Ice Nucleation

Ice can form in the atmosphere in one of two ways. One way is for water vapour to spontaneously form ice (known as homogeneous nucleation) but this only happens at very cold temperatures such as -40 C.

The other way is for a nucleus (such as dust, bacteria, etc) to serve as a starting point for the ice to form. This is the method by which most snow in the atmosphere forms. Nucleation simply refers to that initial transition from below-freezing liquid water (not a crystal) to ice crystal. After that, the ice sort of 'self-nucleates' and crystal growth takes over.

Nucleation by dust, ice, or bacteria helps ice form because of two things:

First, it reduces the amount of surface energy needed to form a crystal. This may seem complicated, but really it's not too bad. If you already know that creating a surface takes energy (specifically, surface energy) then you can probably see that if you freeze half of a water droplet, then you've actually created a new surface — the one in-between the liquid water and the frozen water.

In this way, a difference in phase is like a new surface. This is why 'flash freezing' or homogeneous nucleation only happens at very cold temperatures, or when it can happen all at once to a droplet.

If you add in something — like dust — that provides the surface already (but not one that's so small that the vapour will use a lot of energy wrapping around it, and not one that's so big that it won't have frozen yet) then the water vapour drop doesn't have to create a new surface, it just has to create half of one and then propagate it within the droplet.

The second reason nucleation helps is wetting. Imagine a thick lake and a think lake. Which freezes first? The thin lake.

Now imagine a given amount of water in a droplet. Will that water freeze and crystalize faster in spherical form, or in hemispherical form? Hemispherical form, because it's thinner. When you have a droplet in the air, it is spherical, when you have a droplet on something (let us say wetting the side of your arm) then it becomes thinner. Thinner bulks of liquid can dissipate their latent heat better, and freeze faster.

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