If your bike lives in the garage all winter then summer gear will probably suffice for spring and autumn too. If you venture further out into the cold and the rain then a waterproof shell, long trousers and a base layer may need to be added to your wardrobe. Eventually when you have added lights, hats, full finger gloves, winter jerseys, mudguards, winter tyres and supermarket bags to waterproof your feet you might think the list was complete however even your favourite backpack may need a little help.
A rain guard which can be either water resistant or waterproof is hung over the whole backpack but it needs removing for access and occasionally falls down to be trapped between your rear mudguard and the rear wheel where it rapidly fills with mud. An alternative is to use an internal dry bag but exterior pockets and the sack itself get soaked. The solution is a backpack where the material is waterproof and the zips are water resistant as a minimum specification.
A trawl through the internet found a number of bags available. Gourdon, Overboard, Exped, Kappa, Caribee, Ortlieb, Boblbee, Aquapac and Dakine all made my list but they all had different advantages and, of course, the ideal was a mix of the better elements of all the bags.
The Exped was simple, less expensive and a good colour, may be suitable for many riders but the Overboard floats, and could be used in the summer on the water. A pre-Christmas discount had finished so it seemed poorer value for money. Ortlieb was more expensive but it had looked bulkier and more rigid but a good colour again.
Eventually I settled on the Aquapac, which is one of the more expensive backpacks, but it had exterior mesh pockets for tubes, latex gloves and drinks, and divided insides for separation of layers from food, tools and camera. A transparent pocket for money, house keys and phone seemed very useful and a hook for a car key to avoid fumbling in the dark another good idea. An unusual feature is that the internal pocket is yellow with a white interior to improve visibility when scrambling for items in a darkened wood. There are hooks for attaching things like rear lights and an exterior pocket that you can squeeze in a half full bladder if you remove the support pad/seat.
After a few rides it has proven waterproof, comfortable on short fast and long slow rides, access is easy enough through the roll top closure and the side mesh pockets are secure. The poorer elements are the waist belt missing a central buckle, now added, and the mesh pockets being so deep that you need to take the pack off to pull out a bottle of drink, hence the need for changing the waist belt.
So if you need a bag tested under a waterfall on Youtube to ensure it meets the rigours of Stanmer then buy one quick before the trails turn dusty.