Dry times

If I asked you to name the driest spots on Earth, I wonder if this one would come to mind -- even though it's a top contender for the number one spot.

You might have thought of Chile's Atacama Desert, or possibly somewhere in the Gobi, Sahara, or the Rub' al Khali (Empty Quarter) of Saudi Arabia.  All good guesses, and certainly they're not what I'd call wet climates.  In fact, parts of the Atacama come in second; the high elevation and perpetual clear skies are why it's such a great spot for astronomical observatories -- it's currently home to three of the best, and a fourth is being built.  The La Silla Observatory, the Paranal Observatory (which includes the Very Large Telescope), the Llano de Chajnantor Observatory (which hosts the ALMA international radio observatory), and the Cerro Armazones Observatory (site of the future Extremely Large Telescope), are all in the Atacama Desert.

As an aside, can astronomers please try to come up with better names for their observatories?  I mean, what the hell?  The "Very Large Telescope" and the "Extremely Large Telescope"?  What's next, the "Abso-fucking-lutely Humongous Telescope, No Really I'm Totally Serious You Won't Believe How Big It Is"?

Probably not.  AflHTNRITSYWBHBII would be hard to fit on a grant application.

But I digress.

Anyhow, the top spot for the driest climate on Earth is the McMurdo Dry Valley region of Antarctica, and beats most of the other possibilities by a significant margin.  Some studies indicate the place hasn't had any significant accumulated precipitation in over two million years.  What small amount does fall -- estimates are in the range of a hundred millimeters per year -- almost all evaporates before it reaches the ground because of the fierce katabatic winds.  Katabatic winds occur because air density is strongly dependent upon temperature, and the McMurdo Dry Valleys are surrounded by mountains.  Air masses above the mountaintops lose heat faster, making them become more dense; the air then flows downhill, easily reaching hurricane speed, and pools in the valleys.  Most of the air already started out dry; any humidity it originally had was precipitated out as snow on the windward side of the mountains.  This drops the relative humidity to only a few percent and keeps it there.

Any snowflakes falling into that don't stand a chance.  They don't melt; it's too cold for that.  They sublimate -- turn from a solid to a gas without passing through the liquid phase.

That's how cold and dry it is.

The result is that the McMurdo Dry Valleys are basically nothing but a vast expanse of extremely cold rock, gravel, and sand.

[Image licensed under the Creative Commons Dturme, Labyrinth, Wright Valley, McMurdo Dry Valleys, Antarctica, CC BY-SA 4.0]

The exposed rocks are mostly of Triassic age, and belong to the Beacon Formation, which is largely made of sandstone.  There are a few volcanic intrusions only a few million years old, but by and large, the whole place is just one big bunch of very old wind-eroded sandstone, quartzite, and pebble conglomerate.

And yet... there are living things there.

Not many, of course, but the McMurdo Dry Valleys are home to endolithic bacteria, which live in the cracks and fissures inside rocks, subsisting on the minerals therein and the tiny amount of water in the soil (supplemented from time to time by trickles of glacial meltwater).  They're still poorly understood, but are thought to be metabolically similar to the mid-ocean vent bacteria, which are able to use minerals like sulfur, iron, and manganese as the basis of their metabolism.

All of which makes me wonder if Mars hosts life.  McMurdo has been described as "the most Mars-like environment on Earth;" the site has been used to test equipment for the Mars rover missions.  Hell, if bacteria can survive in McMurdo, it's not much of a stretch to surmise that there might be life underground on Mars -- perhaps a holdover from the distant past, when Mars was a much warmer, wetter place.

I find places like this fascinating.  The idea that we have here on our (mostly) temperate and green planet a spot so profoundly inhospitable is pretty astonishing.  I wonder how (or if) climate change will alter things there?  The entire continent is climatically isolated by the Antarctic Circumpolar Current, one of the hugest oceanic water transporters in the world -- the amount of water flowing through the Drake Passage, between South America and Antarctica, is estimated at around 130 times the volume of all the world's rivers put together -- so it's hard to imagine this shifting in any significant way.

But given that many oceanographers fear that meltwater from Greenland is going to block the Atlantic Meridional Overturning Circulation -- the best-known part of which is the Gulf Stream -- maybe I shouldn't speak too soon.

So that's our look at the Earth's answer to Mars.  Not, I'm afraid, a locale I'm eager to visit, given how little I like the cold.  I'm adventurous, but I draw the line at a place that hostile.

Plus, I like rocks as much as the next guy, but when there's nothing else to see -- well, I can think of a few other places that are higher on the destinations list.  I'm content to appreciate McMurdo from afar.

