Buy a football.... air it up. Weigh it. Soak it in water and then let the air out. It will weigh more than it did before you let the air out.
Not arguing that. But you still need to measure both weight and pressure. If the ball weighs more than 15 oz, you can account for that with moisture, and a heavier ball puts the user at a disadvantage. But if the ball weighs less than 14 oz, then there is an issue, rain or no rain.