In hot weather, your back may sweat more and it may restrict movement a little. If you use a Camelbak, you can put it in the fridge first on hot rides and also use some ice cubes in it. If it is strapped on too tight, it may restrict breathing and the ability to respirate efficiently and get proper chest expansion on inspiration Since it is not attached to the bike, when you dismount you don't have to worry about it if you leave your bike untended. A Rack Pack fits over the rear wheel on an installed rack. This allows carrying a good bulk of material and puts the weight directly over the rear wheel.
Many designs are available for the pack portion.
If you have rear suspension, you need to mount the rack to your seatpost and not your frame. A Seat bag fits under the seat, and is a good spot on the bike for the weight as it is centered and pretty low. This is a good place for tools and some smaller gear. There are several sizes that may fit under your seat, but keep in mind you must have space to lower the seat as needed duriong your ride. A Butt pack is a good alternative to a backpack, and it keeps you back and chest free and has a lower center of gravity.