Just as urban areas can cause problems related to heat, they also can make winters more hazardous. When ice forms on skyscrapers and other tall buildings, a dangerous situation may arise upon thawing. Large sheets of ice have been known to break off of structures and plummet to the streets below. One notable case of this happening occurred in Chicago, Illinois, in April 1995, when the danger of urban ice slabs dropping onto Michigan Avenue compelled authorities to close the street for hours.