Super cooled water in the atmosphere and water

It happens as the initial state is not really unstable. We say: “the system is in metastable state”.

A

B

C

A. Unstable state

B. Metastable state

C. Stable state

H

h

Super saturated water vapor and super cooled water in the atmosphere are metastable thermodynamic systems.

To start condensation or freezing, some nuclei of the new state are needed

(crystallization) nuclei

The nuclei can appear due to density fluctuation (random molecules approach each other)

Random molecular condensation (crystallization)

Energy released due to random condensation (crystallization), Er

Condensation (crystallization) on some larger nuclei releases additional energy EL.n.

Growth the nuclei requires some additional energy EN. It may come from condensation (crystallization)

Process will not go due to energetic disadvantage

There are condensation (crystallization) nuclei

Condensation will proceed, and the energy released will be scarified for formation of larger nuclei

Heterophased nuclei

may freeze becoming ice particles. For the ice phase to appear a nucleus of the ice must be formed inside of the droplet.

Such process is called homogenous phase transfer

The new phase can appear on foreign body (heterogeneous nucleus). Such kind of bodies are called crystallization nuclei. Accordingly this process is called heterogeneous phase transfer

In the atmosphere both homogenous and heterogeneous phase transfer may take place

present, meteorologists believe that condensation nuclei become (at least partly) crystallization ones as the air temperature grows down below 0°C

Crystallization start

No crystallization

Crystallization nuclei are present.
Heterogeneous phase transfer

Crystallization nuclei are not present.
Homogenous phase transfer

It is believed that amount of crystallization nuclei in the atmosphere is rather small. Therefore, the major role in crystallization process is played the homogenous phase transfer. This conclusion is supported by the observed facts of super cooled water droplet clouds (T=-12…-20°C) existence for a long time.

drops and ice crystals

Initial time

W.V. sublimates on ice crystal

Next time

Drops evaporate

And so on until all drops become evaporated. However this process is quite long, practically, it is infinite.

to obtain unknown quantities r1 and r2 and a at a given moment of time.

D denotes diffusion of the water molecules