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. Scattering by particles, also known as Mie scattering, is a different story, particularly as the size of the particles approaches the wavelength of the transmitted light. To compensate for fog, systems can keep the link length short and increase laser power during episodes of decreased visibility.
Scintillation refers to small spatial variations in the refractive index of the atmosphere.
Network planning is simple and links are independent, but the optical link is a single point of failure without redundancy and is not suitable for carrier-grade systems. Rooftop-mounted optical wireless nodes communicate between buildings. Each link is a single point of failure, however, and hub location must be chosen to maximize the number of buildings with line of sight. Hub cost is generally high. Given that attenuation effects require short link lengths to ensure carrier-grade service, this is not a very economical architecture.