Islands Rippling in the Ocean, FROM SPAAAACE (Photo)

The entire universe in blog form
May 11 2014 7:30 AM

Islands Awake!

Relativity is a funny thing. And here I don’t mean Einstein’s relativity, which is both funny and bizarre. No, what I’m talking about is Galilean relativity, where two objects are moving relative to one another, and you can change your frame of reference between them. You experience this all the time; in a car it seems like the scenery is moving past you, not that you’re moving. The scenery thinks the same thing (assuming scenery can think).*

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

For example, when a ship sails through the water, the bow pushes through the fluid and you get a wake, that V-shaped flow as the water gets pushed to the side. That’s even called a bow wave, just to be clear about it.


But you can get the same effect if the ship stays still and the water flows past it. In both cases the physics is the same, so you can look at it either way. And the fluid doesn’t have to be water; it could be, oh, let’s say air. Air flows, so it’s technically a fluid. And if you get some big stationary object with air flowing past it, we should get a bow wave, right?

Right. You do. And in fact it can even be a little more complicated, too, which means you can get some pretty astonishing phenomena, like ship waves forming downwind of the Crozet Islands in the southern Indian Ocean:

Crozet islands
Ripply clouds form downwind of the Crozet Islands due to fun atmospheric physics.

Photo by Jeff Schmaltz/LANCE/EOSDIS MODIS Rapid Response Teamat NASA GSFC

Pretty cool, isn’t it? In this image taken by NASA’s Terra Earth-observing satellite, the islands are outlined so you can see where they are. The wind is blowing to the upper right in that picture, and you can see the V-shaped waves of air as it flows past the islands.

But what about those ripples? Ah, that’s because we have to remember the air is flowing in three dimensions. It gets pushed around the islands, but also up, over them, too (the islands are volcanic, and have peaks hundreds of meters above sea level). The air oscillates up and down as it flows downwind, much like water does as it passes an obstacle.

And in this case the air has moisture in it. When the air moves up it expands and cools. When it cools its ability to hold water vapor weakens, and so water droplets condense, forming clouds. As it sinks it warms up, the water evaporates again, and the clouds go away. Up it goes again, forming clouds, then down it sinks again, and the clouds disappear. This can go on for a long way; in this case, hundreds of kilometers downwind.

In the picture you can see three big islands, and three big wakes. But at the lower left, you can see a fainter wake. Follow it to the head, and you’ll see a tiny island (Île des Pingouins—Penguin Island) just a few kilometers across there. I imagine that would be totally invisible in this image if it weren’t for the huge (though subtle) trail it creates.

I actually see this effect quite often from my house, east (downwind) of the Rocky Mountains, for exactly the same reason. The ripples can sometimes be seen clear across the sky, and it’s pretty amazing. Hmmm … this shot from Terra makes me think it would be fun to look for satellite imagery of those ripples at the same time I see them from the ground. I’ll have to try that the next time I see them from home.

*The best example of this of all time is from the screwball comedy Top Secret! Val Kilmer is sitting on a train, and as the train starts to move you see the station going past the window. When the station clears the window, you see the trees aren’t moving, and realize it really was the station moving. This scene is driven home by Kilmer lurching slightly when the train is supposedly starting to move, making the illusion seem more real (though if you watch carefully, you’ll see he lurches a fraction of a second too late).


Medical Examiner

The Most Terrifying Thing About Ebola 

The disease threatens humanity by preying on humanity.

I Bought the Huge iPhone. I’m Already Thinking of Returning It.

Scotland Is Just the Beginning. Expect More Political Earthquakes in Europe.

Students Aren’t Going to College Football Games as Much Anymore

And schools are getting worried.

Two Damn Good, Very Different Movies About Soldiers Returning From War

The XX Factor

Lifetime Didn’t Think the Steubenville Rape Case Was Dramatic Enough

So they added a little self-immolation.


Blacks Don’t Have a Corporal Punishment Problem

Americans do. But when blacks exhibit the same behaviors as others, it becomes part of a greater black pathology. 

Why a Sketch of Chelsea Manning Is Stirring Up Controversy

How Worried Should Poland, the Baltic States, and Georgia Be About a Russian Invasion?

Trending News Channel
Sept. 19 2014 1:11 PM Watch Flashes of Lightning Created in a Lab  
  News & Politics
Sept. 20 2014 11:13 AM -30-
Business Insider
Sept. 20 2014 6:30 AM The Man Making Bill Gates Richer
Sept. 20 2014 7:27 AM How Do Plants Grow Aboard the International Space Station?
  Double X
The XX Factor
Sept. 19 2014 4:58 PM Steubenville Gets the Lifetime Treatment (And a Cheerleader Erupts Into Flames)
  Slate Plus
Slate Picks
Sept. 19 2014 12:00 PM What Happened at Slate This Week? The Slatest editor tells us to read well-informed skepticism, media criticism, and more.
Brow Beat
Sept. 20 2014 3:21 PM “The More You Know (About Black People)” Uses Very Funny PSAs to Condemn Black Stereotypes
Future Tense
Sept. 19 2014 6:31 PM The One Big Problem With the Enormous New iPhone
  Health & Science
Bad Astronomy
Sept. 20 2014 7:00 AM The Shaggy Sun
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.