metre in the highest part of the troposphere called the free troposphere. The troposphere is the most dynamic layer in terms of chemistry and physics of aerosols and harbors complex chemical reactions and meteorological phenomena that lead to the coexistence of a gas phase, liquid phases (i.e., cloud, rain, and fog water) and solid phases (i.e., microscopic particulate matter, sand dust). The various atmospheric phases represent multiple biological niches. Possible processes in the way atmospheric microbial communities can distribute themselves have recently been investigated in meteorology, seasons, surface conditions and global air circulation. Over space and time Microorganisms attached to aerosols can travel intercontinental distances, survive, and further colonize remote environments. Airborne microbes are influenced by environmental and climatic patterns that are predicted to change in the near future, with unknown consequences. Airborne microbial communities play significant roles in public health and meteorological processes, so it is important to understand how these communities are distributed over time and space. Most studies have focused on laboratory cultivation to identify possible metabolic functions of microbial strains of atmospheric origin, mainly from cloud water. Given that cultivable organisms represent about 1 % of the entire microbial community, culture-independent techniques and especially metagenomic studies applied to atmospheric microbiology have the potential to provide additional information on the selection and genetic adaptation of airborne microorganisms. There are some metagenomic studies on airborne microbial communities over specific sites. Metagenomic investigations of complex microbial communities in many ecosystems (for example, soil, seawater, lakes, feces and sludge) have provided evidence that microorganism functional signatures reflect the abiotic conditions of their environment, with different relative abundances of specific microbial functional classes. This observed correlation of microbial-community functional potential and the physical and chemical characteristics of their environments could have resulted from genetic modifications (microbial adaptation) and/or physical selection. The latter refers to the death of sensitive cells and the survival of resistant or previously adapted cells. This physical selection can occur when microorganisms are exposed to physiologically adverse conditions. The presence of a specific microbial functional signature in the atmosphere has not been investigated yet. Microbial strains of airborne origin have been shown to survive and develop under conditions typically found in cloud water (i.e., high concentrations of H2O2, typical cloud carbonaceous sources, ultraviolet – UV – radiation etc. While atmospheric chemicals might lead to some microbial adaptation, physical and unfavorable conditions of the atmosphere such as UV radiation,