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The atmosphere is host to an omnipresent bacterial community that may influence fundamental atmospheric processes such as cloud formation and precipitation onset. Knowledge of this bacterial community is scarce, particularly in air masses relevant to cloud formation. Using a light aircraft, we sampled above the atmospheric boundary layer—that is, at heights at which cloud condensation occurs—over coastal areas of Sweden and Denmark in summer 2009. Enumeration indicated total bacterial numbers of 4 × 10¹ to 1.8 × 10³ m⁻³ air and colony-forming units of 0–6 bacteria m⁻³ air. 16S rRNA gene libraries constructed from samples collected above the Baltic Sea coast revealed a highly diverse bacterial community dominated by species belonging to the genera Sphingomonas and Pseudomonas. Bacterial species known to carry ice-nucleating proteins were found in several samples. Modeled back …