Based on the long-term data record (1896-2007), the first 90° F day occurs in Phoenix, on average, by April 4th. However, that date has been slowly moving up in the year - most likely due to the increasing urbanization of the region. Based on the last 25 years, the average date is April 1st, and the last 10 years yields a date of March 29th. The earliest 90 °F day occurred on February 24th in 1904 and again in 1986. The latest was May 12th in 1912.

Based on the long-term data record (1878-2007), the first 90° F day occurs in Yuma, on average, by March 22nd. The earliest 90° F day occurred on February 3rd in 1963. The latest was May 3rd in 1975.

The selection of 90º F is fairly subjective. Why not focus on 93° F or 101° F? The graph below shifts the focus away from a single number, instead detailing the higher range of temperatures routinely observed in Phoenix. The individual data can be found in the table below.