Frost Protection - UTL Repository

ACTIVE PROTECTION METHODS
FOAM INSULATION
Application of foam insulation to low growing crops for frost protection was
widely studied mostly in North America and it has been shown to increase the
minimum temperature by as much as 12 °C (Braud, Chesness and Hawthorne,
1968). However, the method has not been widely adopted by growers because of
the cost of materials and labour, as well as problems with covering large areas in
short times due to inaccuracy of frost forecasts (Bartholic, 1979). Foam is made
from a variety of materials, but it is mostly air, which provides the insulation
properties. When applied, the foam prevents radiation losses from the plants and
traps energy conducted upwards from the soil. Protection is best on the first
night and it decreases with time because the foam also blocks energy from
warming the plants and soil during the day and it breaks down over time. Mixing
air and liquid materials in the right proportion to create many small bubbles is
the secret to generating foam with low thermal conductivity. Several methods to
produce foam and apply it are reported in Bartholic (1979). However, Bartholic
(1979) notes that growers show an interest after experiencing frost damage, but
they rarely adopt the use of foam in the long term. Recently, Krasovitski et al.
(1999) have reported on thermal properties of foams and methods of application.
FOGGERS
Natural fog is known to provide protection against freezing, so artificial fogs
have also been studied as possible methods against frost damage. Fog lines
(Figure 7.15) that use high-pressure lines and special nozzles to make small (i.e.
10 to 20 µm diameter) fog droplets have been reported to provide good
protection under calm wind conditions (Mee and Bartholic, 1979). Protection
comes mainly from the water droplets absorbing long-wave radiation from the
surface and re-emitting downward long-wave radiation at the water droplet
temperature, which is considerably higher than apparent clear sky temperature.
The water droplets should have diameters about 8 µm to optimize the
absorption of radiation and to prevent the water droplets from dropping to the
ground. A fairly dense cloud of thick fog that completely covers the crop is
necessary for protection. This depends on the presence of light wind and
relatively high humidity. For example, Brewer, Burns and Opitz (1974) and
Itier, Huber and Brun (1987) found difficulties with the production of sufficient
water droplets and with wind drift. Mee and Bartholic (1979) reported that the
Mee foggers have energy use requirements that are less than 1 percent of heaters,
about 10 percent of wind machines and about 20 percent of sprinklers. They
also reported better protection under some conditions than from use of heaters.
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