The spring sun on thick ice creates a white granular surface we call corn ice. It occurs on all ice types. It can be found in ice ranging from thaw stage 4 (weakening) to 6 ('can't stand on it'). The granules are the well thawed surface layer of the the ice sheet. The has enough freeboard (about an inch per foot of thickness) to keep the top surface above the lake level so melt water drains to the lake level. The air between the granules creates the light color by refraction and reflection.
Ice less than a foot thick with corn ice is likely to be somewhere between dodgy and very dodgy and it should be stayed off of. Corn ice can hide the texture of the underlying ice and can make it difficult to spot rotten candled ice (thaw stage 6, S2 ice). It hardens up on nights near or below freezing as overnight ice. It softens abruptly in strong sunshine. The whole ice sheet can go from hard enough to skate on to too slushy to skate. in an hour or less. The same is true for ice biking. Going out on ice covered with corn ice requires:
- A careful assessment of the ice sheet thickness and state of thaw.
- Careful consideration of the past, expected and possible unexpected weather
- An expectation that you have a higher than usual chance of falling through (claws, life jacket, cold water clothing, dry cell phone, throw rope, buddies, etc)
- Acceptance that you could have to walk a long distance back to shore (take foot traction, wear neoprene or gortex socks, bring shoes if your skates are not removable)
- It is possible to find your self in the water surrounded by thaw stage 7 ice that you can't get back on.
It is common the corn ice to be intermittent at the 'couple inches scale as shown here and at the 'few feet scale. The pole is 1.5" wide (see top picture). The underlying ice is S2 ice (small grain size) that is especially weak at this advanced thaw stage. This corn ice pattern indictiave of S2 ice. Use your test pole a LOT.
Corn Ice close-up. The plant is abouit three inches long. The granules might be better described as flakes.
Corn ice on pancake ice. The picture is about four feet across.
Corn ice on S1 (large grain) ice from a refrozen puddle hole-middle left side to whole right side of the picture. The image is about 4 feet across.
Corn ice can obscure weak ice. The dark area in the center of the image is where I stepped through rotten candled ice (S2 at thaw stage 6). Picture taken about 20 feet from the hole.