# How can microscopic water droplets remain unfrozen to near -40° C?

**URL:** <https://boards.straightdope.com/t/how-can-microscopic-water-droplets-remain-unfrozen-to-near-40-c/682542>\
**Category:** Factual Questions\
**Created:** [February 28, 2014, 8:24pm UTC](https://boards.straightdope.com/t/how-can-microscopic-water-droplets-remain-unfrozen-to-near-40-c/682542 "2014-02-28T20:24:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\
**Post date:** [February 28, 2014, 8:24pm UTC](https://boards.straightdope.com/t/how-can-microscopic-water-droplets-remain-unfrozen-to-near-40-c/682542/1 "2014-02-28T20:24:09Z")

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See [subject](http://twistedsifter.com/2013/03/microscopic-images-of-snow-crystals/).

Also: Graupel. Which is part of the subject but I’m not going to explain nuh-uh because my feelings were hurt being called an unclear OP in another thread.

From cite:  
Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through atmospheric supercooled cloud droplets. These droplets, which have a diameter of about 10 µm, can exist in the unfrozen state down to temperatures near -40° C. Contact between the snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystals.

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**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\
**Post date:** [February 28, 2014, 8:47pm UTC](https://boards.straightdope.com/t/how-can-microscopic-water-droplets-remain-unfrozen-to-near-40-c/682542/2 "2014-02-28T20:47:07Z")

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Wikipedia has a good explanation of it. The phenomenon requires very pure water that is free from contaminates that serve as nucleation sites for ice to form. Without a suitable nucleation site, water can be cooled well below freezing although ice will form as soon as a contaminate is introduced. It could be something as small as a speck of dust. It can lead to a dangerous set of circumstances when airplanes fly through supercooled water droplets. They can suddenly freeze over the wings and control surfaces when they hit them.

> **[Supercooling](https://en.wikipedia.org/wiki/Supercooling)**
>
> Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal structure can form. The supercooling of water can be achieved without any special techniques other than chemical demineralization, down to −48.3 °C (−54.9 °F). Droplets of supercooled water often exist in stratus and cumulus clouds. An aircraft flying throu Animals ...

You can supercool larger quantities of water as well. It has to be very pure and stored in a smooth container that doesn’t provide any nucleation sites for ice to form. As soon as you introduce one, the water will freeze extremely quickly as shown in this video.

[![](https://i.ytimg.com/vi/Fot3m7kyLn4/hqdefault.jpg "supercooled water") ](https://www.youtube.com/watch?v=Fot3m7kyLn4)

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**Author:** ![TonySinclair](https://avatars.discourse-cdn.com/v4/letter/t/49beb7/32.png) [@TonySinclair](https://boards.straightdope.com/u/TonySinclair)\
**Post date:** [February 28, 2014, 8:51pm UTC](https://boards.straightdope.com/t/how-can-microscopic-water-droplets-remain-unfrozen-to-near-40-c/682542/3 "2014-02-28T20:51:20Z")

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I don’t know, but I bet they couldn’t remain unfrozen to near -40° _ **F** _.