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The Arizona monsoon (a.k.a. Mexican monsoon or Southwest monsoon) is finished for the year. While there’s no official monsoon end date declared for Tucson, it’s been at least a couple of weeks since we’ve had any monsoon-pattern weather. Our monsoon is characterized by a shift in winds to a southeasterly direction, which causes moist air from Mexico to flow into Arizona (my understanding is that it’s still not completely clear whether the moisture originates in the Gulf of Mexico or the Sea of Cortez, or both). On hot days, convection of this moist air over mountains results in frequent thunderstorms, some of which drift over the lower valleys (such as that in which Tucson sits). The monsoon provides roughly half of our yearly rainfall, on average.

The monsoon typically lasts from around early July to mid-late September. This year’s monsoon was a mixed bag. While the Tucson airport recorded slightly greater than average rainfall for the season (6.57 inches versus an average of 5.86 inches), this rainfall was heavily concentrated temporally. Despite a slow start to the season, late July was spectacularly wet (there was some intense flooding in Tucson, which we missed seeing because we were on vacation in northern Arizona and New Mexico). However, August and September saw little significant rainfall in Tucson.

Every summer I am completely obsessed with the weather. I check the National Weather Service’s forecast updates probably a dozen times a day and am glued to radar images online. As a result I got a good sense of the storm tracks this summer. We had a very distinct sense that storms were dissipating as soon as they got within a half mile of our neighborhood. At first we thought we were just being bitter about missing out on rain, but day after day it seemed like storms were bypassing us. We suspect that this is a very real phenomenon, and there are at least a couple of possible explanations. One likely contributing factor is that our neighborhood sits near the northwestern end of town, while most of the storms come in from the east or southeast. This means that the storms have a lot of time to dump much of their moisture before reaching us.

A related factor is the urban heat island effect of the city – cities are usually several degrees hotter than the surrounding landscape due to their buildings, asphalt and energy consumption (which generates heat), and Tucson definitely suffers from this. The hot air rising from Tucson probably warms any storms passing overhead, progressively reducing precipitation rates as the storms move north and west towards our part of town. Finally, a third likely factor in our lower rainfall has to do with our lower elevation relative to most of the city. We are near the Santa Cruz River, which is the lowest point in the city. Our neighborhood is therefore probably one of the hotter areas of the city, all else being equal. Like the urban heat island effect, this should contribute to a reduction in precipitation rates.

Do we really get less rain in our part of Tucson than elsewhere? I don’t know for sure, but I plan to test the hypothesis. There’s a great website called Rainlog.org which maps daily rainfall amounts around Arizona. The data is contributed by people with rain gauges. When I get a chance I plan to map total monsoon rainfall for this year’s monsoon using the data from Rainlog. It should be very interesting to see how rainfall amounts varied around the city! Who knows, maybe we’ll find that we got a lot more rain than we thought and that we’re just full of hot air.

The point of all of this is that different locations within a city can have significantly different microclimates, and this needs to be taken into account when designing a site’s water harvesting, food production and energy generation strategies. Up till now we had been treating the average rainfall at our house as equivalent to the “official” reading at the Tucson International Airport. However, this could cause us to design suboptimal rainwater harvesting features at our house if it turns out that our neighborhood is drier or wetter than the airport. It’s important to know your site and it’s individual quirks, and this is something that we’re only beginning to do!

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December 2017
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