Planetary Pastry, Third Course

The Al’s Coffee Shop Astronomy gang is still discussing Jupiter’s Great Red Spot.  Cathleen‘s holding court, which is natural because she’s the only for-real Astronomer in the group…  “So here’s what we’ve got.  The rim of the Great Red Spot goes hundreds of miles an hour in the wrong direction compared to hurricanes on Earth.  An Earth hurricane’s eye is calm but the Jupiter Spot’s rim encloses a complex pattern of high winds.  Heat transport and cloud formation on Earth are dominated by water, but Jupiter’s atmospheric dynamic has two active players — water and ammonia.”

“Here’s your pastries, Cathleen.  I brought you a whole selection.  Don’t nobody sneeze on ’em, OK?”

“Oh, they’re perfect, Al.  Thanks.  Let’s start with this bear claw.  We’ll pretend it’s the base of the weather column.  On Earth that’d be mostly ocean, some land surface and some ice.  They’re all rough-ish and steer air currents, which is why there’s a rain shadow inland of coastal mountain ranges.”pastries 2

“Jupiter doesn’t have mountains?”

“We’re virtually certain it doesn’t, Sy.  The planet’s density is so low that it can’t have much heavy material.  It’s essentially an 88,000-mile-wide ball of helium-diluted liquid hydrogen topped by a 30-mile-high weather column.  Anything rocky sank to the core long ago.  The liquid doesn’t even have a real surface.”

<Al and Sy> “Huh?”

“Jovian temps are so low that even at moderate pressures there’s no boundary between gaseous and liquid phases.  Going downward you dive through clear ‘air,’ then progress through an increasingly opalescent haze until you realize you’re swimming.  Physicists just define the ‘surface’ to be the height where the pressure is one atmosphere.  That level’s far enough down that water and ammonia freeze to form overlying cloud layers but hydrogen and helium are still gases.  It could conceivably look like home there except the sky would be weird colors and you don’t see a floor.”

“If the boundary is that blurry, it’s probably pretty much frictionless — weather passes over it without slowing down or losing energy, right?”


“So there’s way too much slivered almonds and stuff on that bear claw. On this scale it ought to have a mirror finish.”

“Good point.  But now we can start stacking weather onto it.  Here’s my doughnut, to represent the Great Red Spot or any of the other long-lived anticyclones.”

“Auntie who?”

“A-n-t-i-cyclone, Al.  Technical term for a storm that disobeys the Coriolis theory.”

“Uh-HUH. So why’s it do that?”

“Well, at this point we can only go up one level in the cause-and-effect chain.  <pulling out smartphone>  NASA’s Voyager 1 spacecraft sent back data for this this wonderful video

Jupiter seen by Voyager 1 probe with blue filter in 1979. One image was taken every Jupiter day (approximately 10 hours).  Credit: NASA

“Basically, the Spot is trapped between two jet streams, one going westward at 135 mph and the other going eastward at 110 mph.  I’ll use these biscotti to represent them.pastries with arrows

“Hey, that’s like a rack-and-pinion gear setup, with two racks and an idler, except the idler gear’s four times as wide as the Earth.”

“A bit less than that these days, Sy.  The Spot’s been shrinking and getting rounder.  Every year since 1980 it’s lost about 300 miles east-west and about 60 miles north-south.  As of 2014 it was about 2.8 Earth-widths across.  And no, we don’t know why.  Theories abound, though.”

“What’s one of them?”

“Believe it or not, climate change.  On Jupiter, not Earth.  One group of scientists at Berkeley tackled a couple of observations

  • Unlike Earth, which is much hotter near the Equator than near the poles, Jupiter’s Equator is only a few degrees warmer than its poles.
  • Three persistent White Ovals near the Great Red Spot merged to form a single White Oval that recently turned red but only around the edges.

Their argument is long, technical and still controversial.  However, their proposal is that merging the three ovals disrupted the primary heat transport mechanism that had been evening out Jupiter’s temperature.  IF that’s true, and if it’s the case that Jupiter’s jet streams are powered by heat transport, then maybe disrupted heat patterns are interfering with  the Great Red Spot’s rack-and-pinion machine.  And maybe more.”

