That's very wavelength-specific, as this graph shows: http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Atmospheric_Microwave_Transmittance_at_Mauna_Kea

That's very wavelength-specific, as this graph shows: http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Atmospheric_Microwave_Transmittance_at_Mauna_Kea_%28simulated%29.svg/350px-Atmospheric_Microwave_Transmittance_at_Mauna_Kea_%28simulated%29.svg.png http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Atm... Water and water vapor has the same effect on the visual part of the spectrum also, but this doesn't reduce people's enthusiasm for it as a potential communications medium. http://en.wikipedia.org/wiki/Terahertz_radiation http://en.wikipedia.org/wiki/Terahertz_radiation