Four Questions: Hey, How About This Summer Weather?

Christopher Castro, a UA associate professor of atmospheric sciences, is an expert on the North American monsoon and other weather-related phenomena that pass our way.

Monsoon TN 1.jpg

The U.S. Department of Interior calls Arizona's monsoon "spellbinding." Others say it's "amazing." National Weather Service Tucson meteorologist Mic Sherwood says, "We live for this moment. We like our monsoon season."

It's true. The monsoon, which runs from June 15 through Sept. 30, is characterized by periodic heavy rain, high winds and flash floods. And not only do we in Arizona get to experience the monsoon, we get to experience other weather-related phenomena that are equally intriguing, such as dust devils and dazzling sunsets.

According to Christopher Castro, University of Arizona associate professor of atmospheric sciences, our monsoon is similar to India's. In that country, the large-scale atmospheric circulation changes because of the presence of the Tibetan Plateau, Castro says.

"The heating of the land surface associated with the presence of that plateau draws low-level moisture from the Indian Ocean onto the Indian subcontinent, and that happens at a regular time every year," he says. "So, very similar to India, we have a Mexican Plateau here. It's about a third as high. But there's still a regular seasonal reversal of the atmospheric circulation that's accompanied by a regular increase in rainfall."

Here are Castro's responses to questions about Arizona's unique weather-related phenomena, including the monsoon.

Q: What is the monsoon?

A: The monsoon is a season. It's a period of reversal of atmospheric circulation, which results in an increase in rainfall at a particular time of year. The contrast of the heating of the land's surface versus the temperature of the ocean drives warm, moist air toward the land and creates the monsoon. And there's also change in the upper-level circulation pattern. When our upper-level winds shift to an easterly direction, we get moisture at upper levels to facilitate our monsoon thunderstorms.

Q: The monsoon can be a dramatic show of nature, but why is it important from an ecological standpoint?

A: The monsoon is important because it accounts for about half of our annual rainfall. It accounts for less annual rainfall the farther north you go. But it's still important. All of our natural ecosystems here in Arizona have evolved to take advantage of this rainfall. We live in a desert, but we live in a fairly lush vegetative desert relative to other places, and the monsoon rainfall is very critical to maintain the flora and fauna of the ecosystem. For example, saguaro cactus exist here and nowhere else in the world because their root system is very shallow and spread out and specifically designed to take advantage of these very intense monsoon rains. The very symbol of our state evolved to take advantage of these monsoon rains.

Q: Sometimes with the monsoon come haboobs, or strong dust storms. How and why do they form?

A: When we have very active monsoon days with very intense thunderstorms, those thunderstorms have downburst winds associated with them. Those winds hit the ground, and they'll loft any loose dust on the surface of the ground very high, almost a mile up into the atmosphere. It's the lofting of the dust associated with very strong outflow boundaries (microbursts) associated with thunderstorms that will create these walls of dust ahead of these thunderstorms. So when you see a picture of a haboob, you'll see the clouds immediately behind it. The dust storms are pretty bad along Interstate 10, and one of the reasons is the unnatural desert landscape.

Q: Speaking of dust, what are dust devils, and how do they form?

A: They're a local vortex of wind that doesn't occur in association with a thunderstorm. They actually can occur on a clear day, but you need very specific meteorological conditions for them to form. So, they don't form if it's totally clear and calm on a hot day. You have to have a little bit of wind to get them going. That wind will pass an obstacle, and then it will interact with hot air rising from the surface of a thermal, an upward current of warm air, so that the presence of the obstacle will cause a vortex to form, and then it will just keep rising in the thermal. The tallest ones can be a quarter of a mile high. 

The aftereffects of a monsoon storm near the UA's Main Gate on July 1.
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