“Big changes ahead for the Big Planet.”


~~ Rich Olcott

Planetary Pastry, Second Course

We’re still sitting in Al’s coffee shop.  “OK, Cathleen, so Jupiter’s Great Red Spot acts like a hurricane turned inside-out.  Where’s the problem?”

“Just that it goes completely against all the computer models we’ve built to understand and predict hurricane activity.  It’ll take a whole new generation of even more complicated models for Jupiter-like planets.”

“Here’s the doughnuts you asked for, Cathleen.”

“Thanks, Al.  Perfect timing. <drawing on a paper napkin>  Let’s look at hurricanes first, OK, Sy?”


“We’ll start with this doughnut that I’ve just taken a bite out of.  First thing that happens is that warm ocean water heats up the overlying air.  Warmed air rises, so we’ve got an updraft.”

“And then?”

“The rising air is humid (ocean air, remember?).  As it rises it cools and forces moisture to condense out.  Upward flow stops when the warmed air hits the top of the troposphere.  But there’s still more warm air pushing up the plume.  The cooled air has to go somewhere so it spreads out.  That’s where these red arrows on my paper napkin go horizontal.  The cooled air, loaded with water droplets, is heavy so it starts sinking which is why the red arrows turn downward.  They move back across that ocean water again ’cause they’re caught in the inflow.  Full cycle and that’s number 1 here, got it?”


“Hey, Cathleen,  are you gonna need more paper napkins?”Donuts 1
“A couple should be enough, Al, thanks.  Now we get to number 2, the Coriolis thing. That’s always tough to talk students through but let’s try.  The Earth rotates once every 24 hours, right, and its circumference at the Equator is 25,000 miles, so relative to the Sun anything at the Equator is flying eastward at about 1,000 miles per hour.  Any place north of the Equator has to be going slower than that, and further north, even slower.  With me, Sy?”

“Gimme a minute … OK, I suppose.”

“Good.  Now suppose a balloon is floating in the breeze somewhere south of that rising plume.  Relative to the plume, it’ll have eastward momentum.  Now the balloon’s caught in the plume’s inflow but it doesn’t go straight in because of that eastward momentum.  Instead it’s going to arc around the plume.  See how I’ve got it coming in off-center?  Al, would that be clockwise or counterclockwise if you’re looking down from a satellite or something?”

“Umm … counterclockwise, yeah?”

“Mm-hm.  What about a balloon that starts out north of the plume?”

“Uhh … It’ll be going slower than the plume, so the plume gets ahead of it and it’ll arc … hey, counterclockwise again!”

“How ’bout that?  Anywhere in the northern hemisphere, air flowing into a low-pressure region will turn it counterclockwise.  As the inflow draws from greater distances, there’s a greater speed difference to drive the counterclockwise spin.  So that’s number 2 here.  Add those two cycles together and you’ve got number 3, which spirals all around the doughnut.  And there’s your hurricane.”

“Cool.  So how does that model not account for the Great Red Spot?”

“To begin with, the Spot’s in Jupiter’s southern hemisphere so it ought to be going clockwise which it definitely is not.  And there’s no broad band of surrounding clouds — just a lot of structure inside the ring, not outside.  There’s something else going on that swamps Coriolis.”

“So how’s Jupiter different from Earth?  Besides being bigger, of course.”

“Lots of ways, Sy.  You know how labels on healthcare products divide the contents into active ingredients and inert ingredients?  The inert ones just carry or modify the effects of the active ones.  Atmospheres work the same way.  On Earth the inert ingredients are nitrogen and oxygen…”

“Hey, oxygen’s important!”

“Sure, Al, but not when you’re modeling air movement.  The important active ingredient is water — it transports a lot of heat when it evaporates from one place and condenses somewhere else.  The biggest outstanding problem in Earth meteorology is accounting for clouds.”

“You’re gonna tell us that Jupiter’s inactive ingredients are hydrogen and helium, I suppose.”

