Warm temperatures can do as much to internally melt ice as the sunlight, if only over a longer period of time. Thick ice weakens slowly, but it also recovers slowly. While two inches of ice might last only two hours in moderate thaw conditions, a foot or more of ice can last several days. However, safety guidelines for ice thickness should be doubled or tripled after a thaw, until the ice has several days of below-freezing temperatures to recover.
While rain is as bad for snow as it is for cotton candy, it has relatively little effect on ice melt. For example, around one inch of rain at 40°F temperatures will only cause about 1/16 inch of ice loss. One exception to this is the formation of drain holes – if the ice has experienced little internal melting, and the puddle is too large to be blown apart by the wind, it may be funneled through any crack or hole available. As the water drains away these holes are noticeably eroded, becoming large enough to catch an unwary foot, skate, or iceboat runner.
The wind that accompanies the rain often does more damage than the rain itself. Warm wind is the primary driver behind top surface melting. For example, over a 24-hour period at 50°F, strong (20-30 mile per hour) winds can cause over two inches of ice loss. In the same period, heavy rain and only moderate wind result in just one inch of ice loss. Wind can also be responsible for tearing holes in the ice – thin new ice and well-thawed thick ice are particularly vulnerable to a phenomenon called “rafting”, where ice sheets are pushed over themselves at pressure ridges and along shorelines. Wind blowing over puddle drain holes can tear them further open, causing them to be up to 20 feet in size. Both of these phenomena create open water that is then stirred by the wind, bringing warmer, deeper water to the surface to melt the ice.
Just as wind wears away at the ice from above, currents erode it from below. Under-ice melting is common in rivers, lake sections with river characteristics (such as Crown Point, Five Mile Point, and Chipman Point on Lake Champlain), under bridges, and anywhere that water has a strong flow – particularly through narrow spaces. For this reason river ice is considered especially unsafe, and you should always exercise caution before putting weight on it.
