We drove through rain from Wigton to Silloth to have our lunch. The sky was just clearing over the sea as we arrived. Looking south west along the coast there was this long bank of cloud. According to the weather maps it's a cold front. At our latitude, cold air front the Arctic runs into warm air from the Equator. The air masses don't easily mix to give medium temperature air. Where a block of warm air meets a block of cold air you get a weather front. What happens is that as the cold air moves into the back of a mass of warm air, the warm air moves up because it is less dense. A wedge of more dense cold air moves in underneath it. There is more water vapour dissolved in the warm air but as that air rises it cools down. The water vapour can't be held invisible in the air anymore so it condenses out to form clouds. So the boundary between warm and cold air is marked by a line of cloud. These were first drawn on weather maps just after the First World War. They looked like maps of the Western Front battle lines, so they became called weather fronts. It takes about 6 to 8 hours for a frontal system to clear the UK, hence my dad's old weather saying: Rain before 7, fine before 11.
Looking north east. The weather front curls away to the right.
Looking over to Criffel in Scotland. This is looking into the cold air mass. There is less convection to distort the visibility in cold air so the view was very clear.
