Not sure about the Pennines but the Welsh mountain one is definitely weaker in the summer, in fact it virtually disappears a lot of the time. A front from the west in December and January will usually not produce much rain here, often nothing at all: quite often we have sunny spells while places like Capel Curig record 20mm or more of rain. We get much more in winter from Cheshire Gap setups or lows passing to the south than a standard Atlantic westerly. In summer though fronts, rain and cloud come over the mountains as though theyre not there at all, westerly setups can be very cloudy (July 2010) and wet (August 2008). On the MO averages, Shawbury gets about a third the rainfall of Bala from Dec-Feb, but about 70-80% from June-Aug, incredible.