Everyone knows that the Southwest deserts are hot and dry, but those who live there know that this is not always true. Summer visitors may expect July and August to be exceptionally parched, but they are likely to be caught off guard when they encounter something completely different--the monsoon.
During part of the summer, the normally arid climate shifts to a wetter phase when rainstorms pop up up in the afternoon. In some regions, half of the year's total precipitation can fall during the few months of summer because of monsoons. These showers relieve the want of water, but they also bring unwanted extremes. Storms are sometimes violent, bringing with them intense lightning, hail, wind, dust storms, or flash floods. They usually materialize in the afternoon after little to no warning during the first half of the day. Higher elevations are always at greater risk, but storms or floods can sometimes sweep into the valleys quite suddenly.
The monsoon pattern is most pronounced over Southern Utah, Southwestern Colorado, New Mexico, Arizona and inland Southern California. Other mountainous areas such as California's Sierra, Utah's Wasatch, and Colorado's Front Range also get monsoon-pattern storms. If you live or travel in the Southwest, you should be aware of the weather and the way storms can cut short a day of fun in the desert or the mountains.
In meteorology, monsoon is defined as a seasonal reversal of wind direction and resulting change in precipitation pattern. The word and the textbook scenario come from South Asia, where winter winds blow off the mountains and summer winds off the ocean, bringing with them loads of moisture and heavy rainfall. The pattern in the American Southwest and northwestern Mexico is similar, and so the "North American" monsoon earns a shared moniker.
Here's how it works: For most of the year, prevailing winds over the Southwest come from the north and west, bringing air from the mountains and the Pacific Ocean that has an opportunity to dry out as it reaches the desert, so precipitation levels are normally low. The combination of two major factors causes this to change in the summer, however.
First is the seasonal shift of prevailing winds a little bit to the north, driven by pressure outward from the equator as our hemisphere tilts toward the sun and all air north of the equator warms up. With the prevailing winds now running just north of the Southwest deserts, regional weather is left to its own devices, so to speak, and something called the land surface heating effect takes over.
This typically happens in mid June, when summer sun has warmed the desert so much that a low pressure zone builds every day. Essentially, rising hot air creates a vacuum over the desert that must be filled by air from somewhere, and in the absence of the prevailing northwesterly winds, air rushes in from the Pacific Ocean and Gulf of California, which are to the south. Because water heats up more slowly than the land, cooler air and a high pressure zone sit over the ocean. Air tends to move from high pressure to low pressure, meaning moisture-laden air moves from the sea to the desert quite rapidly.
As this cooler air meets the hot desert and the low pressure zone, it shoots up into the atmosphere and forms clouds that hold a lot of energy and moisture, thereby birthing thunderstorms. They typically happen in the afternoon after the sun has heated the land for half the day, and the monsoon air has climbed higher into the sky. Rain that reaches the ground evaporates quickly the next morning, thus feeding a cycle that repeats daily.
Monsoons vary in pattern and intensity by specific location, but in general, expect any desert or mountain environment in the Southwest region to be susceptible to afternoon storms during late June, July, August, and early September. High elevations receive more regular and violent monsoons than low elevations. For example at 7,000 feet, Flagstaff, Arizona, gets thunderstorms nearly every afternoon in July and August, and the same predictable pattern happens over the peaks of the Colorado Front Range. Mountaintops and exposed ridges are dangerous places to be during storms, so you should plan all high country hikes to end by noon.
Even when adventuring in the low deserts, you should pay attention to the daily forecast and be aware that storms can pop up any afternoon. Also realize that rain in one spot can cause flooding within a fairly large radius, so if an isolated monsoon cell is just out of view, canyons beneath blue sky are still at risk for flash floods. Do not enter any narrow canyon if the forecast indicates chance of storms anywhere in the area.
From a safe location, monsoons of the Southwest are beautiful and awe-inspiring to watch. They fill the desert and the mountains with color and water and quell the dehydrating heat of summer. They do present certain dangers to be aware of, however.
As keenly noted by writer and adventurer Craig Childs, "There are two easy ways to die in the desert: thirst and drowning." Such fates can be avoided with proper knowledge and preparation before heading outside in the summertime.
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