WRAL WeatherCenter Blog

Some super cool videos!

Posted August 9, 2010

While the videos of the title are of varying quality, the real common theme here is in demonstrating the behavior of supercooled liquids, in particular water. Supercooling is an important process in meteorology, and plays a part in the formation of aircraft icing, hail, the rapid transfer of water molecules from liquid droplets to snow crystals, and at times in the production of lightning and the occurrence of freezing rain.

Essentially, supercooling means lowering the temperature of a liquid beyond its typical freezing point without actually turning it into a solid, in the case of water, cooling it below 32 degrees F while remaining in liquid form. This happens naturally in the atmosphere when relatively pure cloud or precipitation droplets either form or are carried into areas with colder temperatures in the absence of something called "ice nuclei," which are particles that have molecular properties that favor the formation of ice crystals. In the air, these particles (some types of dust, minerals, even some bacteria) are relatively uncommon compared to the number of "condensation nuclei," which act as sites for the formation of liquid droplets.

In our freezers at home, however, or in the outdoors when the temperature falls below 32 F, most of the time the water contains impurities, or the container holding the water contains impurities or sharp corners or other features that can serve as ice nucleation sites. Given this, water typically freezes pretty quickly after falling below the "freezing point." However, very pure water in a relatively pristine container, left undisturbed and chilled gradually, may also drop well below freezing while remaining in a liquid state. Then, if the water is disturbed by shaking, or a "seed" in the form of a small ice crystal or other favorable nucleating material is introduced, ice crystals will develop at the seeding point, then serve as further seeds themselves such that the entire container will freeze, usually into a slush with a temperature very close to the freezing point.

Quite a few people have documented this process in a variety of ways on video, and have posted the results on the web. It's a neat process to see, and some of you may have experienced it yourselves. It can also occur with liquids like soda and juices, so long as the impurities, in the form of sweeteners,  flavorings and dissolved gasses, are not favorable for ice crystal formation. For those of you who haven't seen it, I've included links to a few videos showing different facets of this process. With temperatures reaching the mid 90s to around 100 again this week, it may be a nice time to think about supercooling!


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