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Alan Stahler: Too dry for rain

Rain falling through dry air, evaporating before reaching the ground, is virga.
Photo courtesy UCAR, Carlye Calvin

The weather’s been dry, most of the month, drying the ground, drying the air. But last weekend was forecast to be wet.

For rain to fall from the sky, many things must happen … all at once, or one after another.

Remember walking through fog: Fog droplets are tiny – so tiny, just walking through them stirs them up to fly every which way.



Fog droplets are not rain drops … fog droplets are cloud droplets. Fog is simply a cloud that touches the ground. To make rain, cloud droplets must somehow clump together. It takes around a million cloud droplets to make one rain drop.

 

Alan Stahler

Before we can make rain drops from cloud droplets, of course, we’ve first got to make clouds.



Blow into a freezer, and the moisture in your breath – hitting the cold air – condenses to liquid … liquid cloud droplets. You’ve made a cloud.

Climb a mountain, and the air grows cold. A common way for nature to make clouds is by lifting air upward.

Air does not weigh a whole lot, but it does weigh something. A deadheading 18-wheeler – rolling home empty – hauls over two hundred pounds of air in the trailer.

For some weeks, a mound of air – a “mountain” of air – had parked off the California coast. It weighed heavily – it pressed down – on the sea, and on the nearby land. Such a mountain of air is a high pressure ridge. This air-mountain resembled others that had parked offshore in the past. Hanging on for weeks, such air-mountains have been nick-named “ridiculously resilient ridges.”

Warm air rises; what goes up, must come down. High pressure ridges form when the air that had risen elsewhere, sinks back down.

To make clouds, air must move upward. The downward-moving air of a ridge does the exact opposite of that. Worse, it pushes down on any other air – say, air warmed by the sun-baked ground – that might try to rise upward. It even pushes down on clouds that have already formed, causing them to warm … and evaporate. Ridges are bad news for clouds.

What we want sitting over us is not a high pressure ridge, but a low-pressure trough. Such a low-pressure trough formed last week, out in the ocean, south of Alaska … and was steered, by the jet stream, toward California.

Unlike the air in a ridge, the air in a trough rises – just what we want, to create clouds. Meteorologists last week saw the trough coming toward us, and forecast rain … lots of rain.

Another mountain of air, besides the ridiculously resilient ridge, influences our weather … and it’s not merely ridiculously resilient – it’s semi-permanent … and it’s humongous. This air-mountain settles into place, mid-Pacific, every summer … and blocks pretty much all storms from reaching California.

With spring drawing to a close, time grows tight, before the semi-permanent North Pacific High sets up for the summer. We need every storm we can get. The forecast for this weekend gave hope for a drenching.

As the jet stream carried the trough toward us, it also shoved the ridiculously resilient ridge out of the way. The approaching low pressure of the trough further weakened the ridge. The barrier to rising air – the barrier to making clouds – was gone.

I spoke by phone, Sunday afternoon, with meteorologist Craig Shoemaker of the National Weather Service in Sacramento. As we spoke, a gentle sprinkle fell. I asked Craig, “What happened to the heavy rain that had been forecast?”

He told me that it was raining … fairly hard … right over our heads.

Backpacking the desert, the landscape is open – no pesky forests of trees block your view of the landscape … or the sky.

It does rain in the desert … but not a whole lot. And, more often than not, it might rain here, but not there … and vice-versa. You can hang out in sunshine, and watch rain fall elsewhere, not far away.

But that rain is falling through super-dry desert air – so dry, the raindrops may never reach the ground: Part-way down, the raindrops evaporate. Rain darkens the sky beneath the cloud, but – as the rain evaporates, part-way down –the sky grows light again. Rain that makes it only part-way to the ground is virga. Virga also falls, occasionally, here in the foothills.

Shoemaker explained that several things had happened … were still happening … this past weekend. The mass of moist air, coming in from the Pacific, had not had a straight shot at California. Disturbances in the air, along the way, had dissipated – weakened – the trough. What remained of the trough did make rain, high over our heads. But that rain had to fall through air that had been drying for weeks … and little rain reached the ground. Falling through super-dry air, the rain drops evaporated. Above our heads, was virga.

IN THE SKY TONIGHT

The north star – Polaris, the pole star – is not very bright. What makes it special is that it sits – 24/7 – above the north pole. Thus, looking toward the north star, we’re facing north. This is a good time of year for finding the north star.

The Big Dipper is now high in the sky – almost overhead – as soon as the sky grows dark. Like a spoon, the dipper has a bowl, and a handle. The last two stars of the bowl are “the pointers.” The pointer stars point – pretty closely, if not perfectly – to Polaris.

All the stars we see with the naked eye are part of our Milky Way Galaxy –a hundred, maybe two hundred billion stars, moving together through space.

If you imagine the bowl of the Dipper as a picture-frame … just within that frame … astronomers have found tens of thousands of galaxies.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at a.a.stahler11@gmail.com.


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