The inter-annual distribution of cloudless days and nights in Abastumani: Coupling with cosmic factors and climate change

•Interannual distribution of geomagnetic Ap index is described by harmonic function.•Yearly maximum of Ap index for cloudless nights coincide with minimum of GCR flux.•Trends of Ap index and GCRs flux are different for cloudless days and nights.•Semiannual variations of lower-upper atmospheric parameters show their coupling.

We examined inter-annual variations and long-term trends of cloudless days (CD) and cloudless nights (CN) in 1957–1993 from Abastumani (41.75N, 42.82E), at different geomagnetic conditions and corresponding galactic cosmic rays (GCRs) flux changes. It showed possible influence of cosmic factors on cloud covering processes and, thus, climate change. It was demonstrated that (1) the inter-annual distribution of monthly mean values of planetary geomagnetic index Ap (for low and moderate disturbances) at CDs can be described by harmonic function with semiannual (with sharp maxima in March and September) and annual (with maximum in August) periodicities; (2) the inter-annual distribution of Ap index for CN has an additional maximum in June, where the largest decrease of GCR flux is observed. This phenomenon is expressed even stronger during Sudden Storm Commencement (SSC) events and strong geomagnetic disturbances (Ap≥50), when their relative numbers are the greatest and are accompanied by bigger reduction of GCRs flux; (3) the long-term trends of mean annual and mean seasonal values of Ap index and GCRs flux at CD and CN are estimated. It was detected that, for the latitudes of this region, long-term decreases (negative trends) of seasonal GCR flux are different at CD and CN, which could affect the radiative balance at the Earth's surface and, as a result, contribute to the climate change.