Ice formations and conditions

Before walking onto a frozen lake, it is important to know what to look out for. Many factors beyond ice thickness can help you decide if it is a good time and place to go on the ice. Factors that affect ice strength and could increase your risk include snow cover, air temperature, sun, time of year, water depth, wind, wildlife, plants, and current. It’s up to you to understand these variables and make responsible decisions when it comes to traveling on ice.

Never assume ice is safe because someone else was out before you. They may not be aware of the many variables that affect ice strength. Conversely, they may know potential problem spots that you don’t. It’s always a good idea to talk to local experts before heading onto an unfamiliar water.

No matter how well you know a waterbody, remember ice conditions change constantly. Always be prepared with the essential safety gear, and check ice thickness as you go.

How does snow affect ice strength?

snow covered lake showing high drifts

Deep snowdrifts on a frozen lake

The insulating effect of snow slows the freezing process. The extra weight also reduces how much weight the ice sheet can support. When the snow melts, it often creates “white ice” on top of the original, clear ice. White ice is much weaker than new, clear ice.

White ice vs clear ice

White ice occurs when snow melts on top of a sheet of ice and refreezes. This type of ice is only about half as strong as new, clear ice because it is full of air bubbles and made up of smaller crystals. As a result, the standard ice thickness guidelines should be doubled when traveling on white ice.

The strongest and safest ice is clear ice—sometimes called blue ice—which typically forms at the start of the ice season on top of calm water. The ice is clear but can appear blue or black when looking down into the water. This ice is formed with large crystals and few or no oxygen bubbles, creating very solid, strong ice.

close up view of clear ice with white cracks

Looking down into clear ice with white cracks.The white cracks in this solid ice are a normal result of changing air temperatures.

Large sheets of ice on land showing the stark contrast of clear ice and white ice

White ice vs clear ice

close up view looking down into a hole cut in the ice showing clear ice and white ice

Look carefully when measuring ice to see how much of it is clear ice. In this image, there are 10 inches of clear ice and 5 inches of white ice on top.

Why does ice melt along the shore first?

partialy frozen shallow water with dead weeds coming through

Shoreline vegetation and shallow water cause
the ice to melt faster near the shore in the spring.

As the temperature starts to rise in the spring and the ice begins to melt, you may think you are safer if you stay near the shore. However, ice typically melts fastest near the shore in the spring. The shallow water and abundant plant life cause the water to warm faster near the shore than in the middle of the lake. Along the shoreline, there are often rocks, logs, docks, and other structures that absorb heat from the sun, causing the water to warm even faster near those objects.

What is honeycomb ice?

rotten ice showing honeycomb texture

Honeycomb ice quickly thawing in the spring

Honeycomb ice is one of the most dangerous types of ice and associated with many fall-throughs during the late-ice season. When ice begins to thaw in the spring from the heat of the sun, the water on the surface of the ice seeps through thin cracks and pores in the ice. These cracks form in a honeycomb pattern due to the hexagonal structure of the ice crystals. If you observe a block of honeycomb ice, you will notice the ice has formed into columns perpendicular to the surface. It takes little or no effort to shatter it and break it into pieces.

Ice thickness guidelines do not apply to honeycomb ice. If ice is honeycombed, stay off.

What is shelf ice or stacked ice?

thick sheets of ice stacked over one another.

Shelf ice

Shelf ice is most common on larger lakes in Minnesota, specifically Lake Superior, Upper Red Lake, Leech Lake, Lake of the Woods, Mille Lacs Lake, and White Fish Chain of Lakes. When these large lakes freeze, they often have areas of open water due to wind and wave action. When portions of the lake freeze, strong wind can break up sheets of ice, sending them towards open water or pushing them onto shore. The accumulation of broken ice chunks washed ashore or piled up on the lake is known as shelf ice and can be very dangerous.

A pile up of shelf ice may appear to be solid, thick ice, but it is very unstable and people should stay off. Because the ice was not created uniformly, the thickness can vary greatly, and large pockets of air often fill the ice. Unsuspecting victims can fall through an air pocket into open water, which may be two or more feet below the surface of the ice. Climbing out can be nearly impossible.

What is a pressure ridge?

long ridge of ice showing ice chunks stack up along a line

Pressure ridge on Upper Red Lake, Minnesota

A pressure ridge or ice heave is a long ridge of ice that forms along a crack in the ice. They can be dangerous for snowmobile riders and other recreationalists. Ice ridges can range from a few inches in height to over 10 feet high, and they can form in a matter of hours, creating an unexpected obstacle for people returning to shore in the evening.

