The Arizona Monsoons

Definition of the word ‘monsoon’ is rather difficult to pin down as it is defined slightly differently depending on the region on the earth to which it is applied. The word monsoon often brings thoughts of torrential rains, like in a hurricane or typhoon. But a monsoon is not a single storm but rather a seasonal wind shift over a given region. The most general definition, in my view, is based on the origin of the word which is the Arabic word ‘mawsim’ meaning ‘a season’, so that ‘monsoon’ would be defined as a major wind system that seasonally changes direction. We frequently use the word ‘monsoon’ in our conversations to refer to the storms that occur during the monsoon season. According to the National Weather Service, a monsoon is caused by the temperature difference between a land mass and a nearby body of water such as an ocean or large lake.

In this discussion, we are focusing on the monsoons that occur in the Southwestern United State (mostly over Arizona) during the summer months.

In 2005, the National Weather Service declared that Arizona’s monsoon season would begin on June 15 and end on September 30. Prior to that declaration, the monsoon season had been defined as beginning when there were three consecutive days when the average dew point temperatures were 55 degrees F or higher (Dew point is the temperature to which the air must be cooled to become saturated with water vapor. Cooling air below its dew point causes liquid water to form in the air and condense to form fog, clouds, and rain.). The end of the monsoon season was determined by when the high-pressure area that characterizes the monsoon season shifts to a position so that the predominant air flow is from the west. The new definition has the advantage of allowing the Weather Service to be consistent in its promotion of weather safety information.

The Sonoran Desert is a North American desert that covers large parts of the Southwestern United State including Arizona, California, and Northwestern Mexico. The prevailing winds across the area are from the west and are subject to phenomena similar to the winds that form the Mojave desert (see my article "Why is the Mojave Desert Hot and Dry? ") during most of the year. That’s why the Arizonan desert, like the Mojave, is also hot and dry.

However, during the summer months, because of the differences in the thermal characteristics of the earth and near-by large bodies of water, the earth heats faster than the water forming an area of relatively low-pressure over the Arizona desert. The more rapid heating of the earth and subsequent lower atmospheric pressure causes the prevailing air flow to shift to a more southerly direction, drawing into the area more moisture-laden air from over the Sea of Cortez/Gulf of California. As the moist air flows into the area, it rises because of the lower pressure and increasing elevation of the terrain in Arizona (Phoenix is about 1000 feet above sea level and some of the mountains to the north and east of Phoenix have elevations well over 10,000 feet), causing thunderstorms and their accompanying lightning and rain, and thus, the Arizona monsoons.

The most dangerous aspects of the Arizona monsoon season are lightning, flooding, and dust storms. Fourteen people have been killed in Arizona by lightning since 2009, according to the National Weather Service, and all of those deaths occurred during the monsoon seasons. Twenty-six people have died from flooding since 2009, ten of those deaths occurring in a flash flood near Payson in July of 2011. Dust storms can cause traffic accidents which in turn can result in traffic fatalities. But heat is by far the biggest weather-related killer in Arizona. Not all heat-related deaths in Arizona occur during the monsoon season, but that season does include the hottest months of the year.

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