First and foremost, the fabric that makes up the main body of the backpack should be made of high-quality nylon. To withstand downpours, you may want to treat the fabric with a water-repellent coating, too. As far as extra features, some backpacks have a rubberized bottom that won’t get wet if you set it on a damp surface. Reinforced corners and zippers are also a plus in wet weather because they also keep out water.