We have a polar stream that should be a this time of year to the north of scotland but is actually closer to the equator and hence riding over us. this just sends all the wet rain over us. It is thought that the stream is moving further north every year (apparantly 2kms every year just not this year or the last 2 years!!!) but during the so called summer months the tropical stream follows the suns path and the stronger rays of the sun. This in theory (well my theory anyway) should push the polar stream further north and then high pressure should build between the 2 and then we get some nice weather. if you go to wikipedia and search for jet stream it explains it quite well with diagrams.