Icing on The Brownie

“So what you’re saying, Sy, is that Jupiter’s white stripes are ammonia snow clouds that go way up above a lower layer of brown clouds like the white icing stripes I put on my brownies.”

“That’s what I’m saying, Al.”

“But why stripes? We got white clouds here on Earth and sometimes they’re in layers but they don’t make stripes.”

“Well, actually they do, but you need the long-term picture to see it. Ever notice that Earth’s forests and deserts make stripes?”

“How ’bout that? I guess they do, sorta. How’s that work?”

“It took us five hundred years to figure out the details. Quick summary. Sunlight does its best year-round heating job at the Equator, where the oceans humidify the air. Warm air rises. Rising warm air cools, releases its moisture as rain, and you get a rainforest belt. The cooled, dried air spreads out until it sinks at about the 30th parallels north and south. Dry air sucks moisture out of the land as it returns to the Equator and you get desert belts. Repeat the cycle. More loops like that center around both 60th parallels. The pattern’s not completely uniform because of things like mountain ranges that block some of the flows. Basically, though, as the years accumulate you get stripes.”

“Jupiter does that, too, huh?”

“On steroids. In one way it’s simpler — no underlying continents mess things up. On the other hand, Jupiter’s got more than a hundred times Earth’s surface area so there’s room for more loops. Also, Jupiter’s interior is still shedding a lot of heat, almost as much as the planet gets from the Sun. Here’s a diagram on Old Reliable.”

“So you’re saying that the upward loops push Jupiter’s atmosphere to where it’s colder and those white ammonia snow clouds form. Then the downward loops move the clouds to where it’s warmer and the ammonia evaporates to show us the brown stuff. Makes sense. But what’re those side-to-side arrows about? We got anything like those on Earth?”

“Sort of, a little bit. Remember the Coriolis force?”

“Uhh, that’s what makes hurricanes go round and round, right? Something to do with the Equator running faster than places further north or south?”

“That’s the start of it. The Earth as a whole rotates 360° eastward in 24 hours, but how many miles per hour that is depends on where you are. The Equator’s about 25000 miles long so Quito, Ecuador on the Equator does a bit more than 1000 miles per hour. Forty-five degrees away, the 45th parallels are only 70% as long as that, so Salem, Oregon and Queenstown, New Zealand circle 70% slower in miles per hour. Suppose a balloon from Salem travels south as seen from space. As seen from the Equator, the balloon appears in the northeast rather than straight north. Winds work the way that balloon would. All around the world, winds between 10° and 30° north and south come from an east-ish direction most of the time.”

“What about the winds right at the Equator? You’d think the northerly part and the southerly part would cancel each other out.”

“That’s exactly what happens, Al. We’ve got a more-or-less equatorial belt of thunderstorms from humid air cooling off as it goes straight up, but not much of a prevailing wind in any direction — that’s why the old sea captains called the region ‘the doldrums’.”

“An equator belt like Jupiter’s, eh?”

“Not quite. Jupiter has a lovely white equatorial zone all right, but that one doesn’t stand still. It roars eastward, 300 miles per hour faster than the equator’s own 28000 miles per hour. All Jupiter’s white zones move east at a pretty good clip. Its dark belts sprint westward at their own hundred-mile pace. Then there’s the jet streams that run between neighboring bands, and lots of big and little vortices carried along for the ride. The planet’s way too segmented and violent for Coriolis forces to build up enough to play a part. The scientists have a couple of heavily-simplified models, but nowhere near enough data or computer time to fill them in.”

“Earth’s atmosphere is messy enough, thanks. My brain’s hurting.”

Voyager I video of Jupiter, processed by JPL,
from Wikimedia Commons

~~ Rich Olcott

The Titanic Winds of Titan (And Venus)

Last week we saw that the atmosphere of Saturn’s moon Titan wasn’t quite as weird as we thought.  But there another way it’s really weird, completely unlike Earth but yet very much like Venus.  Titan’s a superrotater, a world whose atmosphere circles the planet much faster than its surface does.

