  The atmospheric turbulence is the main factor that influences quantum
properties of propagating optical signals and may sufficiently degrade the
performance of quantum communication protocols. The probability distribution of
transmittance (PDT) for free-space channels is the main characteristics of the
atmospheric links. Applying the law of total probability, we derive the PDT by
separating the contributions from turbulence-induced beam wandering and
beam-spot distortions. As a result, the obtained PDT varies from log-negative
Weibull to truncated log-normal distributions depending on the channel
characteristics. Moreover, we show that the method allows one to consistently
describe beam tracking, a procedure which is typically used in practical
long-distance free-space quantum communication. We analyze the security of
decoy-state quantum key exchange through the turbulent atmosphere and show that
beam tracking does not always improves quantum communication.
