Be prepared, carry extra.

When you know you will be dealing with different conditions, you will always have a plan B. That being said, when living in a city that has changeable weather in the summertime, I always carry two pair of glasses. One clear and the other is of black base. As much as I love the original ones, I also like to have some aftermarket lenses. Having more than one pair of lenses for my Ray-Ban frame always gets me prepared. I don't need to worry about not having my sunglasses for the day because the lenses are damaged or tainted.