Today is the halfway point in July; we’re firmly in summer’s grip with just as much of the season behind us as we’ve got in front of us. It’s a lonely, miserable time of the year for us heat haters, but for folks in the southwestern United States, it marks the glorious time of year when monsoon season ramps up.

It’s a Dry Heat (For Now)

Temperatures in this part of the world get a little toasty in the summer. The high temperature in Phoenix has hit 100°F every day for the past 35 days (including today’s impending high of about 105°F), and the average high temperature stays above 95°F until the end of September.

The only thing that saves desert-dwellers from the scorching heat is the cool, refreshing blast of a nearby downpour, a pattern that begins in July thanks to the monsoon.

Look West

It’s common to use the term monsoon casually—”oh, we’re getting monsoon rains right now”—but the term refers to a seasonal shift in the winds that allow wetter or drier conditions to prevail over a certain area. The most pronounced wet monsoons occur over southern Asia, where moist winds in the summer blow high levels of moisture over a landmass experiencing intense daytime heating. The combination of extreme heat and moisture to match can lead to extraordinarily heavy rainfall—Cherrapunji, India, situated in the hills north of Bangladesh, once received 366 inches of rain in the year between August 1860 and July 1861 as a result of the moist winds interacting with the higher terrain.

Rains that affect the southwestern United States aren’t that extreme (could you imagine?), but the storms can grow intense, leading to heavy rain and damaging winds that create some dangerous situations.

The Winds of Change

The makings of the monsoon start in the spring and early summer as the southwest turns into a natural blast furnace. Hot air is less dense than cool air, so there are fewer air molecules over one spot (say, Yuma) when it’s 110°F than there are when it’s 60°F in the same city. Since atmospheric pressure is the weight of the entire atmosphere pressing down on you, if there’s less atmosphere above you, there’s less weight and lower pressure at your location. This leads to the formation of what’s known as a “thermal low,” and this feature is key to the southwestern monsoon.

Starting around July, we typically see a ridge of high pressure begin to build-in across the southern United States, keeping much of the Deep South hot and dry, exacerbating the drought in some years while letting residents air out their basements in others. The combination of clockwise flow around the ridge and slight counter-clockwise flow around the thermal low allows tropical moisture to filter into the southwest from the Gulf of California and the Gulf of Mexico.

Liquid Sunshine

The result is rain, and rain in amounts that aren’t too shabby, all things considered. Above is a graph for Phoenix’s Sky Harbor Airport that shows normal yearly rainfall (brown line) and observed rainfall (green line/shading) between January 1, 2010 and July 14, 2015. You can see that the year starts off with some precipitation before coming to an abrupt halt between April and June. Once the wet monsoon arrives, moisture filters back in and rainfall starts ticking up after the beginning of July. The city sees about eight inches of rain each year on average.

Moisture levels in this region of the country don’t get all that high—it is the desert, after all—but dew points can climb high enough to evade the “dry heat” cliché on particularly juicy days. Temperatures still reach well into the 100s on most days (barring clouds or cold air from nearby storms), and the extensive heat and high moisture create instability that allows storms to form without too much of an issue.

Evaporated Paradise

What the region has to watch out for, however, is the remnants of hurricanes in the eastern Pacific. Every once and a while, a tropical cyclone will make landfall on the Baja Peninsula and move north/northeast through the Gulf of California and enter the southwestern United States. The influence of these systems can push atmospheric moisture levels well above normal, sometimes pushing record territory on some days.

If a thunderstorm is able to tap these (relatively) extreme levels of moisture, the storms can produce extensive flash flooding.

Flooding in thunderstorms that form during monsoon season is a major issue. The ground in desert regions has far less permeability than the soil we have out east, so rainwater can’t absorb into the ground like it does when Mobile, Alabama, sees four inches of rain in an hour and the ground is dry by nightfall. Just a short burst of heavy rain can create flooding issues in urban areas, but it doesn’t even have to rain for flooding to affect certain areas.

Arroyos are one of the most dangerous features of monsoon season. An arroyo is a dry creek bed that only fills with water when it rains. Fills is a understatement, here: arroyos usually gush after a thunderstorm, even one that forms miles upstream. Clear-air flash floods in arroyos are very dangerous to hikers if they’re not paying attention to their surroundings or flash flood warnings.

Enjoy the rain, watch out for floods, and don’t drive through a flooded roadway—not only is it stupid, but you run the risk of going broke after they pluck your dumb self out of the water.

[Images: AP, GREarth, Tropical Tidbits, xmACIS2]

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