How do pressure ridges form?

Pressure ridges form as a result of contracting and expanding ice due to air-temperature fluctuations. Ridges are more likely to form during years with little snow coverage and extremely cold stretches followed by warm days. When the air temperature drops well below freezing, the ice contracts and causes cracks to form. The lake water seeps up into those cracks and freezes, filling the gaps. When the air temperature rises, the ice needs to expand but can no longer fill out the gaps.

The pressure from the expanding ice typically causes one sheet of ice to slide on top of the other. When this happens, it can result in a solid wall of thick ice. The sheet that gets pushed under often becomes flooded with water, resulting in a layer of thin, weak ice or even creating a patch of open water along the ridge.

large pressure ridge showing one sheet of ice on top of another.

Pressure ridge with ice buckled up and hidden by
deep snowdrifts.

Sometimes, however, both sheets of ice buckle and fold down or up. If the ice buckles and folds down, it can result in a stretch of open water with ice several feet beneath the surface. This type of pressure ridge is much harder to see and may be hidden by a thin layer of ice or snow.

Pressure ridges are always dangerous. Talk to local experts to learn about known ridges and always be on the lookout for newly formed ridges.

How do I get around a pressure ridge?

Because these ridges can form in just a few hours, snowmobilers and recreationalists must always be on the lookout, even when returning on the same path they took earlier in the day. If you see a pressure ridge, follow it at a safe distance until you find a safe path/opening to cross.

large pressure ridge showing one sheet of ice on top of another.

Pressure ridge can form in just a few hours, snowmobilers and recreationalists must always be
on the lookout, even when returning on the same path they took earlier in the day.

How does wind affect ice?

Strong winds have the power to quickly break up a layer of thin ice. If there is any open water on the lake, wind creates wave action that can deteriorate an ice sheet quickly. On large lakes such as Upper Red Lake and Lake Superior, people commonly choose to go on the ice before the entire lake freezes. While this is never recommended, it is especially dangerous on a windy day. If the wind is blowing out from shore, it can cause the ice you are standing on to break away from shore and float toward the middle of the lake. You will become trapped on the floating ice sheet, which also will continue to deteriorate from the wind and waves. This will put not only your life at risk but also the emergency responders who have to come out to save you.

Tip: On large lakes, you may not know if the entire lake is frozen, so pay attention to wind speed and direction. If there are strong winds early in the ice season, do not go on the ice. If winds pick up or change direction while you are on early ice, return to shore quickly.

long ridge of ice showing ice chunks stack up along a line

Rescue personnel blow up inflatable raft near open
water during a cold water rescue training.

long ridge of ice showing ice chunks stack up along a line

Ice sheets can quickly break away and float toward
the middle of the lake on a windy day

What are gas holes?

long ridge of ice showing ice chunks stack up along a line

Possible gas hole in the middle of a frozen lake

Decaying vegetation that releases gas bubbles causes gas holes. When the bubbles rise to the surface, they bring warmer water from the bottom of the lake, which slowly melts a bell-shaped hole into the ice. These holes can be several feet in diameter at their widest and only a few inches across at the surface. They are often hidden by a thin layer of ice or snow.

Gas holes tend to form in the same areas each year, often near stream inlets where vegetation and debris collect or in shallow water with heavy aquatic vegetation. If you are not familiar with the area, talk to the local experts to learn if there are any known gas holes on the lake.

How can wildlife affect ice?

long ridge of ice showing ice chunks stack up along a line

Flocks of waterfowl such as these swans can warm up
the ice and cause it to thin.

Schools of fish or flocks of waterfowl can adversely affect ice. The movement of fish can bring up warm water from the bottom of the lake. Cars and snowmobiles have broken through the ice in these instances. Groups of Canada geese or other waterfowl sitting on the ice also can cause thin patches.

What area of a lake might have unsafe ice?

  1. Areas with flowing water:
    1. Streams
    2. Under bridges
    3. Culverts
  2. Close to shore in late season
  3. Areas where gas holes are more likely:
    1. Stream inlets
    2. Shallow water with heavy aquatic vegetation
  4. Near logs, rocks, bridges, and docks (anything that can absorb heat from the sun). These structures often have thinner ice or open water near them.

view of open water under a bridge.

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