Icicles may look beautiful hanging off buildings and other structures during the winter, but they bring a danger as they grow larger and fall.
There are no icicle injury statistics in the United States. The U.S. Occupational Safety and Health Administration reports that it investigated 16 fatalities or serious injuries connected with snow and ice removal from roofs in the last 10 years.
In most cases, icicles form when snow on a roof is melted from below due to heat traveling upward from the heated space inside the building. The resultant melted water flows out to the eaves of the roof and refreezes when the outside temperature is below freezing, AccuWeather.com Forensic Meteorologist Steve Wistar said.
Icicles can become very dangerous when they grow large, especially when they have a long distance to fall, Wistar said.
"Over a long falling distance (at least 30 stories), they will accelerate until reaching terminal velocity, which has been calculated by physics professor Andreas Schroeder at the University of Illinois-Chicago to be 80 to 90 mph," he said. "A half-pound icicle, 3 inches in diameter, falling at this terminal velocity exerts a 1,000-pound force on whatever it hits. People have been seriously injured or killed by such projectiles."
The risk of being hit by falling icicles would be greatest when the wind is strong or when the sun is shining directly on the icicles which can partially melt and loosen where they are attached, Wistar said.
Besides the risk of injuries and fatalities, large icicles falling with this force can cause significant property damage.
Massive icicles fell from buildings in Boston last Sunday, shattering windshields of cars, amid a winter thaw, according to CBS Boston. The incident resulted in street closures.
Ice can build on high wires, towers and skyscrapers, posing serious threats to people and objects below when it is windy or sunny.
"It is common for pedestrian areas below such structures to be closed when there is a risk of falling ice," Wistar said.
"This could happen anytime in the winter after a significant snowfall. Loosening by sunshine would be most common later in the winter, in February and March, when the sun is higher in the sky and thus causing more radiative warming," he said.
It is safe to knock down little icicles at any time, Wistar said.
"With bigger ones, it is safe when neither you nor anyone else is underneath them. I stand well off to the side if I am knocking down a large icicle," he said. However, hiring professionals to help clear snow and icicles off roofs is always the safest bet.