With July 2009 now over it will go down as not only the hottest July on record but the hottest month of all-time in both Phoenix and Yuma (as determined by average temperature, see tables below). The data below represent the top ten warmest Julys for Phoenix and Yuma by average high, mean, and low temperatures. Records for Phoenix began in 1896 and for Yuma in 1878.

Why was July 2009 so hot? Figure 1 shows what the average weather pattern was in the mid-levels of the atmosphere, near 20,000 feet, during most of the month. As is often the case, there was a large ridge (Monsoon High) over the southwest states. More unusual was a persistent trough over eastern Canada extending into the eastern U.S. When looking at how this pattern differed from the long-term means (Figure 2), it can be readily seen that the atmosphere was slightly warmer than normal over the western parts of the U.S. and Canada while it was much cooler over the eastern U.S., centered over the Great Lakes region. Looking at the air temperature at about 5,000 feet above the surface, this pattern clearly showed up of cool in the east and warm in the west. In fact, while Phoenix and Yuma experienced the warmest July on record, locations such as Detroit, MI, Chicago, IL, Indianapolis, IN, and Marquette, MI had one of the coolest Julys on record.

Another factor in the record heat locally was the role moisture plays in observed temperatures. First, due to the physical properties of water vapor, it requires more of the sun's energy to warm a piece of "wet" air versus "dry" air by the same amount. Likewise, "wet" air can retain heat longer than "dry" air. Figure 4 shows that the amount of moisture in the air was above normal this month, which helped keep low temperatures well above normal. The high low temperatures create a positive feedback loop in that it is easier to have warmer max temperatures when starting from an already warm point in the morning. This helps negate the fact that "wet" air is slower to warm than "dry" air. This was more of a factor in the Yuma area. In Phoenix, a significant attributing factor was the urban heat island influence, which has been contributing to increasingly warmer overnight temperatures for several decades (Figure 5).

Second, with the exception of localized areas, the monsoon season had gotten off to a slow start. The newly created Phoenix Rainfall Index (PRI) had only measured 0.11" of rain across the Phoenix area, which was the driest start to the monsoon season since at least 1998. Since 1998, the average PRI by the end of July is 0.57". Figure 6 shows the percent of normal precipitation across the region through July. Without the widespread rainfall it is difficult for temperatures to cool down much.