Flying birds, trees, and tall buildings can temporarily block a single beam when it appears in line of sight (LOS) of transmission in an FSO system.
There can exist temperature variations in the free space between a transmitter and receiver in FSO communications; they manifest as pockets of air that are unusually warm compared to the air around them. The source of these variations is usually due to heat rising from the earth or from man-made devices like heating ducts or exhaust vents. These variations can cause fluctuations in the amplitude of a transmitted optical signal as it is seen by its receiver, causing the signal’s amplitude to fluctuate randomly. This effect is called scintillation.