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I notice in the EMS chart that only half of the IR range will travel through the earth's atmosphere. So in the range of frequencies that penetrate the atmosphere what frequency travels farthest?

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I suppose what you want is the graph at http://en.wikipedia.org/wiki/Radio_window, provided you are talking about vertical transmission from space to the earth's surface. Results would presumably be different for horizontal transmission because the pressure and chemical composition of the atmosphere would be much closer to constant, but I can't immediately source those transmission coefficients.

As you see, radio waves transmit best, 10 micron comes through pretty well, etc.

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For "horizontal" transmission reflection of the ionospheric regions to radio waves is the problem. Not tranparency of the atmosphere. This is why at vertical incidence a fraction of radio waves below 10 meters reach ground, whereas there is no tranmission at lower angles. Example is FM band waves from Jupiter, only to hear when Jupiter is close to zenith. –  Georg Jul 24 '11 at 13:59
    
Interesting, Georg, thanks. I didn't know that reflection was so important at low incidence angles. I meant horizontal between a transmitter and detector on the earth's surface, since the OP seemed to be not only about sources outside the atmosphere("what frequencies travel farthest?"). Short wave radio messages can famously be transmitted many thousands of miles because of internal reflection from the ionosphere, with very little absorption, making it the frequency of choice for long ranges. Absorption of near-visible frequencies at sea level depends on smog, dust, water vapor, etc. –  Peter Morgan Jul 25 '11 at 0:36
    
Right, the same reflections in SW band directing the waves back to earth, reflect incoming waves from outside, keeping them out. –  Georg Jul 25 '11 at 18:36
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