February 2021 - 25

FROST CONTROL
temperature experienced by trees will not
necessarily be the same as the reported
ambient temperature.
To measure the temperature of the flower
buds, farmers must take into account
the cooling effect of evaporation of the
moisture on the trees, which is referred to
as the wet-bulb temperature. The wet-bulb
temperature is the lowest temperature
experienced at the measured relative
humidity (RH) of the air. The wet-bulb
temperature takes into account the cooling
caused by the evaporation of air moisture.
Generally, the wet-bulb temperature will
be a few degrees lower than the dry-bulb
temperature except at the 100% RH.
Recently, Knox et al. (2017) at the
University of Georgia showed that wetbulb
temperature can be simply derived
by " subtracting one third the difference
between ambient temperature and dew
point, from the ambient temperature. "
Modern frost alarms measure wet-bulb
temperature, which can directly transmit
real-time data to the user's mobile phone
or computer.
Relative humidity
The RH is often overlooked in orchard
frost management. However, it plays a
significant role in determining the extent
of frost damage. As RH increases, moisture
condenses near sensitive flower parts,
which at subfreezing temperatures begins
to form ice crystals. According to Levitt
(1980), it is likely that freeze injury results
from ice forming outside the cell wall and
inside the plant tissue. Higher RH can
cause greater frost damage compared to
low RH, for the same freezing temperature.
Dew point
Dew point is commonly reported
as the atmospheric temperature below
which water droplets begin to condense,
forming dew or fog. At low temperatures,
the moisture in the air transitions from a
gas to a liquid, as air temperature reaches
the dew point. A higher dew point is
associated with relatively slow drops
in temperature and vice versa. A lower
dew point also generally means surface
moisture will evaporate quicker.
Cloud coverage
Clear skies normally result in lower low
temperatures than when there is cloud
cover trapping radiant energy from the
ground.
Wind speed
Wind is a major determinant of the
rate of loss of thermal energy. Almost all
of these FP methods will have reduced
efficiency at wind speeds above 10 mph,
and are therefore not recommended when
winds are above 10 mph.
Effective FP requires weather stations
and sensors for each of the fields you are
seeking to monitor and protect. Data
from nearby weather stations may not
always be reliable. On-farm weather
stations are the most accurate source of
weather data and should be linked with a
frost alarm. This alarm will sound based
on the wet temperature and should be
precise down to 0.1˚ F. FGN
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Your Edge - And Ours - Is Knowledge.
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Visit our website: www.trece.com or call: 1- 866-785-1313.
© 2019, Trécé Inc., Adair, OK USA * TRECE, PHEROCON and CIDETRAK are registered trademarks of Trece, Inc., Adair, OK USA * TRE-1387, 1/19
FGN | FEBRUARY 2021 | 25
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February 2021

Table of Contents for the Digital Edition of February 2021