Nice day? Nasty Day?

I don't often comment on the weather, as it's dull as shit.
However, today, when I left for work, it was beautifully sunny. So much so, that I didn't bring a jumper, just my thin jacket.
As the train hit the M60 (the big one round Manchester), it suddenly got very foggy and cold. When I got off the train, I could see my breath in the air.
What the hell is going on? It's about 20 miles from my house to Manchester, but the difference in weather is so stark I'm dress inappropriately.