Atmosphere is the transmission medium for an FSO link. Like all transmission mediums, the atmosphere is subject to attenuation loss. This loss can have a major effect on the availability of an FSO link in that region. Attenuation in the atmosphere is dependent upon several conditions, primarily the weather. The region in which a link is being established has specific weather conditions, allowing for easily predictable attenuation sources. For example, fog and snow are the two primary weather conditions in temperate regions. In tropical regions, heavy rain and haze are two main weather conditions. Some of the weather conditions and their limitations with regards to FSO applications are described below.
Fog is a free space vapor that consists primarily of tiny water droplets that can partially or completely hinder the passage of light. Fog is a medium that combines signal attenuation from absorption, scattering and reflection, making it a particularly challenging medium to work with. Fog’s effective attenuation is strongest in the visible spectrum. Non-selective scattering produces fog’s white color.
Rain is a non-selective scattering phenomenon that causes fluctuation effects in laser delivery. The visibility of an FSO system depends upon the quantity of the rain. In the case of heavy rain, for example, water droplets have solid composition which can either modify the beam characteristics or restrict the transmission of the beam as it is absorbed, scattered, and reflected by falling water. 
Haze particles stay in the air for a longer time and can lead to prolonged atmospheric attenuation. Attenuation values of haze depend upon the visibility level at that time. There are two ways to gather information about the performance of an FSO system in haze: by installing a temporary system at the site and checking its performance during haze, and/or by using a Kim & Kruse model as a guideline metric.
Smoke is generated by the combustion of different substances like carbon, glycerol, and household emissions. It affects the visibility of the transmission medium by scattering and reflecting transmitted light. Since the components of combustion effect the chemical properties of smoke, there is no general absorption (and therefore transmission) curve for smoke. However, common wood smoke has high absorption in the low visible and UV region and low absorption in the infrared region.
Sandstorms are a well-known problem in outdoor link communication. Sandstorms can be characterized by two ways: first, the size of the wind particles (which depends on the soil texture), and second, by the necessary wind speed required to blow the particles up during a minimum period of time. Since sand has variable particle size, all three types of scattering can occur depending on the classification of the sandstorm.  
Cloud layers are a main part of Earth’s atmosphere. The formation of clouds is done by the condensation or deposition of water above the Earth’s surface. It can block, in whole or in part, sections of an optical beam transmitted from Earth to the space. The attenuation caused by clouds is difficult to calculate because of the diversity and inhomogeneity of the cloud particles, being a mixture of water, dirt, rocks, and other materials with differing optical properties.
In 2019, researchers in Australia produced a paper (Chedzey, et. al) analyzing 40 decades of cloud climate to optimize the positioning of ground stations in clear-sky conditions. With enough predictive modeling, the ground stations will be able to continuously operate in areas of low cloud cover, maximizing laser throughput and mitigating attenuation and interference from clouds.
Paper: Kim & Kruse Model (Section 2)
Chedzey, Helen C.; Herne, David E.; Lynch, Mervyn J.; Nener, Brett D.; Grant, Kenneth J.; Mudge, Kerry A.; Clare, Bradley A. “A 40-Year Cloud Climatological Study for Australia: Implications for Siting of Laser Communication Infrastructure.” Conference paper; National Institute of Information and Communications Technology, October 29, 2019: 37th International Communications Satellite Systems Conference (ICSSC), Okinawa, Japan.