With the recent extremely cold weather, ponds across the country have begun to freeze over. Invariably, questions arise such as, “How quickly does the ice thicken?” and “How thick does ice need to be to support a person, car?” The answers to these questions vary greatly.
Ice accumulation rates can be determined by a number of complex formulas that include temperature, snow cover, wind, and radiational cooling. A simpler method looks at Frost Degree Days (FDDs). This is calculated similar to growing degree days. Take the average temperature and subtract it from the freezing point of water (32 F). Each degree that the average is below freezing is one FDD. After the first ice layer forms, studies have shown that the ice will accumulate at about 1 inch per 15 FDDs. For the questions regarding depth to support various objects, there are tables that indicate 2-4 inches of ice would support a single individual, while 8-12 inches of ice would support a car (http://www.dnr.state.mn.us/safety/ice/thickness.html). However, all of those guides mention that this is for “clear, solid ice”, and are based on regions with consistent, long-term cold weather, such as Canada, Alaska, the northern United States, and Russia.
It should also be noted that these calculations are for still water. Rate of accumulation will be much slower in moving water, and the strength of the ice will be less.
The Michigan Department of Natural Resources (DNR) has this advice, “The DNR does not recommend the standard "inch-thickness" guide used by many anglers and snowmobilers to determine ice safety. A minimum of four inches of clear ice is required to support an average person's weight on the ice, but since ice seldom forms at a uniform rate it is important to check ice thickness with a spud and ruler every few steps.” They also note, “Be especially cautious in areas where air temperatures have fluctuated. A warm spell may take several days to weaken the ice; however, when temperatures vary widely, causing the ice to thaw during the day and refreeze at night, the result is a weak, spongy or honeycombed ice that is unsafe.”