Warm air rises and, as it does, it cools and condenses. It then falls but may not reach the surface. That moisture, now warmed and perhaps boosted by more warm air from the surroundings, rises again.
Virga – an observable streak or shaft of precipitation that falls from a cloud but evaporates or sublimes before reaching the ground. (Wikipedia: Virga)
Think donut. The cyclic movement of air, down in the middle, up all around the edges, is what builds a thunderstorm. Over time, it builds in intensity getting bigger, moving more and more moisture up and down, and with higher and higher velocities.
During this process, of course, the water may freeze and turn to snow or sleet. And if it repeats this vertical roller coaster ride, all fed by new incoming moist warm air, it builds up into hail and keeps getting bigger and bigger.
Until one of two things happens:
- the solids become too heavy for the air currents to lift, or
- the bottom of the donut touches the ground.
In either case, the core down draft and all its load pours down onto the ground where, like water poured from a bucket onto the ground, it mushrooms outward.
But with a thunderstorm, there’s a huge amount of air as well as water — it’s the air that has been lifting the water each time around — and that “deluge” of air being poured out is what causes what you see in the picture below.
That is, if the land is dry and the soil loose, that down spout of air will create a haboob and sweep up the dry land, pushing it up and rolling it back on itself in the characteristic reverse sweeping motion.
In wetter climates or where the soil is held in place by grass and protected by trees, you feel and see the effects of the sudden outward rushing of air that precedes the arrival of the precipitation.
Same thing, just no loose dust to make it visible.
A haboob is, technically speaking, a dust storm but only one of several types.
Other dust storms are created by incoming cold fronts sweeping up the dirt, but these tend to be spread out over much larger distances. The dust isn’t as thick because the front isn’t as concentrated as a thunderstorm.
And simple gusty days can also kick dirt into the air and hold it aloft but the drama is even less so. It’s windy and dusty yes, but not as bad as a cold front’s approach and, in comparison, inconsequential to a haboob.
Haboobs can be bad on the ground. Lawn furniture goes flying, outdoor curtains and awnings take a beating, dust coats everything in sight and traffic may even come to a stand still for a minute or two.
Up in the air, however, haboobs are much, much more dangerous, and it’s not because of the dust.
There’s a company in upstate New York I know that makes a “wind shear” detector for airplanes. That’s what the thunderstorm creates near the ground and it is especially dangerous when landing or taking off.
In a nutshell, if you fly into the haboob as it blows outward, your airspeed jumps up. When you reach the center of the mushroom, it returns to normal but with a strong downdraft. But as you fly away from the center, that air becomes a tail wind and steals lift.
And lift is what keeps the airplane up there.
Inside, you’ll feel the plane dropping. A downdraft at 35,000 feet may be unsettling but the pilot has lots of room — about seven vertical miles of it — to recover. But if the plane has descended to 500 feet and slowed in anticipation of landing, that mushrooming air behind the airplane can suddenly remove almost all lift.
The wind shear detector monitors air speed, altitude and the plane’s angle of attack. In a wind shear, the computer in the detector would first see a sudden rise in air speed. This happens as the plane flies nose first into the outward mushrooming air. A few seconds later it then senses the airspeed drop to normal but then continue dropping. If that low airspeed condition persists at a very low altitude, the detector automatically announces “Wind Shear! Add Power! Wind Shear! Add Power!” It also shakes the control stick to be sure it gets the pilot’s attention.
Because at that point, the airplane may become a brick.
And bricks don’t fly.
I was recently in one such landing coming home to Phoenix. The pilot had warned us that dust storms were in the area so we might encounter a few “bumps”. Indeed, as we passed over the Phoenix Zoo less than three miles from the end of the runway, I felt the plane sink. Accustomed to small bumps I waited but, several seconds later we were still sinking.
Looking out my window and knowing the area well, it was clear we weren’t going to make the runway.
Fortunately, warned by the automatic shear detector or just the seat of his pants, the pilot knew what to do. He ran the engines up to full throttle and, in a second or so, that thrust alone seemed to be rocketing us skyward again. (Thank you, Rolls Royce and General Electric, makers of those powerful engines.)
They call it a “missed landing.”
We made a five minute circuit of the city during which we had a clear view of the haboob and its leading yellow edge. Our second attempt at landing was successful. Just a couple of small bumps in the unusually quiet cabin.
Air and water do all this.
Them and that giant energy source called the sun.