Saturday, July 23, 2016


I have prided myself on writing about our summer rainy season twice (see El día de San Juan and Waiting, below) without using the word "monsoon."

In the last few decades the word has come into use by local journalists and television weather people, but when I was a kid in Phoenix in the 1960s I never heard it. And when it began to be used to refer to our summer rains, I found it laughable. "Monsoon" conjured up images of what occurs in India, which is a June-through-September rainy season producing an average of 35 inches across the subcontinent, whereas the summer rains in Phoenix have averaged 2.71 inches according to weather records going back to 1896.

Now 2.71 inches is a big deal in our parched corner of the Sonoran Desert, but "monsoon"? Gimme a break.

But it turns out that our weather does indeed follow the same pattern as the one that forms over the Arabian Sea and which affects India. The word "monsoon" derives from Arabic mausam, meaning "season," as the rains are caused by a seasonal shift in winds. What happens there also happens here in the Southwest, though our inputs of moisture-laden air are lower. (And what you get in the Southwest is dependent on location. Usually, the higher up you are, the wetter. Thus Tucson, 1200 feet higher than Phoenix and tucked into a corner formed by two high mountain ranges, averages 6.08 inches, June through September.)

So, OK then -- The Arizona (a.k.a. Southwestern or North American) Monsoon.

Here's how it works.

In May a dome of high pressure develops and is centered over the Mexican Northwest, and this high pressure system continues to affect the Phoenix area well into June. It pushes away inflows of air. It is during this time, under a bright and relentlessly cloudless sky, that we will have single-digit humidity readings and often set records for high temperatures. June is the month in which a local TV station will typically perform the stunt of attempting to fry an egg on an incandescent Phoenix sidewalk.

But by late May mountain surfaces in Mexico heat up and cause air to rise, drawing in moister, denser air from the Gulf of California. The resulting low-pressure area pushes the high pressure system off to the northeast, from the Mexican highlands eventually to west Texas.

The clockwise flow of air around this high pressure area begins to move moist air from Mexico and the Gulf of California into Arizona and New Mexico. Higher-level winds circling around the high and carrying moisture from the Gulf of Mexico also contribute to this flow. Meanwhile, the low-pressure area that has formed in Mexico -- often over the northern Baja Peninsula and having a counterclockwise flow -- also serves to move moist ocean air northward. A great atmospheric funnel, mouth end over Mexico and spout end over the U.S. Southwest, has formed.

The specifics of the locations of high- and low-pressure areas may vary, but the basic mechanism is a seasonal shift in air flows, from dry westerlies to moisture-laden southerlies. Propelled by differences in ocean and land surface temperatures, pressure systems are created which serve to move moist air from south to north.

The change always begins in Mexico. Heavy rains begin to appear over the Sierra Madre Occidental. By late June, as the high pressure system moves to the north and northeast, rains typically begin to appear over southeastern Arizona's high, isolated mountain ranges. By early July the pattern moves into Phoenix and the central Arizona highlands as well as into New Mexico. The summer rains come to Tucson, Phoenix, Flagstaff, El Paso, Las Cruces, Albuquerque, and Santa Fe -- the fabled Seven Cities (if not of Gold at least of Fast Food and Subdivisions).

A thunderstorm boils up over the high country north of Phoenix on July 1, 2016.
Taken by the author from the entrance to the Musical Instrument Museum

at North Tatum and East Mayo Boulevards.
Individual thunderstorms develop when moist air, heated by the desert floor, expands and rises. As it moves with the seasonal flow to the north and northeast, it encounters mountains and is thrust up into colder air at high altitude, causing condensation -- that is, clouds. This is why on hot monsoon afternoons in Phoenix I can see towering thunderheads over distant mountains.

Sometimes those masses, now heavy with water droplets, will collapse, causing a storm to charge out of the mountains and onto the deserts, often pushing air ahead of it -- a microburst.

The location and timing of the rains is highly variable, both from year to year and within the season itself, and so, lacking consistency, are the subject of endless speculation, conversation, and hope. Recent arrivals laugh at our obsession with what would be piddling rainfalls back in Illinois. They don't get it yet.

Find more detailed explanations of the mechanism at South West Weather (with nice graphics showing the pressure systems and air flows) and the CLIMAS Project.

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