This no longer occurs because the water table levels have dropped far below the surface due to groundwater pumping. Air over the equator heats, rises, loses moisture and then sinks at about 30 degrees north latitude, creating large cells of stable high pressure. With low moisture content, and little to disturb the airflow, precipitation is sparse and infrequent along this global desert zone. Tucson Precipitation Figure 1-2 Tucson area mountains and rivers. An annual average of 12 inches of rain falls within the Tucson Basin.
Rainfall varies greatly according to the season and the location within the area.
The east side of the mountain range gets less rain than the rest of the range. The precipitation arrives in two distinct seasons. In the summer, as land temperatures rise, dense, moist air from the Gulf of California and Gulf of Mexico is drawn inland. In the afternoon, as temperatures reach their peak, the moist air is pushed upwards, forming large thunderheads. Figure 1-3 Summer storms in the area often are confined to isolated locations.