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Crossing Ice

There is no such thing as “safe ice.” Conditions and unseen or unknown factors can render seemingly safe ice suddenly dangerous. Crossing ice should not be done without planning, consideration of risks, employing safety devices and having a rescue plan ‘just in case’.

Ice conditions can be greatly affected by water currents, snow depth, and recent weather. As a general guideline, two inches of black or white ice will probably hold you up 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. The table below provides weight bearing guidelines for various thicknesses of ice.[1]

Thickness

Safety

<2 inches (5cm)

Dangerous

4 inches (10 cm)

Suitable for individual weight

7 inches (18 cm)

Will bear a snowmobile

8-12 inches (20-30 cm)

Will bear an ATV or small car

If you want to get geeky about estimating ice thickness in advance you can use the following formula:[2]

Z = ice thickness in inches

S = degree days accumulated below 32 F

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 5F. To find degree days, subtract 5o F from 32F for a value of 27. If on December 17 the temperature is 4o F, subtract 4F from 32F for a value of 28. S would then have a value of 55 by December 17 (27F + 2 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 32F 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 32F.

Tips for Crossing Ice

Stay off the ice if you have any doubts about safety.  Walk in single file and stay spread out. Stay clear of any moving water caused by currents.

Slip sliding along

Carry a long pole to poke and use it to check on the ice.  Held horizontally a long pole can arrest a fall through the ice.

Is this safe?

Keep ice picks readily available by carrying them on a cord around your neck or wear a sheath knife to help you crawl back on ice if you break through.

Ice Pick closed

[1] Environment Task Force Newsletter, Ohio State University Extension, Winter 1998, V1 #4, http://ohioline.osu.edu/enviro/wi98/enviro_7.html

[2] Outdoor Action Guide to Winter Camping, http://www.princeton.edu/~oa/winter/wintcamp.shtml

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