Nathan Hydration Vapor Air Hydration Pack
At the top of the priority list for running during a hot Summer in Phoenix is staying hydrated. Nathan Hydration has always made great gear, but we think the Vapor Air (Airess for the women's version) is far and away their most comfortable hydration pack yet. With a bladder capacity of 2 liters and extra storage covering every inch of this pack, the Vapor Air is equipped to carry everything you need for your longest workouts.
From the Nathan Hydration web site: