The Osprey Ultralight Medium backpack rain protection is a waterproof shield for your backpack that compresses into a tiny stuff sack and deploys quickly when you need it most. Made of 40D high tenacity ripstop recycled nylon, this cover will keep water and dust out of your pack and protect your valuables by keeping everything dry so that you can fully enjoy your hike. It is recommended to get the larger size if you’re on the fence. The medium size fits 30 to 50 litre backpacks.