When there's a chance of rain in the forecast, we pack along the Castelli Tempesta Lite Jacket. The fully waterproof shell is extremely lightweight and packs down to fit into a pocket, so we can break it out anytime clouds turn dark overhead and the drops start to fall. Internal wrist gaiters create a seal to keep rain from seeping through, and the rear vent allows any excess heat to escape on long, wet ascents.