Even by the standards of the desert Southwest, the past several weeks there have been scorching. Phoenix had its third hottest June on record, with an average high temperature of 107.9°F (42.2°C), less than a degree below the top spot (108.6°F in 1974). There were 11 consecutive days at or above 110°F during the month, and only four days were cooler than normal.

July started hot, too — seven of the first eight days of July were at least 110°F (43.3°C). And new record highs were set on July 7 and 8, when both days reached 118°F (47.8°C).

However, a small easing of the heat is getting underway, as the annual monsoon is returning to the Southwest, playing a big role in the weather in the middle to late summer.

An increase in the number and intensity of heat waves is one of the clearest signs of climate change. The long-lasting Southwest heat hasn’t been specifically tied with climate change, but it certainly fits a pattern of increasingly common extreme heat. On average, Phoenix now has seven more days above 110°F (43.3°C) each year than they did in 1970.

Even at night, relief has been tough to come by. The temperature on the morning of July 8 only dropped to 95°F (35°C), the second hottest low temperature on record for the city.

Part of the reason the low temperatures are so high is that the Southwestern Monsoon is introducing more humidity and increasing cloud cover. These two ingredients help keep temperatures higher at night by locking in daytime warmth. But unlike the more intense monsoon in India, the Southwestern Monsoon brings much more localized showers and thunderstorms during the middle to late summer.

The driest and sunniest part of the year in Phoenix is May and June, with the hottest time of the year coming in late June and early July. The intense heat isn’t just a threat to health and a stress on air conditioning demand, it affects the ability of airplanes to fly. The hotter air has a lower density, preventing an airplane’s wings from generating enough lift to take off.

The lower density air that results from the incessant heating of the landscape in early summer induces an area of low pressure in the atmosphere. And like a traditional storm system circulating around an area of low pressure, the air rushes in from somewhere else to replace it. In the case of the Southwestern Monsoon, moisture-laden air rushes north into Arizona from the Gulf of California.

The influx of moisture is not enough to cause widespread daily rains, but it is enough to raise the humidity and kick-start showers and thunderstorms during the afternoon and evenings, bringing temporary relief to the the otherwise sweltering conditions of the desert.
 

 

While normally small, some of the storms can produce torrential rain, which can lead to flash flooding, as the desert soils do not absorb the rainfall as easily as the forested areas and softer soils of the eastern U.S.

As the storms decay in the evening, the remnant winds can create huge dust storms, known as haboobs. While not especially common, they generally occur a couple of times a year, and when they effect an urban area like Phoenix, they are especially disruptive.

haboob

Haboob timelapse from KNXV-TV (Phoenix)

Credit:

As the planet continues to warm from the increase in greenhouse gases, the Southwest Monsoon will likely be impacted. The change in the amount of annual rainfall from the monsoon is still being actively investigated, but in a warming world, there is more evaporation, so individual storms may produce more rainfall, which could lead to more occurrences of flooding. There is some evidence that the monsoon will arrive later and persist later in the annual cycle as there has already been some observed delay in the start and end of the monsoon season.  

In turn, these changes may affect the way resources are managed in desert cities, areas with growing populations where water is a limited commodity.