Before I moved to Arizona to live full-time, I thought the only serious weather issue was the temperature.
The Phoenix area is extremely hot. We average 110 days a year over 100 degrees, and it doesn’t cool off much at night. That heat is relentless.
The Phoenix area is extremely hot. We average 110 days a year over 100 degrees, and it doesn’t cool off much at night. That heat is relentless.
I’m sure you’ve seen the postcard.
In early July, just when I feel I can’t take it anymore, I will scream if I see another temperature reading of 116 on my car dash, and I swear I’m moving someplace cold before next summer, something changes. Our single-digit humidity begins to creep up to 30% or more.
It’s the first hint that the monsoon season is coming to the desert.
I grew up in the Midwest and was used to lightning and thunderstorms. But when I experienced my first monsoon storm in Arizona, I discovered just how awe-inspiring life in the desert can be.
In early July, just when I feel I can’t take it anymore, I will scream if I see another temperature reading of 116 on my car dash, and I swear I’m moving someplace cold before next summer, something changes. Our single-digit humidity begins to creep up to 30% or more.
It’s the first hint that the monsoon season is coming to the desert.
I grew up in the Midwest and was used to lightning and thunderstorms. But when I experienced my first monsoon storm in Arizona, I discovered just how awe-inspiring life in the desert can be.
Though the word monsoon is often misused to refer to a single long, drenching thunderstorm, it is actually the name given to a large-scale weather pattern. The word derives from the mausin, Arabic for “season” or “wind shift.”
Why does Phoenix have summer monsoons? Doesn’t it seems counter-intuitive to have the majority of the rainfall when the temperatures are the highest?
Why does Phoenix have summer monsoons? Doesn’t it seems counter-intuitive to have the majority of the rainfall when the temperatures are the highest?
The scientific answer lies in our distance from the equator. You see, we live on the edge of what is called the “horse latitudes,” two subtropical atmospheric high-pressure belts that encircle the Earth around latitudes 30 degrees North in the Northern hemisphere and 30 degrees South in the Southern hemisphere. The horse latitudes are those bands that sailors dread because their ships are becalmed when they hit these high-pressure belts of no wind and clear skies.
Those high pressure ridges are the product of global air circulation known as the Hadley Cell. The air at the equator heats and rises to the lower atmosphere, where it diverges, moving away from the equator, both north and south, until it cools and sinks around the 30th parallel in both hemispheres. At the surface level, the sinking air diverges again with some returning from the equator, completing the Hadley circulation.
The end result is that little or no moisture ever reaches the ground below this climatological high-pressure area. Thus, the horse latitudes produce arid climates in the areas below them. The Sahara, for example, is situated in a horse latitude. So is our own Sonoran desert.
But luckily for us, we get a reprieve.
Those high pressure ridges are the product of global air circulation known as the Hadley Cell. The air at the equator heats and rises to the lower atmosphere, where it diverges, moving away from the equator, both north and south, until it cools and sinks around the 30th parallel in both hemispheres. At the surface level, the sinking air diverges again with some returning from the equator, completing the Hadley circulation.
The end result is that little or no moisture ever reaches the ground below this climatological high-pressure area. Thus, the horse latitudes produce arid climates in the areas below them. The Sahara, for example, is situated in a horse latitude. So is our own Sonoran desert.
But luckily for us, we get a reprieve.
That the high pressure ridge starts migrating poleward in late spring. The ridge across North America typically migrates far enough northward to begin monsoon conditions across the Desert Southwest in July and continue through September.
On average, Arizona gets about half of its annual rainfall during the summer monsoon.
On average, Arizona gets about half of its annual rainfall during the summer monsoon.
In addition, the wind shifts direction in the summer. Arizona winds usually come from the west, but they shift to southeasterly in the summer, bringing moisture up from the Gulf of Mexico.
The wind shift and increase in moisture combine with the surface low pressure from the desert heat to produce storms in a cycle of “bursts” (heavy rainfall) and “breaks” (suddenly reduced rain).
The wind shift and increase in moisture combine with the surface low pressure from the desert heat to produce storms in a cycle of “bursts” (heavy rainfall) and “breaks” (suddenly reduced rain).
Before the rain, the wind shift can trigger dust storms known as haboobs, which appear as loose swirling walls of dust several hundred feet high. The name comes from the Arabic word habb, meaning “wind.” They’re formed when air is forced downward and pushed forward by the front of a thunderstorm cell, picking up dust as it travels across the desert.
Driving through a haboob can be terrifying. Drivers are instructed to pull off the road, turn their lights off, and wait until it has passed.
The downpour from the monsoon thunderstorms spurs plant growth. Desert plants absorb moisture to sustain themselves after the wet season ends.
Driving through a haboob can be terrifying. Drivers are instructed to pull off the road, turn their lights off, and wait until it has passed.
The downpour from the monsoon thunderstorms spurs plant growth. Desert plants absorb moisture to sustain themselves after the wet season ends.
Prickly pear fruit ripens at this time, turning a dark shade of pink.
Every year I watch, amazed, as my trees grow tall and lush in the midst of such heat, and the plants in my landscape bloom in magenta, bright orange and red.
Who knew the desert had such vibrant plant colors?
Nothing like the delicate pastel pinks, white and lavender blooms of my Midwest yard.
The monsoon rains are always welcome, but also dangerous. Flooding and wind damage is common. All over Arizona, you will see dry riverbeds called washes. During monsoon season, these washes can fill with water very suddenly.
The monsoon rains are always welcome, but also dangerous. Flooding and wind damage is common. All over Arizona, you will see dry riverbeds called washes. During monsoon season, these washes can fill with water very suddenly.
In fact, it may not even be raining where you are for a wash to fill with water, because the rainstorm may be happening upstream from you.
Every season, flash flooding happens and cars, people and even cattle become stranded or washed away.
Sadly, monsoon season also can make a tender box fire season even worse, due to the increased amount of lightning, up to tens of thousands of strikes per night in some areas.
Yes, life in the desert can be harsh. The unique weather patterns and monster storms remind me of the volatility of this rock we’re living on. The vivid bursts of plants, cacti and wildlife show how life can survive, and flourish, in the harshest environment. And the brief monsoon proves with stark reality how precious our water supply is to our survival.
Good lessons, all, don't you think?
Every season, flash flooding happens and cars, people and even cattle become stranded or washed away.
Sadly, monsoon season also can make a tender box fire season even worse, due to the increased amount of lightning, up to tens of thousands of strikes per night in some areas.
Yes, life in the desert can be harsh. The unique weather patterns and monster storms remind me of the volatility of this rock we’re living on. The vivid bursts of plants, cacti and wildlife show how life can survive, and flourish, in the harshest environment. And the brief monsoon proves with stark reality how precious our water supply is to our survival.
Good lessons, all, don't you think?
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