Why doesn’t Niagara Falls freeze in weather that’s tens of degrees below zero?

Water is a typical crystalline material. The mechanism of solidification of water molecules is not just related to temperature. In microscopic thermodynamics, the transition point between the solid and liquid states of molecules is often combined with the expression of intermolecular Gibbs free energy. When the water flow is too large, the Gibbs free energy between water molecules is easier to exhibit liquid properties due to the greater mobility.

The macroscopic explanation is: water will form relatively large crystal nuclei when the temperature is low, but due to the stirring effect of the flow, the large crystal nuclei are broken, making it difficult to form large crystals. However, as long as the time is long enough, there will be more and more fine crystal nuclei in the water, making the water more viscous and less fluid, and it will eventually freeze, but the process will take a longer time. . If impurities such as fine sand are added to the water, the degree of supercooling when the water becomes ice can be greatly reduced. This is because crystal nuclei are formed during crystallization, and adding fine sand increases the nucleation rate and reduces the risk of freezing water. required temperature.

The growth rate of crystal nuclei and the water flow rate are a key criterion for determining whether supercooled water freezes. Under the premise that the temperature is low enough and there are enough condensation nuclei, if the growth rate of crystal nuclei can be made greater than the flow rate of water, water will definitely freeze. This is why some low-rise waterfalls freeze in winter. When the height of a waterfall is large enough and the water flows fast enough, it is more difficult to freeze.