As Jock has already stated, predicting the weather can be a lottery.
This year, here in the south of Scotland the spring was exceptionally dry, with a spell of warm sunny weather in late March, yet the summer months of June, July and August were mostly cool and wet (though fortunately a nice sunny day for the World Pipe Band Championships in Glasgow in August).
If you want to do some walking I would advise avoiding the coldest winter months which are usually from around mid-November till mid-February.
During the peak summer months of June, July, and August Transatlantic airfares are usually more expensive, with no guarantee of good weather.