Let’s start with a relatively simple Earthside phenomenon, a hurricane.  Warm air rises, right?  When the warmth comes from bathtub-temperature sea-water, it’s wet warm air.  As the air rises it cools and releases the moisture as rain.  But the air can’t just keep rising forever or we’d squirt out all our atmosphere. So where does it go?

From a physicist’s perspective, that’s the key question.   If we can track/predict the path of a small parcel of air molecules through a weather system, then we’ve got at least a rough understanding of how that system works.

For the past half-century, atmosphere physicists have been engaged on a project grandly entitled the General Circulation Model (GCM), a software mash-up of the Ideal Gas Law, Newton’s Laws of motion, thermodynamic data for solid/liquid/gas transformations, the notoriously difficult Navier-Stokes equations for viscous fluids, and careful data management for input streams from thousands of disparate sources.  Oh, and it’s important that the Earth is a rotating spheroid rather than a flat plane.

hurricane-en-svg
How a tropical cyclone works
Illustration by Kevin Song, from Wikimedia Commons

Kevin Song’s diagram summarizes much of what we know about hurricanes.  An air packet rises until it hits the tropopause (the top edge of the troposphere), then expands horizontally.  While the packet’s spreading out, the planet’s rotation generates Coriolis “forces” that bend straight-line radial paths into the spirals we’ve seen so often in satellite photos.

A hurricane may look big on your weathercaster’s screen, but it’s less than 0.1% of Earth’s surface area.  Nonetheless, many of the same principles that drive a hurricane underlie global weather patterns.

wind-cells-and-jets-2Air warmed by the equatorial Sun rises, only to sink as it heads poleward.  Our packet loops between the Equator and about 30ºN (see the diagram).

Actually that loop is a slice through a big doughnut that stretches all the way around the Earth.  Another doughnut lies southward just below the Equator.  Two more pairs of doughnuts reside polewards of those as indicated by the other arrows in the diagram.  The doughnuts act like a set of interlocking gears, each reinforcing and moderating the motion of its neighbors.

Thanks to the same geometric phenomenon that spins a hurricane, air packets in these doughnuts don’t loop back to the points they started from.  The Earth turns under the packets as they journey, so each packet takes a spiraling tour around the planet.

Because of all those doughnuts, on average Earth wears a set of cloud-top necklaces.  Regions within 15º of the Equator are rain-forested, as are the Canadian and Siberian forested belts near 60ºN.  The world’s most prominent deserts cluster beneath the dry downdrafts near the 30º latitudes.  Jupiter, “the Easter egg planet,” gets its pink and blue bands from similar doughnuts except that Jupiter has room for many more of them.

Those green circles in the diagram are important, too.  They also represent Earth-circling doughnuts, but ones whose winds flow parallel to Earth’s surface rather than perpendicular to it.  The ones close to the surface give rise to the trade winds.  The high-altitude ones are the jet streams that steer storm systems and give the weathercasters something to talk about, especially in the wintertime.

Jet streams flow briskly — 60 to 200 mph, on a par with a middling hurricane.  Here’s a benchmark: Earth’s equatorial circumference is 25,000 miles, so Ecuadorian palm trees circle the planet at (25000 miles/24 hours)=1041 mph.  Our jet streams go about 15% of that.  Theory and GCM agree that the jets are powered by the Coriolis effect — spiraling air packets in the primary donuts cooperate to push jet stream air packets like oars on a galley ship.  That adds up.

Titan and Venus can’t possibly work that way.  Both of them rotate much more slowly than Earth (Titan about 30 mph, Venus only 4), so Coriolis forces are negligible.  But Titan’s jet streams do 75 mph and Venus’ race at 185.  What powers them?  The physicists are still arguing.

~~ Rich Olcott