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Al Stahler: Come the rains

Mountains play a major role in climate. As moist, winter winds blow into California from the Pacific, the winds are forced upslope. As the moist Pacific air rises, it cools, and the mountains wring out its moisture. Forcing air to rise, mountains coerce the air into dropping rain and snow.

Having rained-out its moisture, the air blowing over the mountains is now dry. Blowing down the leeward (downwind) side – blowing downslope – it’s the same trick backwards: The air warms, and thus dries out yet more. Lying in California’s mountainous rain shadow, Nevada a desert state.

California, too, lies in the shadow of a mountain … though only part of the year … and it’s a very different sort of mountain. The mountain that dries-out California, each summer, floats over the Pacific.

The mountains of California are easily visible – they’re made of rock. But the island floating on the Pacific cannot be seen. This mountain that dries out California, in summer, is made of air – it’s a mountain of air.

Last Tuesday, Earth passed through the equinox – the day when hours of daylight equal hours of night (“-nox” from Latin, “night”). The directly-overhead sun has crossed the equator, into the southern hemisphere. Now we must wait for that mountain of air to follow.

The sun shines brightly over the tropics. Intense sunshine warms the ground, warms the waters. Warm ground, warm water, in turn, warm the air that sits over them.

Warm air rises. Just as moisture is wrung out of the air rising over mountains, moisture is wrung out of the air rising over the tropics. Day after day, afternoon thunderstorms drench the tropical rain forests.

The rains may fall, but the air keeps rising upward … keeps rising, miles into the sky … until it can rise no more. Topping out, the rising air then turns sideways. Now the air flows north and south. To keep things simple, we’ll look only at air flowing north … toward us.

Anything with any amount of warmth emits infrared radiation. Our skin feels infrared, coming off a hot potato, coming off the wires in a toaster. Down here, near the surface, infrared is absorbed – by rocks, by trees, and, in the air, by water, carbon dioxide, and a dozen other greenhouse gases. But miles up, where air flows north from the tropics, the air is thin – too thin to breathe – too thin to keep infrared from escaping, out into space. As it emits infrared energy, the north-flowing air cools down.

Warm air rises, but cool air sinks. Like air flowing downslope on the Sierra’s east side, the sinking air grows warm. And, like the air that dries out Nevada, the sinking, warming air sucks up moisture. Reaching the ground, this warm, dry air – air that had started out in the tropics – this warm, dry air creates great deserts: the Saharan Desert of Africa … the Chihuahuan Desert of Mexico.

And this sinking air piles up. It builds a mountain of air, over the Pacific, large enough to keep summer storms out of California.

The tropics are bathed in sunlight, but some tropical regions are bathed more strongly than others. It depends on the season.

Summer happens when the sun climbs high in the sky. In the northern tropics – just north of the equator – the summer sun doesn’t just climb high … the sun can climb straight overhead – making the northern tropics very efficient at sending warm air up into the sky, and then northward, where the air will cool, sink, and build that mountain of air.

In winter, though, the sun heads south, warming most intensely the southern tropics. Again, warm air rises, and then turns north and south. But starting out so far south, the air cannot reach so far to the north – cannot build its mountain of air, so far to the north. As the sun heads south in winter, the mountain of air follows the sun – it, too, heads south.

No longer blocked by that mountain of air, winter storms – Hallelujah! – can reach California.

Lots of variables go into “deciding” how big that mountain of air will grow. One factor is the climate over the tropics (which itself can be tweaked by many factors affecting temperature and moisture – most immediately, how much tropical rain forest gets chain-sawed). Lots of variables also go into how far north the mountain of air builds up, and when it starts moving south. The sooner the mountain heads south, the sooner our winter storms arrive.

Days now are growing shorter, nights, longer. Last Tuesday, Earth passed through the equinox – the day when hours of daylight equal hours of night (“-nox” from Latin, “night”). The directly-overhead sun has crossed the equator, into the southern hemisphere. Now we must wait for that mountain of air to follow.

In the sky, as soon as the sky grows dark (which it does quickly now, close to the equinox):

Thursday night: Moon is close to planet Jupiter.

Friday night: Moon is close to Saturn.

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


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