Turbulence is a phenomenon in fluid mechanics created by unpredictable changes in pressure and flow velocity. Atmospheric turbulence follows the same mechanics. For optics, since pressure and temperature are directly related, turbulence also means changes in temperature. This corresponds to changes in refractive index, producing damaging effects to signal transmission through the atmosphere. Turbulence can be studied using chaos theory, but a model for its flow characteristics and behavior is an unsolved problem in physics.
Atmospheric turbulence is primarily created from conditions existing in the Earth’s atmosphere, up to 30 km from the Earth’s surface. Since atmospheric turbulence induces divergence in an uplink beam for the first 30 km of its path, the divergence will be compounded as the beam travels the rest of the distance to its target satellite. Conversely, for downlink transmissions, the beam is unaffected by divergence until the last 30 km of its path. Considering the immense distances for satellite communication, the divergence of the beam must be less significant for downlink propagation. Mitigation techniques are therefore especially critical for uplinks. 
Turbulence in FSO
FSO is directly affected by attenuation in the atmosphere. Since the atmosphere channel is not ideal, light undergoes attenuation, dispersion, and divergence. Turbulence contributes to all three and is the primary source of wavefront distortion. When a signal is sent from transmitter to receiver, either by uplink or downlink, the received wavefront will not be perfectly flat. The wavefront will reflect the distortion received by turbulence and will require correction via adaptive optics. For this reason, uplink mitigation is much more important than downlink mitigation.
Eddies are off-shoots of large packets of turbulent air that have broken up and distributed their kinetic energy. The temperature in eddies varies greatly and the size of these eddies can be very small. As the light beam propagates in free space, its interactions with eddies will alter sections of the wavefront.  Instead of all sections of the wavefront arriving at the same time, sections of the wave will arrive earlier or later than other sections. This arrival delay causes an image jitter commonly referred to as scintillation.
Paper: Effects of Clear Atmosphere Turbulence on FSO Quality in West Asia
Paper: Fundamentals of FSO; Section 3: Turbulence Mitigation