Here in Flagstaff, by about July 1st, we wait to smell the first hint of rain falling, somewhere. Sometimes distant peels of thunder tease us, but the building clouds don’t deliver. It will rain in some areas of Flagstaff and not others. Then the glorious soaking rains arrive on many afternoons turning hillsides green, helping nourish the forests, and washing every-thing clean.
The National Weather Service has tried to define the monsoon season on the calendar, but most of us begin to hope for it about the 4th of July, to last into September.
The word “monsoon” is derived from the Arabic word “mausim” which means season. Ancient traders sailing in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer. Thus, the term monsoon actually refers solely to a seasonal wind shift, and not to precipitation.
Even though the term monsoon was originally defined for the Indian subcontinent, monsoon circulations exist in other locations of the world as well, such as in Europe, Africa, and the west coasts of Chile and the United States. Arizona happens to be located in the area of the United States that experiences a monsoonal circulation. During the summer months, winds shift from a west or northwest direction to a south or southeasterly direction. This allows moisture from the Gulf of California and the Gulf of Mexico to stream into the state. This shift in the winds, or monsoonal circulation, produces a radical change in moisture conditions statewide.
This monsoonal circulation is typically referred to here in Arizona as the Arizona monsoon.
This change in wind direction is the result of two meteorological changes:
• the movement northward from winter to summer of the huge upper level subtropical high pressure system, specifically known as the Bermuda High
• the intense heating of the Mohave Desert creates rising air and surface low pressure, called a thermal low.
These two features combine to create strong southerly flow over Arizona. The southerly low-level winds help to bring in moisture from Mexico, originally coming from a combination of moisture from the Gulf of Mexico, the Gulf of California, and the Pacific Ocean. When this moisture encounters the higher terrain of Arizona, it gets lifted and forms thunderstorms. These thunderstorms can contain very heavy rainfall, hail, strong gusty winds, or a combination of these conditions.
By the way, the term “monsoons” as typically used in the phrase “when the monsoons arrive . . . ,” or “we had some monsoons move through town last night . . . ,” is wrong. There is no such thing as monsoons. The word should be used the same way that the word “summer” is used. Thus, the proper terminology is “monsoon thunderstorms” not “monsoons.” Remember, the monsoon is just a seasonal shift in wind direction, the thunderstorms occur because of the moisture moving over the state from the south.
Until the late 1970s, there was serious debate about whether a monsoon truly existed in North America. However, considerable research, which culminated in the Southwest Arizona Monsoon Project (SWAMP) in 1990 and 1993, established the fact that a bonafide monsoon, characterized by large-scale wind and rainfall shifts in the summer, develops over much of Mexico and the intermountain region of the U.S. Published papers at the time called this pattern by different names, including the “Summer Thunderstorm Season,” “The Mexican Monsoon,” “The Southwest Monsoon,” and the “Arizona Monsoon.” The generally accepted name is now “North American Monsoon.”
We love our “monsoon,” as overall, Flagstaff averages 276 days without precipitation each year.
