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Al Stahler: Blowing bubbles over our heads

Air flowing from north and south meets – converges – near the ground in the ITCZ (Inter-Tropical Convergence Zone), then rises to form the clouds which rain down on the tropical rain forests.
GOES Project Science Office

If it’s to warm the air, the sun has to first warm the ground. But winter days are short, and the sun never climbs very high in the sky. Short days and low sun make for cold days.

Down in the tropics – close to the equator – winter barely makes a difference. Days are long, and the sun climbs high. The tropics suck up sunlight and grow warm. But bathing in all that sunshine, the tropics should broil. How does our planet cool its tropical regions?

What do you or I do when we’ve got too much of something? We ship it out – haul it from here, over to there.



Warm air rises. Look out over a sun-baked parking lot to see warm air rising. As light moves between bubbles of warm air rising and cold air sinking, it bends: The air shimmers. Warm air rising, cool air sinking also makes stars twinkle.

Huge bubbles of warm air rise upward, over the tropics. As the bubbles rise, they cool. Just as water vapor in your breath condenses into tiny droplets when you blow into a freezer, water vapor in the rising air condenses to form clouds … which rain down onto the tropical rain forests.



The huge bubbles of warm air continue rising over the tropics, higher and higher, until the bubbles hit a ceiling – a layer of warmer air that dictates: “This high, but no higher.” Unable to move upward, the air moves sideways – it moves north.

The tropics thus toss humongous balls of air toward the north – inviting northern regions to play catch.

What goes up … must come down. Air, tossed northward from the tropics, sinks. A lot of air sinks over the Pacific.

Besides the fact that the tropics are tossing balls made of air, something else distinguishes this game of catch from the game we play in our backyard. Our planet spins – spins like a top. So this game of catch is played on a carousel.

Play catch on a carousel, and every pitch becomes a curve ball. Air that started out moving from south to north – from the tropics to mid-Pacific – swerves toward the east. The wind no longer blows south-to-north, but west-to-east.

Light some incense – or blow out a candle – and watch the smoke rise. (The “smoke” from the blown-out candle is actually tiny droplets of wax, condensing from the wax-gas rising off the wick – best not to breathe the waxy cloud).

The smoke, at first, rises smoothly. But, just a few inches up, the smoke stream becomes turbulent – it breaks up into swirling curlicues, dancing this way and that.

Air blowing west-to-east over the Pacific starts out smoothly … but soon becomes turbulent – it still blows toward California, but in curlicues, swirling this way and that.

As the smoke from the incense swirls, the curlicues carry air from right to left, left to right. Curlicues in the airstream blowing over the Pacific carry air from north to south, south to north. The curlicues thus continue the process of carrying tropical heat north, even as they carry colder, northerly air south.

By mixing warm air from the south with cold air from the north, the swirling air sets up a situation similar to blowing into a freezer. As warm air meets cold, water vapor condenses, forming clouds. With luck, these cloudy curlicues evolve further … into full-blown storms, bringing rain and snow to California.

But not … in … summer. In summer, the tropics toss so much air at us – so much air sinks offshore, over the ocean – it blocks the curlicues heading for California. The mound of air pushes the curlicues north, to water the temperate rain forests of the northwest.

Come winter, though, and the tropics lighten up on how much air they toss north, allowing the mound of air to shrink … to drift south … allowing curlicues over the Pacific to reach California.

One more kicker … actually, lots of kickers. The game of catch, between the tropics and the Pacific, is not the only game in town. Many masses of air and water also play catch. The games they play have various names. The most famous games, no doubt, are El Niño, and El Niño’s big sister: La Niña. A lively game of La Niña is now playing out in the tropical Pacific, sending warm water here, cold water there … knocking the usual game of catch – tropics to mid-Pacific – into a cocked hat. When La Niña is in play, California’s winter is often dry.

As I write this, early in the week, the National Weather Service in Sacramento has forecast a curlicue to blow over the Pacific northwest, bringing rain to northerly regions … and maybe a bit of rain here, as the very edge of the curlicue brushes over the foothills.

After that, NWS in Sac (and NWS’s Climate Prediction Center) forecast few curlicues blowing our way. With smooth airstreams, and lots of sunshine, it’s gonna get warm.

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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