Published on: 2.12.2019

Information produced by Finnish Environment Institute

Is the ice strong enough?

Fast ice, black ice, pack ice, drift ice, slush ice, grease ice. Ice comes in many forms, and you should be familiar with them before attempting to walk on it.

A lake freezes over gradually, starting with light grease ice in sheltered bays. The ice then grows thicker and spreads further from the shore, until the entire lake finally has a solid ice cover. The ice may remain thin and fragile throughout the winter over straights and areas with a current, however, and cracks may appear in it on large lakes and out at sea.

Transparent, glass-like ice that forms on undisturbed water is known as black ice, or clear ice. Black ice is sturdy and will already carry a person at a thickness of five centimetres.

If the early winter is mild, the snow load may press down on thin ice and water may rise on top of it, creating slush which freezes when the temperature drops. The cloudy white slush ice resembles solidified milk, and its bearing strength is only one half of what could be expected of clear ice.

Fast ice on the coast is stagnant and attached to the shores and islets. Drift ice is carried along by winds and currents and sometimes accumulates into large stacks known as pack ice.

When walking on ice you should keep an eye on its quality and thickness, for example using a stick with a metal tip, and always carry ice picks. Before your excursion, you should familiarise yourself with safety guidelines and equipment. Care is needed especially in late winter: spring ice is fragile and may become treacherous within a few hours.

Image: © Anne Saarinen, Vastavalo