  The annual distribution of the planetary geomagnetic Ap index monthly mean
values on visually cloudless days and nights at the Abastumani Astrophysical
Observatory (41.75N, 42.82E) in 1957-1993 is analyzed. Cloudless days were
selected from day-time observations of the total ozone content and cloudless
nights from night-time measurements of the mesosphere-thermosphere nightglow
intensity. Both series of observations were carried out almost uninterrupted
during the referred time period. The distribution of long-term monthly mean Ap
values on cloudless days shows a semi-annual variation with maxima on the
equinoctial months (March and September), while on cloudless nights it displays
a seasonal-like character. The semi-annual and seasonal-like variations of Ap
modulate the annual distribution of long-term monthly numbers of cloudless days
with maximum in August and that of cloudless nights with maximum in September.
The lowest monthly mean Ap value for cloudless days and its highest value for
cloudless nights fall on June. Different annual distributions of long-term
monthly numbers of cloudless days and nights, when sudden storm commencements
are present, have been found. This coupling between the visually cloudless
days/nights and the magnetically disturbed days may be considered as a
manifestation of the influence of space weather/cosmic factors on the regional
and global climate.
