  Absorption of terahertz radiation by atmospheric water vapor is a serious
impediment for radio astronomy and for long-distance communications.
Transmission in the THz regime is dependent almost exclusively on atmospheric
precipitable water vapor (PWV). Though much of the Earth has PWV that is too
high for good transmission above 200 GHz, there are a number of dry sites with
very low attenuation. We performed a global analysis of PWV with
high-resolution measurements from the Moderate Resolution Imaging Spectrometer
(MODIS) on two NASA Earth Observing System (EOS) satellites over the year of
2011. We determined PWV and cloud cover distributions and then developed a
model to find transmission and atmospheric radiance as well as necessary
integration times in the various windows. We produced global maps over the
common THz windows for astronomical and satellite communications scenarios.
Notably, we show that up through 1 THz, systems could be built in excellent
sites of Chile, Greenland and the Tibetan Plateau, while Antarctic performance
is good to 1.6 THz. For a ground-to-space communication link up through 847
GHz, we found several sites in the Continental United States where mean
atmospheric attenuation is less than 40 dB; not an insurmountable challenge for
a link.