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Ill winds

When you think about it, wind is a strange phenomenon.

In its simplest form, wind occurs when uneven heating of the surface of the Earth causes higher pressure in some places than in others, and the air flows from highs to lows.  But it's considerably more complex (and interesting) than that, because as surface-dwellers we often forget that there's a third dimension -- and that air can move vertically as well as horizontally.

I got to thinking of this because I've been reading Eric Pinder's fascinating, often lyrical, book Tying Down the Wind: Adventures in the Worst Weather on Earth.  Pinder is a meteorologist who was stationed as a weather observer on Mount Washington, New Hampshire, which one in every three days clocks hurricane-force winds (greater than 119 kilometers per hour) and is the spot that holds second place for the highest anemometer-clocked wind speed ever recorded on the Earth's surface (an almost unimaginable 372 kilometers per hour; the only higher one was on Barrow Island, Australia, which on April 10, 1996, during Cyclone Olivia, hit 407 kilometers per hour).

The fact that air moves vertically, of course, is why air moves horizontally.  When the Sun heats a patch of ground, the air above it warms and becomes less dense, causing it to rise.  This creates an area of low pressure, and air moves in from the side to replace the air moving upward.  This process, writ large, is what causes hurricanes; the heat source is the ocean, and the convection caused by that tremendous reservoir of heat energy not only generates wind, but when the water-vapor-laden air rises high enough, it undergoes adiabatic cooling, triggering condensation, cloud formation -- and torrential rain.

The process can go the other direction, though.  A weather phenomenon that has long fascinated me is the convective microburst, something that most often happens in hot, dry climates in midsummer, like the American Midwest.  The process goes something like this.  Rising air triggers cloud formation, and ultimately rain clouds.  When the droplets of water become heavy enough that the downward force of gravity exceeds the upward force of the air updrafts, they fall, but they drop into the layer of warm, dry air near the surface, so they evaporate on the way down, often not making it to the ground as rain.  Evaporation cools the air that surrounds them, making it denser -- and if the process happens fast enough, it creates a blob of air so much denser than the air surrounding it that it literally falls out of the sky, hits the ground, and explodes outward.  Windspeeds can go from nothing to 100 kilometers per hour in a matter of fifteen seconds.  Then -- a couple of minutes later -- it's all over, the dust (and any airborne objects) settle back to Earth, and everyone in the vicinity staggers around trying to figure out what the hell just happened.

A convective microburst in Nebraska [Image licensed under the Creative Commons Couch-scratching-cats, Downburst 1, CC BY-SA 4.0]

Microbursts aren't the only weird weather phenomenon having to do with density flow.  Have you heard of katabatic winds?  If you haven't, it's probably because you live in an area where they don't happen, because they're really dramatic where they do.  Katabatic winds (from the Greek κατάβασις, "falling down") occurs when you have significant chilling of a layer of air aloft -- on top of a mountain, for example, or (even better) over an ice sheet.  This raises the density of the air mass, creating a huge difference in gravitational potential energy from high to low.  The superchilled air pours downward, funneling through any gaps in the terrain; the effect is accentuated when there's a low pressure center nearby.  The katabatic winds off Antarctica (nicknamed "Herbies," for no reason I could find) and the ones off Greenland (known by the Inuit name piteraq) can be unpredictable, fast, and frigid, often driving layers of snow horizontally and creating sudden whiteout conditions.

Then there's the foehn (or föhn) wind, created when onshore air flow is pushed up against a mountain range.  This occurs in the southern Alps, central Washington and Oregon, parts of Greece and Turkey, and south-central China.  On the windward side of the mountains, the air rises and cools; this causes condensation and higher rainfall.  But when the air piles up and gets pushed over the mountain passes, it warms for two reasons -- the pressure increases as it goes downhill on the other side, and the condensation of water vapor releases heat energy.  The result is a warm, dry wind that pours downhill on the leeward side of the mountains -- the source of the "Chinook winds" that desiccate the northwestern United States east of the Cascades.

Interestingly, foehn winds are associated with physiological problems -- headaches, sinus problems, and mood swings.  It's documented that prescriptions for anxiolytic medications go up when the foehn is blowing; and a study at the Ludwig Maximilians Universität München found that suicide and accident rates both go up by about ten percent during periods when there's a strong foehn, and no one knows why exactly.

In any case, there are a few interesting tidbits about a phenomenon we usually don't think about unless we're in the path of a hurricane or tornado.  Something to think about next time your face is brushed by a warm breeze.  We live at the bottom of a layer of moving fluid, driven by invisible forces that usually are benign.  Only occasionally do we see how powerful that fluid can be -- preferably, from a safe distance.

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