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SYFY WIRE Bad Astronomy

Our Planet Has the Vapors

By Phil Plait

I’ve posted a lot of images of our planet as seen from space, but I wonder if you’ve ever seen it quite like this: in the thermal infrared, where water vapor glows by day and night.

First to orient you, here are visible light images of the Earth from the same satellites, so you can see where they’re pointed:

The satellite GOES-15 (also called GOES West) is on the left, and GOES-13 (East) is on the right. The West view is almost entirely the Pacific Ocean while East mostly shows North and South America.

The video is the same view, but shows the Earth at a wavelength of about 7 microns, 10 times redder than what our eyes can see. At that “color,” warm things glow brightly while cooler objects are dark.

… kinda. That’s part of the picture (literally), but there’s more. Water vapor in the atmosphere is actually opaque to this wavelength, so it prevents the light from the warm Earth to get out into space where the satellites can see it. I know the white parts look like clouds, and that might be where you think they are, but it’s actually the opposite: The clouds (or at least the water vapor in the air) are where it looks dark in the video.

The animation was put together using the GOES images by James Tyrwhitt-Drake, whom you may remember from previous video of the Earth from space, and of the ridiculously cool video of the Sun in ultraviolet. He sent me a note about this, and we discussed these images. He told me something I didn’t know: On the GOES site, they take the data from the satellite and invert them, swapping black for white, so that clouds look white. I’m not sure why, though it may be as simple as us being used to seeing them as white. That’s obvious to me now; the space around the Earth is cold and should look black, but in the GOES images on their server it’s white.

Tyrwhitt-Drake took the images and reinverted them, so space (and anything else that’s cold) looks black. That makes it harder to interpret by eye, but it better represents what the Earth would actually look like if you could see it from space in the thermal IR.

Very—puts on sunglasses—cool.

He also pointed out something pretty neat: Keep your eyes on the west coast of South America in the satellite view on the right. You’ll see it pulsing white, about once per second; that’s the Atacama Desert, and you’re most likely seeing it heating up every day as the Sun hits it, causing it to glow in the IR. Deserts are very dry (by definition) so they cool off rapidly at night as well. By the way, right after that bright pulse, you can see the rest of South America get dark as clouds form over land.

It’s interesting that you have to go looking for differences between day and night in the video; it’s not as obvious as it would be in visible light.

Incidentally, water vapor is a pretty good greenhouse gas, even more efficient than carbon dioxide. CO2 is more worrisome because every year there’s an extra 40 billion tons of it in the atmosphere, pumped there by humans. Water vapor is relatively stable over time. But this video makes it clear—irony!—why water vapor traps heat as well: It’s literally opaque to it. The ground warms up during the day, but the water in our air (as well as CO2 and other gases) absorbs it, preventing it from radiating away in space.

The result? Well, you know what the result is. Unless you’re a Republican senator, of course.

Our world is an amazing and intricate machine, and a lot of its delicate parts can’t be seen unless you get away from the planet itself, and look back with different eyes. Their effects are certainly felt, but it’s because we’ve left this planet that we can see it so much more clearly.

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