Monsoon Season in the American Southwest

August 19, 2012

It is autumn, and the Southwest Monsoon Season is coming to a close.  The rains still fall, but not as often. Sometimes the clouds pour their precious cargo of moisture onto the mountain ranges and it rushes down into the valley, filling our arroyos. Just as Texans know to turn their trucks around when they see water on the road, New Mexico residents know to stay away from the arroyos when there are rain clouds over the Sandias. And yet, it is so tempting to hike through these ditches that crisscross the land, because now, at the end of the monsoon, they are filled with flowers of every color imaginable.

The monsoon season brings magic to these desert regions, reviving plants that have suffered for months in extreme heat, sparking life into wildflower seeds that spring up overnight into towering plants with tiny orange flowers that sway and dance in the wind, and saving the lives of many little creatures–snakes, lizards, spiders–that help sustain the delicate ecosystem. The monsoon season is more than just rain, though. It is a daily display of spectacular storms that sometimes bring rain so quickly, so intense, that drivers are forced to park on the roadsides until the deluge subsides because they cannot see out their windshields. The monsoons bring winds that tear shingles from rooftops and flood the intricate pattern of arroyos in the valley. The monsoons also bring a shocking and fearfully dangerous amount of lightning strikes to the area.

The Southwest Monsoons bring their life-giving forces to the states of California, Arizona, New Mexico, and sometimes southern Nevada, Colorado, and Texas. Monsoons are gifts from God to the desert, and they do not understand state boundary lines. There is a reason, however, that I have chosen to focus on the New Mexico monsoon, and it is not because I live here at the moment. In fact, I spend most of my year traveling between Colorado, New Mexico and Texas. The New Mexico monsoon is unique, and I knew this before I began to study the monsoon season. It is a fact I picked up through observation and verified later by researching the New Mexico weather patterns. The fact is, New Mexico does not have a pattern for monsoons.

The National Weather Service has determined through yearly studies that the Arizona monsoon season generally begins in early July. According to the statistical data, however, the monsoon start date for Arizona seems to be changing–it once began around July 3rd, but after the year 2000 the Arizona monsoons moved back a few weeks to mid-July. One possible reason for this could be climate change, but it’s important to remember that we do not have centuries of data on weather studies to determine if there is another pattern involved. New Mexico, on the other hand, does not have a determined start date for its monsoons, though the plants, birds, and other wildlife seem to know they are coming, and when they finally arrive the desert breathes a sigh of relief. Arizona and New Mexico both receive nearly half of their annual rainfall during the monsoon season, which explains why the monsoons are so vitally important to the ecosystems of these regions. The rain, the growth of vegetation–it is all part of the fascinating monsoon process. When summer nears its end, after months of frying in 100-degree-plus heat like eggs in a pan, the American Southwest once again begins to bloom.

So, what causes the Southwest Monsoon? The answer is actually quite complex, and yet, simple. It is nature at war with itself, a great series of battles that begin with the Sun.

In the desert springtime, the winds tend to move in a westerly direction. As summer progresses and the sun warms the land and ocean at different rates, this creates a “tug of war with the winds,” according to the Southwest Climate Change Network. When the land reaches its peak of heat late in the summer months, the winds tend to change direction and the monsoons begin.

The monsoons start in northern Mexico, which receives moisture from two sides: California and the Gulf of Mexico. The land is hot, the air coming in from the ocean is full of moisture, and this produces rain. The desert vegetation explodes in growth across the landscape sending moisture back up into the sky through a process called evapotranspiration. When Arizona and New Mexico reach their peak of parched southern air–and New Mexico couldn’t get much hotter than it’s been the past few months!– the cooled air from Mexico and hot air in the Southwest begins to pull the moisture north where Arizona and New Mexico patiently wait for their share of the rain.

Soon it is July, and the Four Corners–the point where Arizona, New Mexico, Colorado and Utah meet–is so dry it is parched. The hot air rises, creating a trough in the the lower atmosphere. Down in Baja, ocean temperatures are hovering around 85 degrees, moderating local temperatures, keeping them lower than that of the desert Southwest, but eventually the imbalance becomes too great. The winds above 30,000 feet in the Southwest turn back around, carrying moisture from the Gulf of Mexico with them, and the surface air over the Gulf of California races north into Arizona and New Mexico with all this rain.

As the moisture-heavy air hits the mountains in Arizona and New Mexico it begins to rise, expand, and cool. The temperature of the air decreases below dew point (the temperature that is so low that the air can no longer hold the moisture and it condenses and falls as rain). Lightning strikes the parched land, thunder crashes loudly in the desert skies, and once again, the glorious desert flowers begin to bloom. The land is humid, the sagebrush sparkles with raindrops, and the cycle continues until the temperature difference between land and see begins to drop sometime in the fall. The battle has ended, and the desert has once again come to life.



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