Many other investigators have shown pronounced weekly cycles in atmospheric composition, particularly in large urban settings. A substantial body of literature shows that the varying concentrations of fine atmospheric aerosols (PM2.5) impact precipitation processes; generally, higher concentrations of these aerosols tend to depress winter precipitation especially in short-lived, shallow, and orographic clouds. Phoenix, Arizona has a large population relying heavily on motor vehicles as the primary means of transportation. This results in a strong weekly cycle of PM2.5 concentrations with a maximum on Wednesday and Thursday and a distinctive minimum on the weekend. To determine any influence on rainfall, we analyze daily precipitation records from 116 stations in the Phoenix area and find a strong weekly cycle in winter precipitation frequencies with maximum values on Sunday and minimum values on Thursday. The weekly cycle in precipitation frequency strengthens slightly moving eastward (downwind) across the metropolitan area as well as with increasing proximity to the metropolitan area. These results strongly suggest that human activity is influencing winter precipitation primarily by the suppressing effect of PM2.5.