Ice Travelers, Beware!

One of Nature's nasty tricks for the unwary traveler is the little known fact that there is a particularly dangerous speed for crossing lake or ocean ice. Moving either more slowly or more rapidly than this dangerous critical speed makes for a safer crossing.

It is not immediately obvious why there is a critical speed at which a moving object--a truck, a snow machine, an ice skater or a taxing aircraft-- is most likely to break through the ice. The reason is that the object moving across the ice at other than very slow speed creates a wave in the ice, i.e., a moving deflection of the ice. Just as a sound wave moves at fixed speed through air, the wave in the ice moves at fixed speed. The speed of the wave in the ice increases with the ice thickness and the depth of the water beneath the ice. A typical speed for the wave in foot-thick (30 cm) sea ice over water 20 feet deep (7 meters) is 20 mph (32 kph).

Now if the object creating the wave moves at the same speed as the wave, the object stays with the wave, just as a successful surfer stays with a water wave riding into the beach. Staying with the wave is great fun for the surfer, but it is disaster in the making for the object riding with the ice wave. The trouble is that the effective weight of the object on the ice builds up and becomes many times the static weight.

Engineers have known about this dangerous phenomenon for many years, but only this year a new method of calculating the stresses on the ice has made it possible to learn how much the effective weight increases when an object moves at the critical speed across the ice. Devised by Professors Howard Bates and Lewis Shapiro of the Geophysical Institute, the new method shows that a 10-ton truck moving at critical speed across ice can create the same breaking stress as a slowly-moving truck weighing 100 to 150 tons. Moral: drive slowly on ice to avoid hitting the critical speed.