Francisco has a cylindrical barrel 3 feet tall that he uses to collect rainwater. The barrel is not empty when rain begins to fall at a steady rate. Francisco checks the level 2.5 hours after the rain begins falling and finds that the water level in the barrel is 31.5 inches. He checks again 2.5 hours after his first check and finds that the water level is 32.5 inches. By how many inches does the water level increase for each additional hour of rainfall?