Al Stahler: Fall rain | TheUnion.com
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Al Stahler: Fall rain

Earth’s tropical regions feel more intense sunshine than regions elsewhere, and soak up serious amounts of solar energy: The tropics get hot. What’s a planet to do, with all that heat, building at the beltline?

Hey! If you’ve got too much of something … find a place to dump it.

An obvious place to dump excess heat is in the arctic, up by the north pole. But how can the planet move all that heat from the tropics to the pole?



Shipping hot rock out of the tropics would be not be easy, nor would it be fast … and if we don’t get all of today’s heat out of the tropics, tomorrow will be even hotter.

As moist air – air that brings rain – moves across the Pacific in autumn, it is jostled – pushed this way and that – by other air masses … masses of air steered by what’s around and what’s beneath them … including the temperature of the water beneath them.

Water is easier to move than rock, and Earth does ship tropical water north, in ocean currents. But that’s still not fast enough.



What remains is the atmosphere – winds blow way faster than water. The tropics can pump heat into the air, then blow that air north.

(I’m writing as if inanimate objects – the planet, and places on it – make decisions … as if they “think.” But heat “wants” to flow from hot to cold, much as a rock “wants” to fall when you drop it – no thought needed.).

How do we move heat from Earth’s surface, into the air?

Just sitting on warm rock … warm vegetation … warm water … air will soak up heat. But it’s a slow process.

When you and I need to get rid of heat – when we’re running, jumping, dancing – we turn on our sweat glands. We coat our skin with water, which quickly evaporates. It takes energy to evaporate water, to turn liquid water to gaseous vapor – think of evaporation as super-slow boiling. As sweat evaporates, it sucks energy – heat – from our skin, and cools us off.

The tropics, too, sweat … sort of. Water evaporates off wet rocks, wet leaves, wet seawater. It takes energy to evaporate all that water, and this sucks heat from the tropics.

The water vapor coming off rocks, leaves, ocean, mixes with the air above. Warm, moist, tropical air then climbs into the sky. Cooling as it climbs, water vapor condenses – it transforms from gas, back to liquid, and forms cloud droplets (the same sort of droplets that form when you breathe out on a cold winter’s day … or breathe onto a mirror).

When water evaporates – when liquid becomes gas – it sucks up heat. When water vapor condenses – when gas becomes liquid – it’s the same trick backwards. When vapor condenses, it releases energy (the exact amount of energy, in fact, it sucked up in evaporating).

This energy, released into the air when vapor becomes liquid, energizes the tropical atmosphere – creates storms that rain down on the tropical rain forests; powers hurricanes; and it is this energy that the atmosphere will carry away from the tropics, toward the pole. The warm, moist tropical air heads north.

Setting out on a long journey, I’m sure you’ve noticed, it’s easy to get distracted, to get turned in another direction. The journey from tropics to pole is, indeed, a long one, with many opportunities to be diverted … diverted, for instance, into California.

Spinning like a carousel, our planet puts its spin on pretty much everything sitting on it. It puts a twist on hurricanes, getting them going counterclockwise. It spins up huge domes of hot air – domes of air we experience as heat waves – giving these airy domes a clockwise spin. The heat wave that baked us last August was the result of a humongous dome of hot air, hanging over western North America, over California … and spinning clockwise.

That hurricanes and high-pressure domes of hot air spin in opposite directions has consequences.

Hold your hands out in front of you, one hand palm up, the other palm down. Slowly spin them, both in the same direction. Very soon, there’s a collision – a smash-up – as the thumb of one hand collides with the little finger of the other. When parcels of air, both spinning in the same direction, meet up, it’s messy – turbulent.

Now put your hands out again, but this time, both palms up. Start them spinning again, but now in opposite directions. Very soon, your two thumbs meet. But now, both thumbs move in the same direction … they meet smoothly. Spinning in opposite directions, air masses can smoothly interact.

Last August, off the coast of Baja, remnants of Hurricane Elida spun counterclockwise … even as the humongous dome of hot air over our heads spun clockwise. When the two masses met, off Baja, there was no problema: The spinning remnants of Elida easily feed moisture … water vapor … energy … into the spinning dome. Reaching the Coast Range, and then the foothills, the energized air generated electrical storms … which sparked fires … the Jones fire.

When will the rains come?

As moist air – air that brings rain – moves across the Pacific in autumn, it is jostled – pushed this way and that – by other air masses … masses of air steered by what’s around and what’s beneath them … including the temperature of the water beneath them.

Sea surface temperatures across the Pacific fall into patterns – warm here, cool there. Typically, winds create a narrow tongue of cold seawater, lying close to the equator, between the tropics of the northern hemisphere and the south: the “cold tongue” (CT).

Every so often – every few years – warm water sloshes east, from the Asian side of the Pacific, submerging the cold tongue. The sea surface between northern and southern tropics grows warm, which changes how the ocean steers the air. Submersion of the CT by warm water signals an El Niño event.

At other times – as warm water sloshes back toward the Asian side of the Pacific – the equatorial cold tongue grows larger … and colder. The opposite of El Niño, this is a La Niña event.

The tropical Pacific has been in a La Niña state since mid-summer … and will likely continue in a La Niña state through the winter. Looking at how similar La Niñas have affected U.S. weather in the past, the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration is forecasting a dry fall for southern California … and a wet fall for the Pacific Northwest.

For Northern California, the CPC sees “EC” – equal chances that the coming fall will be dry … or wet … or something in-between. We’ll have to wait and see.

Tonight, after sunset, the moon will sidle up close to planet Jupiter. A few hours later – once the sky is really dark – planet Mars will rise over the trees in the east. Mars now is close to our planet, and it’s “full” – fully-lit by the sun. And Mars is now showing its south pole to Earth; allowing its south polar ice cap to reflect sunlight back in our direction. Mars glows bright orange tonight, in the dark eastern sky.

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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