The divergence of the transmitter beam essentially sets this fraction, assuming the receiver aperture is smaller than the diameter of the beam at that point. Typical values are rejection of 25 dB or so compared with a nonfiltered system. In the atmosphere Mesh architecture provides redundancy for carrier-grade systems. A good rule of thumb for determining link range is that if you can see it, you can communicate with it. This rule of thumb also gives potential users an intuitive feel for the relative importance of fog, snow, and rain in preventing link operation.
Fog sometimes can obscure the building across the street.
Rain hardly ever comes down hard enough to limit visibility severely. And snow lies somewhere in between the two. Many species of gases in the atmosphere can cause absorption, but water is the dominant factor for operating wavelengths of WONs. By using a transmission wavelength outside of the so-called water window and keeping the path lengths short, absorption can largely be ignored. Due to its low cross-section, Rayleigh scattering is really only significant for very long path lengths.