I carry some ultralight insulation and rain wear, all of it together weighs about 20 ounces and easily fits into a day pack with room to spare.

http://www.amazon.com/Driducks-Rain-Suit-Khaki-sm-Gear/dp/B001C3YJI8

http://www.gearzone.com/Montbell-Thermawrap-Parka-p/165-15710.htm
http://www.gearzone.com/Montbell-Thermawrap-Parka-p/165-15710.htm

That said, I do dress appropriately for the expected conditions and for extreme cold like my Swiss Army surplus wool pants.
http://west.loadup.com/military/surplus/23259.html