2014 October – Why and How of Rainfall in the Sonoran Desert



A system of winds actually brings about summer’s dramatic monsoon storms. When heat bakes the surface of the desert, dry air rises, creating a vacuum that generates a flow of wind from the Pacific Ocean. Sea wind carries moisture. The moist air moves across the land and expands, rising off the desert floor in vast columns called thermals.

Thermals are beasts, zooming upward at 50 feet per second in some cases. The upward thrust generates strong downdrafts that collide explosively with the ground throwing up huge walls of dust. These are the haboobs that we dread.

The thunderheads of August afternoons rise to enormous heights, piling up to 40,000 feet into the air. While we bake in the heat below, snowstorms rage in the tops of thunderheads.

In perfect conditions, the massive clouds drench the parched landscape with sheets of rain, spawning orgies of life in puddles and washes. But often, the moisture evaporates in the superheated air before it reaches the ground. The lovely and gauzy curtain of vapor that hangs tantalizingly above the desert is called virga.

In contrast to the explosive nature of monsoon rains, the rains of winter are gentle. They develop in a civilized manner, with a low pressure system that pushes storms normally destined for the Pacific Northwest south across a broad band of the Southwest. This flow of air becomes established and one storm will follow another for several weeks. Soft winter rains fall for days, soaking the soil and bringing snow to the higher elevations. This is how our reservoirs refill. The El Nino conditions that are predicted for this year encourage this pattern.

As we recently experienced, there is a third type of rain in the Sonoran Desert. Every so often we are visited by a tropical storm in the fall. A cyclone or a hurricane from the Pacific Ocean can trundle across land and drop huge rainfalls in the Southwest, contributing half or more of the total overall moisture for the year. The most recent fall storms have been in 1970, 1983 and 2014.

So it is – this element of life, which every organism requires, comes on its own terms. Unique conditions must materialize, like a miracle happening every time. But this is normal for desert dwellers. Here, life has adapted to wait, to conserve, to hunker down and survive. When our rainy seasons do arrive, every plant and every animal is poised to respond.

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