A backpack rain cover is one of the best pieces of hiking gear you can buy. It protects not only your pack but any gear stored inside it such as tents, sleeping bags, stoves and food. Some packs come with a built in pack cover and others have to be purchased separately.
The most important thing is that the rain cover is waterproof. The material needs to be either polyester or a sil/ripstop nylon with a tight weave that stops practically all water from getting into your backpack. It is also helpful if the material is coated with a polyurethane.
It is also a good idea to get a backpack rain cover that is lightweight and can be stowed in a small package when not in use. Since it will be stowed in your backpack anyway you don’t want to carry extra weight around if you don’t need to.
You should be able to get a quality backpack rain cover for about $20 to $25. The price is a bit higher for the more durable and longer lasting models.
The most common way to attach a rain cover is to have it attached to the top of the pack with straps that wrap around the top. There are also models that have an elasticated or drawstring border that you can fasten around the edge of your backpack. These are the most versatile but also add a little more fabric to the overall weight of your backpack.