As a general guideline, 1 inch of black or white ice will probably hold you up. Two inches is safe, and six inches will hold up a moose. Thickness of suspect ice can usually be determined quite quickly by using an ice axe or auger to drill through. However, for advanced trip planning, you can use the following formula to estimate the thickness:
Z = ice thickness in inches
S = degree days accumulated below 32 oF
A = a coefficient which varies as follows:
(.8) -windy lake with no snow
(.5 to .7) – average lake with snow cover
(.2 to .4) – sheltered small river with rapid flow
S is calculated as follows: Suppose ice is formed December 15 and the mean temperature for December 16 was 5o F. To find degree days, subtract 5o F from 32o F for a value of 27. If on December 17 the temperature is 4o F, subtract 4o F from 32o F for a value of 28. S would then have a value of 55 by December 17 (27o F + 28o F = 55). Next take the square root of 55 (7.4). To determine ice thickness, multiple 7.4 by the appropriate coefficient A (say .8 for a windy lake with no snow), and your answer is 5.9 inches of ice. If you don’t know the date of ice formation, you can estimate by the following technique:
– For lakes 3 – 10 feet deep, freezing occurs very close to the date when the 3-day running mean temperature is 32o F and where temperatures remained mostly below that for the rest of the winter.
– For lakes 20 – 50 feet deep, the date of freeze-over occurs when the 40-day running mean temperature reaches 32o F.
Alternatively you can tie a rope around the largest group member and if they make it over safely, most should be good to follow.
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