The problem with riding up mountains in summer is actually descending mountains. You ride up, you get hot and sweaty, you descend then you instantly get cold and the moisture from the sweat keeps you cold. Shivering on a bike on a fast descent is not a good thing. In the days of old school racing – spectators at the top of the mountain pass would give riders newspaper to put down the front of their jerseys to block the wind for the descent, thus keeping them warmer and safer.
Below is a modern day take design for a versatile jersey with side panels that can transform into a wind vest.