Sunday, August 6, 2017

Hodgepodge 281/365 - American Monsoon

We've been having strange weather lately: hot (well, for here, anyway—high 70s) and very muggy. Humidity isn't unusual here; after all, we live on the coast, and summertime upwelling creates cool, moist air as a rule. But mugginess—hot and humid—is unusual. As is rain in summer, but yesterday morning on my way to the Fiesta I even had to turn my windshield wipers on. Sadly, there was no accompanying thunder. I love thunderstorms, which are a rare but wonderful treat hereabouts.

Turns out we've been enjoying the edge effects of the North American/Southwest/Mexican/New Mexican/ Arizona monsoon, a regular occurrence every July and August and into September, a period of pronounced increase in thunderstorms and rainfall in the southwestern U.S. and northern Mexico. It is responsible for fully half the rainfall in these areas. According to Wikipedia, "During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon–early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily." This must be a blessed relief from the otherwise high temperatures of summertime in this region.

Rather than try to summarize the causes and effects, I'll just copy from Wikipedia wholesale, because I find this most interesting. (Yes, I did supply the link above, but it's easier if it's right here, right?)
The North American Monsoon is not as strong or persistent as its Indian counterpart, mainly because the Mexican Plateau is not as high or as large as the Tibetan Plateau in Asia. However, the North American Monsoon shares most of the basic characteristics of its Indian counterpart. There is a shift in wind patterns in summer which occurs as Mexico and the southwest U.S. warm under intense solar heating. As this happens, the flow reverses. The prevailing winds start to flow from moist ocean areas into dry land areas.
The North American monsoon is associated with an area of high pressure called the subtropical ridge that moves northward during the summer months and a thermal low (a trough of low pressure which develops from intense surface heating) over the Mexican Plateau and the Desert Southwest of the United States. The monsoon begins in late May to early June in southern Mexico and quickly spreads along the western slopes of the Sierra Madre Occidental, reaching Arizona and New Mexico in early July. The monsoon extends into the southwest United States as it matures in mid-July, when an area of high pressure, called the monsoon or subtropical ridge, develops in the upper atmosphere over the Four Corners region, creating wind flow aloft from the East or South-East.

Pulses of low level moisture are transported primarily from the Gulf of California and eastern Pacific. The Gulf of California, a narrow body of water surrounded by mountains, is particularly important for low-level moisture transport into Arizona and Sonora. Upper level moisture is also transported into the region, mainly from the Gulf of Mexico by easterly winds aloft. Once the forests of the Sierra Madre Occidental green up from the initial monsoon rains, evaporation and plant transpiration can add additional moisture to the atmosphere which will then flow into Arizona. Finally, if the southern Plains of the U.S. are unusually wet and green during the early summer months, that area can also serve as a moisture source.

As precipitable water values rise in early summer, brief but often torrential thunderstorms can occur, especially over mountainous terrain. This activity is occasionally enhanced by the passage of tropical waves and the entrainment of the remnants of tropical cyclones.

Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations and feeding desert streams. Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk. A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought.

Flash flooding is a serious danger during the monsoon. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away; it is therefore wise to avoid camping in a dry wash during the monsoon. Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.
It seems this year the monsoon is centered a bit more westerly than usual, which creates a greater effect out here on the margin of the continent (though it's more complicated than that, of course).

One of my favorite books from last year's reading binge was Craig Childs's The Desert Cries: A Season of Flash Floods in a Dry Land. That was the monsoon calling. It is beautiful and refreshing, as well as destructive and deadly. "I sometimes think that, in the perfect Eden, there are no floods," writes Childs. "In Utopia, a person never dies fighting beneath muddy waves. Then I am glad to not live in such barren places as these. Floods belong to a fertile and dynamic land. We cannot control elements of danger, magnificence, and prowess in the world. To wish them away or to tear them into survivable pieces is to wish for a less genuine Earth."

P.S. I had no clue what to write about today, so I resorted to a website of 365 writing prompts. The first one was "1. Outside the Window: What’s the weather outside your window doing right now? If that’s not inspiring, what’s the weather like somewhere you wish you could be?" I might be using that website more often: it's useful!

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