Freezing temperatures, cold winds and fresh snow: layers of ice have already formed on the lakes, ponds and reservoirs. However, even if the icy surfaces are tempting for ice skating, the Climate and Environment Department strongly warns that it is extremely dangerous to walk on any water surfaces in Aachen's parks and forests. In the case of fenced-in ponds such as the Diepenbenden or Kupferbach dams, it is even completely forbidden to enter them.

Even after several consecutive nights of sub-zero temperatures, the dangers of ice skating should not be underestimated. There is a risk of collapsing, especially in open water. According to experts, layers of ice are only considered safe if they are at least 15 centimetres thick. For flowing water, the minimum thickness is 20 centimetres. The ice thickness is and remains very thin at the inlet and outlet points. And the fenced-in dams in particular pose a great danger. Steep banks and rapidly changing water levels during rainfall can cause the ice to break or form treacherous cavities under the ice.

Fish also live in many of Aachen's ponds. A large number of skaters startles the fish out of their hibernation, their metabolism is boosted and their breathing intensifies. This leads to higher energy and oxygen consumption and puts an enormous strain on the fish's energy balance. In the following spring, they often die without symptoms if they lack the energy they need to compensate for the stress situations associated with rapidly rising temperatures.