  We investigate estimation of fluctuating channels and its effect on security
of continuous-variable quantum key distribution. We propose a novel estimation
scheme which is based on the clusterization of the estimated transmittance
data. We show that uncertainty about whether the transmittance is fixed or not
results in a lower key rate. However, if the total number of measurements is
large, one can obtain using our method a key rate similar to the
non-fluctuating channel even for highly fluctuating channels. We also verify
our theoretical assumptions using experimental data from an atmospheric quantum
channel. Our method is therefore promising for secure quantum communication
over strongly fluctuating turbulent atmospheric channels.
