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?
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,
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
Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
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.
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.
PG Research Foundation, Darien, Illinois
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Update: June 2012