Spraying Plants to Prevent Cold Damage
Name: Irwin W.
Why do farmers spray crops with water to prevent them
from freezing? My weather book says "when the water freezes on the
vegetation, the latent heat released imparts just enough heat to warm the
plant." Doesn't the phase change, from water into ice, result in
continued absorption of heat energy from both the outside air and the
inside plant liquids?
Te Handy Weather Book; Lyons, Walter A.; 1996; ISBN 157859054x
Your weather book is correct. However, what's often overlooked (or left out)
is the fact that when the spraying and freezing stops, the ice overlayer
(and the covered plant tissue) will fall to ambient temperature. Thus, the
process only works so long as water is continuously added, and continuously
freezing. The trade-off comes into play if/when the weight of ice load on
the plant is great enough to damage it. In general, the process is a short
term solution applied in the hope that the freezing weather won't last too
Regarding your question: Yes, the ice formation on the outside of the plant
does cool the liquids inside the plant to a temperature of 32 F. However,
the liquid inside the plant is not pure water. Rather, it is a solution of
various substances dissolved in water. As such, those dissolved materials
serve as a kind of natural anti-freeze that depresses the freezing point of
the water to below 32 F. How far below 32 F is dependent on the
concentration of those dissolved substances. This enables some plants to
withstand the lowered temperature.
When liquid water touches the plant and the air is below freezing, the water
freezes instead of the plant. When there is liquid water on the plant, the
temperature cannot drop below freezing - the water all has to freeze first.
So if farmers spray their plants throughout the time of frost danger, the
plants will not freeze.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
I do not think it is the latent heat of freezing is the reason that crops
are sprayed with water. The two reasons that this technique is useful are:
1. Ice has a rather low thermal conductivity, especially when sprayed on to
form a "snow" on the leaves and branches. 2. the coating of ice / snow
protects the plants from wind which carries heat and moisture away from the
plant, just as it does to people -- i.e. the wind chill factor you hear and
read about in weather forecasts. These are the same reasons people burrow
into the snow if they are caught in a snow storm on some mountain, and why
The phase change from water into ice requires a RELEASE of energy. The
matter must cool down. This energy is what goes to protect the plant. If
the ice were melting, then it would require heat from the surrounding air.
This is the same reason that the weather near a body of water is a little
more temperate. Cooler in the summer, (takes a lot of heat from the air to
heat up Lake Michigan), as well as warmer in winter, (Lake Michigan takes a
long time to freeze giving up energy as it does so.).
I hope that clears up things a little.
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Update: June 2012