The Swimline HydroTools Pool Cover Air Pillow is made to withstand low winter temperatures and help reduce wear and tear on your winter cover. Simply inflate and place in center of pool under cover. It absorbs freezing water pressure and helps protect your pool walls. The air pillow also reduces excess rain, snow and leaves on your winter cover. For use with above ground pools. Available sizes include 4x4, 4x8 and 4x15.