“Precisely, Sy.  Jupiter has two active ingredients, water and ammonia, plus smaller amounts of sulfur and phosphorus compounds.  Makes for a crazy complicated modeling problem.  I’m going to need more pastries.”

“Comin’ up.”


~~ 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.

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

The Force(s) of Geometry

There’s a lot more to Geometry than congruent triangles.  Geometry can generate hurricanes and slam you to the floor.

It all starts (of course) with Newton.  His three laws boil down to

Effect is to Cause as Change of Motion is to Force.

They successfully account for the physical movement of pretty much everything bigger than an atom.  But sometimes the forces are a bit weird and it takes Geometry to understand them.

Side forceFor instance, suppose Fred and Ethel collaborate on a narwhale research project.  Fred is based in San Diego CA and Ethel works out of Norfolk VA. They fly to meet their research vessel at the North Pole. Fred’s plane follows the green track, Ethel’s plane follows the yellow one.  At the start of the trip, they’re on parallel paths going straight north (the dotted lines).  After a few hours, though, Ethel notices the two planes pulling closer together.

Ethel calls on her Newton knowledge to explain the phenomenon.  “It can’t be Earth’s gravity moving us together, because that force points down to Earth’s center and this is a sideways motion.  Our planes each weigh about 2000 kilograms and we’re still 2,000 kilometers apart.  By Newton’s F = G m1m2/r2 equation, the gravitational force between us should be (6.7×10-11 N m2/kg2) x (2000 kg) x (2000 kg) / (2,000 m)2 = 6.7×10-11 newtons, way too small to account for our speed of approach.  Both planes were electrically grounded when we fueled up, so we’re both carrying a neutral electric charge and it can’t be an electrostatic force.  If it were magnetic my compass would be going nuts and it’s not.  Woo-hoo, I’ve discovered a new kind of force!”

See what I did there?  Fred and Ethel would have stayed a constant distance apart if Earth were a cylinder.  Parallel lines running up a cylinder never meet.  But Earth is a sphere, not a cylinder.  Any pair of lines on a sphere must meet, sooner or later.  Ethel’s “sideways force” is a product of Geometry.

Images extracted from NOAA’s SOS Explorer app, available from

Hurricanes, too.  This video shows a day in the life of Hurricane Sandy.  Weather geeks will find several interesting details there, but for now just notice the centers of  counter-clockwise rotation (the one off the Florida coast is Sandy).  Storm centers in the Northern Hemisphere virtually always spin counterclockwise.  Funny thing is, in the Southern Hemisphere those centers go clockwise instead.

The difference has to do with angular momentum.  We could get all formal vector math here, but the easy way is to consider how fast the air is moving in different parts of the world.

We’ve all seen at least one ice show act where skaters form a spinning line. The last skater to join up (usually it’s a short girl) has to push like mad to catch the end of that moving line and everyone applauds her success. Meanwhile the tall girl at the center of the line is barely moving except to fend off dizziness.

YellowknifeThe line rotates as a unit — every skater completes a 360o rotation in the same time. Similarly, everywhere on Earth a day lasts for exactly 24 hours.

Skaters at the end of the line must skate faster than those further in because they have to cover a greater distance in the same amount of time.  The same geometry applies to Earth’s atmosphere.  The Earth is 25,000 miles around at the equator but only 12,500 miles around near the latitude of Whitehorse, Canada.  By and large, a blob of air at the equator must move twice as fast as a blob at 60o north.

chain 2Now suppose our speedy skater hits a slushy patch of ice.  Her end of the line is slowed down, so what happens to the rest of the line?  It deforms — there’s a new center of rotation that forces the entire line to curl around towards the slow spot.  Similarly, that blob near the Equator in the split-Earth diagram curls in the direction of the slower-moving air to its north, which is counter-clockwise.

In the Southern hemisphere, “slower” is southward and clockwise.

If not for Geometry (those differing circle sizes), we wouldn’t have hurricanes.  Or gravity — but that’s another story.

~~ Rich Olcott