Sounds like going over the Pyrenees mountains between France and Spain, even in summer you see cyclists at the bottom in shorts and teeshirts and at the top putting on winter coats.
We took a trip to the picos de Europa, in northern spain, we were on the beach in the morning and dressed for winter at the top in the afternoon, amazing. We obviously aren't as fit as some of you, we drove up in the car !!
S2S 2015 -24€