 Rainfall tends to be associated with Atlantic depressions or with convection . The Atlantic depressions are more vigorous in autumn and winter and most of the rain which falls in those seasons in the south @-@ west is from this source . Average annual rainfall is around 980 millimetres ( 39 in ) . November to March have the highest mean wind speeds , with June to August having the lightest winds . The predominant wind direction is from the south @-@ west . 
