Football shoulder pads are constructed of a foam-like substance that is pressed against the player's body. While some contemporary shoulder pads employ heat-dispersing materials, it's still vital to wear a layer of clothes below the shoulder pads to ensure the padding doesn't trap too much heat against your body. This isn't necessary for modern helmets which have vents designed into them. In fact, wearing a helmet will actually prevent you from needing a shirt under your pads because the headgear will act as its own barrier between you and the elements. However, this convenience option comes at a price: not all helmets are created equal, so make sure you get one that fits properly and offers the right amount of protection before you go out in heavy rain or snow. The bottom line is that you need to wear enough clothing on game day to stay comfortable but not so much that you're wasting time removing it during stoppages in play. Consult with your team trainer if you have any questions about what kind of clothing should be worn under your pads.