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Name: Tim
Status: student
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Prior to the formation of certain forms of precipitation the text book I have states that water exists in a "supercooled" state, one that is below freezing but has not begun to crystalize, how does this process occur?


Replies:
Dear Tim-

If I understand your question correctly, you want to know how water vapor attains its "supercooled" status.

One of the properties of water is its reluctance to condense or crystalize without some nucleii present for that purpose. Condensation nucleii can be dust, or salt particles. Without sufficient nucleation particles, the water vapor will continue to cool, reaching temperatures as low as -20C before condensing or crystalizing.

The method for cooling the water vapor in the atmosphere is almost always an adiabatic process, that is, one in which heat neither added or extracted from the parcel. The parcel cools due to the expansion of the gas as the pressure is reduced as it ascends in the atmosphere. There are different physical processes that cause the air to rise, such as circulations across different kinds of frontal boundarys, convective ascent in thunderstorms, and others.

It is the task of the meteorologist to determine where these areas of rising air will occur, and then determine the type of precipitation that will result. This is one of the more interesting jobs of the weather forecaster.

Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO


Tim,

A flat, undisturbed surface of water has a vapor tension that is dependent on the temperature of the water. However, the vapor tension (often called the surface tension) of a droplet is greater because it is not a flat surface. The smaller the droplet, the greater the curvature of the surface and thus the greater the vapor tension. Vapor tensions above that of a flat water surface prevents freezing at 32 degrees F. The greater the tension, the lower the temperature the droplet can be cooled to before it will freeze. This is called "supercooling" or "undercooling". Since most water droplets form on a nucleus, usually of particulate nature that is hygroscopic (dissolves in water), most droplets are actually a weak solution. The solution often has a greater vapor tension than pure water, thereby making the freezing temperature even lower.

David Cook
Meteorologist
Argonne Nat. Lab.


It's true that below its freezing point liquid water has more potential energy than solid water (ice), and that it is energetically favorable for the water to freeze. However, there is a significant difference between a process being physically favorable and being easy. To change a state of matter, whether boiling, freezing, condensation, or melting, it is usually easiest if there is some surface that the molecules of the phase-changing substance can adhere to. This allows them to reconfigure into different arrangements, including some resembling the "normal" arrangement of the new state. These "nuclei" can then recruit other molecules to join them, and soon the whole sample will change its state.

Without these nuclei, matter can exist in an unfavorable or unstable state for a long time before it spontaneously converts to another state.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


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