Ice Feathers?

[Steve M]

Can someone please remind me how super cooled water droplets stay in a liquid state in an atmosphere well below freezing?

[bclark]

Can someone please remind me how super cooled water droplets stay in a liquid state in an atmosphere well below freezing?

The droplets don't have anything to crystallize onto until they impact "something" (in the case of rime ice formation) or until they find something to "seed" that crystallization (in the case of high clouds or snow) like a speck of dust or other minute particulates in the air.

In general, water droplets on the order of magnitude of those that make up fog will remain in a super-cooled state down to somewhere around -40 F, if I remember correctly from cloud physics class (Ryan, Jim, Bill O or someone else please correct me if I am wrong!), before they will spontaneously crystallize.

[Bill O]

Yes, -40F is the magic number, which also happens to be the same in F as it is in C.

I remember learning all the details about why you need an ice nuclei. All I know now is that you need one.

[Patrad Fischroy]

I think that it has to do with the energy required to shift all of the jostling water molecules into an ordered structure. Imagine this analogy, you have a roomfull of children and you want to get them all lined up. Just think of the energy that you would expend running from one end to the other. Once you get them in line and tied securely together it isn't so hard to keep them there.