and steers mid-latitude cyclones outside of the region (winter).