Some agencies have already done this by establishing thickness benchmarks. A small study by the Bartlett Park District in 1991 indicated that ice thicknesses of four to ten inches were being used as benchmark measurements by survey respondents. Snow ice is only about half as strong and melts quicker. During warm periods, ice can melt from both the top and from the bottom. In fact, the water temperature below the surface can be warmer.
Water flooding up through the hole is a dangerous sign.
Ice can be weakened by salt runoff and other impurities. Water current or velocity can reduce the thickness of ice especially near a spring or outlet. We remember the young George Bailey rescuing his younger brother, Harry, who has broken through the ice at the end of a sled run. It appears that the ice was too thin because of a natural outlet to a stream. In pulling the Baileys to safety, the other boys use the proverbial human chain to disburse their total weight over the unbroken, adjacent ice.