Los Angeles herald., 1908-03-09
HOW TO PROTECT
FRUITS FROM FROST
BEST METHODS KNOWN TO SCI
ENcE DISCUSSED
Growers Should Study Weather Con-
dltions In Their Locallty--Pro-
eess Explained by
Officials

The problem of protection from frost
may be considered under four sections.
as follows:
1. The issuance of adequate frost
warnings.
2. A discussion of the physical pro-
cesses operative in the formation and
dissipation of frost.
g. The construction of devices and
apparatus based upon the principles
enumerated in No. 2.
4. The intelligent use of these de-
vices so that maximum efficiency may
be obtained at the critical hour.
A period of strong north winds fol-
lowed by clear skies is period of
danger. Frost is essentially a problem
in air drainage and some knowledge
of the movement of the air over the
ground, or In other words, the local
air circulation is essential for success
full frost forecasting. The chief cause
of cooling is undoubtedly radiation, and
this is controlled to a large degree by
the relative purity and stiffness of the
air. When, therefore, the lower air
strata are quiet and dust free, also
vapor free, conditions are favorable for
intense radiation; and consequently
rapid cooling of both vegetable fiber
and the soil. All fruit growers should
carefully study the weather conditions
preceding frost in their localities. The
district forecaster at San Francisco ana
observers at any weather bureau sta
ton will gladly receive suggestions in
the matter of local air drainage anu
frost formation, and will, so far as
their time permits, investigate and re-
port upon local features.
Under the second heading. discussion
of the physical processes involved in
frost formation, we would call atten-
ton to the following ways of prevent
ing the fall in temperature:
l. Adding heat
2. Adding water vapor.
3. Adding both heat and water vapor.
4. Ventilation, or mixing of the air.
5. Irrigation, or use of water.
8. Use of heated water.
Z. Use of screens or covers, prevent
ing loss of heat by radiation.
8. Spraying.
All of the above processes can be
found explained in detail in publica-
tons of the weather bureau. The ac-
ton of water in any of its forms;
whether solid, as ice or frost; fluid. as
water; or gaseous, as water vapor, Is
protective. Large amounts of heat
are given OFF in the change from vapor
to liquid and from liquid to solid. The
oretically it is possible by the applica-
ton of sufficient water to prevent In
jury by frost. AS the plant cools, ow-
ing to radiation. and an indraft of cold
air, the vapor present condenses, giving
free a large quantity of heat, some
thing like 900 calories; and If the
water turns to frost. more heat is set
free. about so calories. If. for example.
in an ordinary rain gauge enough frost
has formed to weigh, when melted,
about an ounce, then the latent heat of
condensation is approximately 90S
times $50, or 617, 440 calories, enough to
raise the temperature of the air at
freezing 4 degrees over a space of ten
feet square and to a considerable
height.
Note also that the water plays an
important part when the sun rises and
the warming OF the vegetable fiber be-
gins. Here the water retards the heat
ing, and is thus very beneficial. Chilled
plants sprayed with cold water about
sunrise are less liable to injury. The
latent heat of evaporation and fusion
now comes into action in an opposite
direction, the heat being utilized In
changing the frost into water or the
water into vapor. This, in the writer's
opinion, is the critical period, and she
believes that more injury results from
the too rapid warming of plant fiber
after 1t has been chilled than from the
chilling itself. He has known of cases
where delicate flowers have been in
cased in ice and yet no injury resulted,
largely because the thawing out was
gradual. In examining many orchards
1t has been ascertained that. the por-
ton most exposed to the suns heat in
the morning is, other things being
equal, the portion of greatest injury.
Up to the present time the chief effort
In frost protection has been to pre-
vent loss of heat by radiation from the
ground; and protective methods have
been rightly based upon the possible
saving and storage of heat But 1t may
be none the less profitable to study the
problem from a later stage, namely.
the period of the chilled fiber. While
1t Is not definitely known what action
during frost causes the death of the
cell tissues, whether 1t be a rupture of
cell walls or a separation and with
drawal of water content, 1t would seem
that we could not err in guarding the
plant fiber from any strain due to rapid
and unequal heating when in chilled
condition. Beginning before sunrise,
some screen or covering should be in
terposed between the fruit and the
suns rays. For the reasons given
above 1t is also advisable to apply wa
ter. 1t may interest fruit growers to
point out to them that the actual tenn
perature of the air Is probably of less
Importance in frost formation than the
cooling of the plant surface due to ra-
diation. Nearly all fruit growers now
understand that after sunset, If there
be no clouds present and but little mo-
ton of the air Though it is we'll to test
this by watching the flow of smoke
near the ground) the plant surface
emits heat rapidly into space. The un-
der surface of the foliage of course in
tercepts heat from the ground, and so
we seldom find frost forming under a
tree. The connective currents of the
air are generally sluggish and there Is
as a, rule little mixing of the air strata,
or no thorough circulation at such
times. Helmholtz has shown that lay
ers of air of different temperatures may
lie close to one another without mix
ing, and indeed this is known to be the
case not only near the ground, but
even in free air Doubtless herein lies
the explanation of the streakiness of
frost. We have examined orchards
which were apparently level; and yet
upon close inspection it was found that
although there was no noticeable slope
of the ground, there were certain cham
nels of air motion and certain stagnant
zones. The frost streaks were found to
coincide with the stagnant areas Much,
however, depends upon the nature of
the radiating surface.
The warming or the chilling of the
ground has not much effect upon the
air at a distance of ten or twelve feet.
That is to say, the rate of heat con-
duction from one layer to another is
very slow. Therefore 1t is not unusual