These differences in the expectation of summer rain versus winter rain define the boundaries of the deserts of North America. Then some reach the Sonoran Desert, but none or almost none make it to the Chihuahuan Desert. Winter storms strong enough to break through the Pacific coast mountain spine hit the Great Basin, Mojave, and then Colorado Deserts first. Although, if conditions are just right, they might get a wet air mass flowing up the Gulf of California between the Mexican states of Baja California and the Sonoran mainland. The Colorado and Mojave Deserts are last in line and often do without. The Sonoran Desert is next, getting monsoons more often than not, but still there are some years when the monsoons are weak or never show up. The Chihuahuan Desert gets first dibs, and so regularly receives summer rain (but little or no winter rain). Only when those air masses still have moisture after drenching the Chihuahuan and Sonoran Deserts, will they then reach the Colorado and Mojave Deserts. The monsoons first reach the Chihuahuan Desert, and if strong enough will reach to the Sonoran Desert. Hot air rises, and the void created must be filled by cooler air, usually coming from an air mass flowing over oceans.įor most of the arid west of North America, that somewhat cooler, water-saturated air comes from the Gulf of Mexico. By July the desert temperatures are at their peak. ![]() ![]() None of that is true for summer monsoons. Cooler winter temperatures mean less evaporation and so more rainwater can be absorbed by the soil, and eventually plant roots. Sometimes strong storms, strong enough to cause flooding, squeak through but they are rare. If a cell reaches the California deserts and is strong enough to still have water after the mountains try to wring as much of that precious liquid as possible, then usually the desert may get pelted with a gentle, steady rain across a broad front. Winter rains begin as low-pressure cells above the Bering Sea, saturated with water vapor and then shifting south and east pushed along by the whims of the jet stream. The 1976 flooding left the sand dunes of what is now the Coachella Valley National Wildlife Refuge looking like an archipelago of sand islands surrounded by water several feet deep. 1n 1939 and again in 1976, both in September at the tail end of the monsoon season, hurricanes hit the Coachella Valley causing widespread flooding. Occasionally, a hurricane will bring especially strong winds and heavy rains. When you see the cut banks of desert washes, the force needed to move that much water, rock, gravel, and sand was almost always due to a monsoonal storm. But monsoonal storms are sometimes much stronger, dropping more rain per minute than the often-gentler winter rains, so even with increased evaporation, there can still be enough to make a difference for desert plants. That means a lot of the rain hitting the baked earth evaporates before it can quench the thirst of desert plants. Monsoons occur when it is especially hot, when that hot rising air creates enough of a vacuum to pull in the wet air from the Gulf. If you are where the rain is falling, the temperatures can drop 20-40 degrees, from well above 100 to maybe 75 degrees, refreshing and exciting because with the rain there is almost always thunder and lightning, and sometimes flash floods. ![]() If you are not within the rain zone, all you get is humidity. The rains can be heavy but isolated to maybe a few square miles more or less. When they do show up, it is typically sometime between July and early September, when the desert is at its hottest. Some years they stick around for weeks and months at a time, while other years (and seeming increasingly so) they do not show up at all. Summer rains, monsoons, in the California deserts are persnickety at best. ![]() “Why are the desert blooms that spring to life after a monsoon so magnificent? The answer is – their impermanence …” - Alaric Hutchinson
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