E. Palle et Cj. Butler, Sunshine records from Ireland: Cloud factors and possible links to solar activity and cosmic rays, INT J CLIM, 21(6), 2001, pp. 709-729
The records of sunshine hours obtained since the late 19th century from fou
r stations distributed throughout Ireland were analysed. A gradual decrease
in sunshine hours has occurred at all four sites since records began. Incr
easing cloud factors, resulting from enhanced evaporation rates over the At
lantic 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 g
round-based observations and satellite-based cloud factors over Ireland. In
addition, it was found that cloud factors over Ireland correlated well wit
h cloud factors over large oceanic areas such as the North Atlantic and mid
-high latitudes generally. Thus cloud factors land similarly sunshine facto
rs) from regions on the boundaries of large oceans which lie in the directi
on of the prevailing wind could be useful in determining the long-term chan
ges in cloud factors over more extended areas. Knowledge of such lent: 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 betwe
en total cloud factor and cosmic rays over non-tropical oceans between 1984
and 1991 previously reported, it was found that this correlation did not h
old in the subsequent period 1991-1994. Other work has established a link t
hrough 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, w
as mostly negative. Although a dip at seven years past sunspot minimum is e
vident in the sunshine factor for all four sites and in most seasons, it is
of marginal statistical significance. Copyright (C) 2001 Royal Meteorologi
cal Society.