Layers, boys. Dress in thin layers.
Leaving vertical climate zones out of it...
One time, I was driving up on Whidbey Island in the middle of the night, in the summer, between dives, just wearing speedos, and I saw some weird searchlights in the sky, just ahead. I came around a corner into four inches of sleet. The searchlights were the headlights of cars in the ditch. I remember walking around barefoot in the ice, getting dressed so I could help pull cars back onto the road.
Another time, I left home on a dive trip. It was about 100 F when I left. By the time I got to the boat, it was cold and rainy. I realized that I didn't have any shoes with me. Had to wear wetsuit booties all weekend. The fashion didn't catch on, though.
Slightly off-topic: Not long ago, due to loss of luggage (Not to be libelous, but a certain large airline, based in Denver, whose name starts with a "C" looses my bag every single time.) I wound up in October in Spain with only what I was wearing on the plane - light cargo shorts, tee and tevas. The rain in Spain also falls vigorously in the mountains, as well as the plain. It was several days before I had time to drive to Sevilla and buy clothes. Meanwhile, from overheard conversations, it seems that I spawned a great reputation for northwest guys. ("They must love the rain... no matter how cold it gets, they never wear shoes...")