Sunshine records from Ireland: cloud factors and possible links to solar activity and cosmic rays

The records of sunshine hours obtained since the late 19th century from four stations distributed throughout Ireland were analysed. A gradual decrease in sunshine hours has occurred at all four sites since records began. Increasing cloud factors, resulting from enhanced evaporation rates over the Atlantic as sea surface temperatures have risen, is one possible explanation for the decline in sunshine. A strong negative correlation was confirmed between sunshine factors from ground‐based observations and satellite‐based cloud factors over Ireland. In addition, it was found that cloud factors over Ireland correlated well with cloud factors over large oceanic areas such as the North Atlantic and mid‐high latitudes generally. Thus cloud factors (and similarly sunshine factors) from regions on the boundaries of large oceans which lie in the direction of the prevailing wind could be useful in determining the long‐term changes in cloud factors over more extended areas. Knowledge of such long term variability in the Earth's cloud cover is important input information for modelling past climate change. The importance of cosmic rays as a link between solar activity and climate was assessed from a study of the ISCCP‐D2 satellite cloud factors and Irish sunshine data. Whilst these results confirmed the strong correlation between total cloud factor and cosmic rays over non‐tropical oceans between 1984 and 1991 previously reported, it was found that this correlation did not hold in the subsequent period 1991–1994. Other work has established a link through specifically low cloud. Indirect evidence of cloud formation by cosmic rays from a variation in the sunshine factor following Forbush decreases, and over the sunspot cycle, was mostly negative. Although a dip at seven years past sunspot minimum is evident in the sunshine factor for all four sites and in most seasons, it is of marginal statistical significance. Copyright © 2001 Royal Meteorological Society