Ice-in & Ice-out

When does ice form?
Ice-in dates vary widely depending on the size and location of the waterbody, as well as the weather. While high-altitude shallow ponds may be fully frozen by mid-November, the larger and deeper lakes typically freeze in stages rather than all at once. This process occurs during December, January and early February, giving us frequent opportunities to skate on new black ice, as long as we’re willing to travel and we know where to go.
The widest and deepest sections of big lakes such as Champlain and Sebago sometimes don’t freeze until early March, and sometimes not at all. As the climate warms up, this list may grow to include shallower lakes like Winnipesaukee and Newfound. As of mid-February 2024, all four of these lakes still have large expanses of open water.
How does ice form?
Usually a cold, clear, still night is required for ice formation.
Wind, waves, current, and water bird activity can all retard ice formation.
Usually, ‘fall turnover’ must be complete before ice can form. Water is densest at a temperature of 39 degrees Fahrenheit, and the entire lake must cool down to that temperature or below before ice can begin to form.
Once the ice has formed across the entire waterbody, it thickens most quickly in cold, windy weather. However, if the lake is only partially frozen, wind and waves (even a bitterly cold wind) can destroy the ice as rapidly as it formed.
Once ice has formed, clouds and precipitation (snow, sleet, and even frost feathers) interfere with radiational cooling and retard ice formation.
For the first few cold nights at 20 degrees or below, ice thickens at an average rate of 3/4 inch per night. However, once the ice thickness has reached 3 inches, the rate of ice growth decreases as the ice gets thicker.
Freezing Degree Days (FDD) can be tracked as a rough yardstick to predict the rate of ice formation and growth.
How and when does the ice go out?
Ice-out in northern New England used to occur in April or the first half of May. Nowadays, the ice goes out between mid-March and mid-April. This is partially because the ice just doesn’t get as thick as it used to, and also because there’s less snow cover to insulate the ice from the warm spring sun.
A much more rapid process than ice-in, ice-out can take just hours instead of days. I’ve see a pond go from six-inch-thick skateable ice in the morning to open water by sunset, without a speck of ice in sight. All it takes is a warm, windy day to soften the ice and then blow it away.
Step 1 is for the ice to ‘candle’. Candling is when the ice crystal boundaries melt and the water drains out, leaving each crystal floating in the water unattached to its neighbors. Since ice crystals form vertically, each crystal is shaped like a tall, skinny candle or pencil, and it’s only held in place by its neighbors. Once one pencil tips over, they all do.
Step 2 is for the ice to melt away from the shore. This happens first on the north shore of the lake, which gets more sun than the other sides. Eventually the sand and rocks along the shore soak up enough of the sun’s heat to cause the ice to recede along the entire shoreline.
Step 3 is for the wind to start blowing and cause the entire ice sheet to start drifting downwind. As the ice sheet crunches ashore, the pencil-shaped ice crystals break away from each other and fall apart, leaving room for the ice sheet to continue drifting. Occasionally piles of ice crystals get pushed up on shore or up on top of rocks, but they don’t last long. Exposed to the air on a warm, windy day, they only last a few hours or even just minutes. By the end of the day, the entire ice sheet may have vanished without a trace. However, if it’s a cool day or the wind isn’t strong enough, ice-out may continue over two or three days instead of just one.