  We present results of a comprehensive study of the distribution of absorption
dips with orbital phase in Cygnus X-1. Firstly, the distribution was obtained
using archival data from all major X-ray observatories and corrected for the
selection effect that phase zero (superior conjunction of the black hole) has
been preferentially observed. Dip occurrence was seen to vary strongly with
orbital phase \phi, with a peak at \phi ~ 0.95, i.e. was not symmetric about
phase zero. Secondly, the RXTE ASM has provided continuous coverage of the Low
State of Cygnus X-1 since Sept. 1996, and we have selected dip data based on
increases in hardness ratio. The distribution, with much increased numbers of
dip events, confirms that the peak is at \phi ~ 0.95, and we report the
discovery of a second peak at \phi ~ 0.6. We attribute this peak to absorption
in an accretion stream from the companion star HDE 226868. We have estimated
the ionization parameter at different positions showing that radiative
acceleration of the wind is suppressed by photoionization in particular regions
in the binary system. To obtain the variation of column density with phase, we
make estimates of neutral wind density for the extreme cases that acceleration
of the wind is totally suppressed, or not suppressed at all. An accurate
description will lie between these extremes. In each case, a strong variation
of column density with orbital phase resulted, similar to the variation of dip
occurrence. This provides evidence that formation of the blobs in the wind
which lead to absorption dips depends on the density of the neutral component
in the wind, suggesting possible mechanisms for blob growth.
