Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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temperature will fall to as low as the wet-bulb temperature. If the dew-point
temperature (humidity) is low, then the wet-bulb temperature can be considerably
lower than air temperature, so the temperature of wet plant tissue can fall well
below air temperature and cause damage if insufficient water is applied. Also, if the
rotation rate is too slow or if the sprinklers are stopped too early, temperatures can
drop below the critical damage temperature and cause damage.
In older literature on the use of over-plant sprinklers, it was common to warn
against a sharp drop in temperature when sprinklers are started during low-dewpoint
conditions. Under windy low-dew-point temperature conditions when the
air temperature is relatively high (e.g. 10 to 15 °C), evaporation from the
droplets causes the water droplet temperature and hence the air temperature to
drop rapidly when the sprinklers are started. However, the water droplet
temperature commonly falls to near 0 °C as they pass from the sprinkler heads
to the plant surfaces. Consequently, there is no reason why the air temperature
would drop below 0 °C when the sprinklers are started. As shown in Figure 7.10,
the plant surface temperatures will drop below 0 °C as water sublimates from
the plant surfaces. But the temperature rises again when hit with a new droplet
of liquid water.
FIGURE 7.10
Leaf-edge temperature changes when wetted by a sprinkler system applying
water at 2.8 mm h -1 with rotation rates of 120, 60 and 30 s, air temperature
near 0 °C, a wet-bulb temperature near -2 °C and a wind speed near 5.5 km h -1
(after Wheaton and Kidder, 1964)
LEAF EDGE TEMPERATURE (° C)
6.0
3.0
0.0
application rate = 2.8 mm h -1 wind speed = 5.5 km h-1 = 1.53 m s-1
air temp.
wet-bulb temp.
-3.0
0 20 40 60 80 100 120 140 160
T I M E ( S )
freq = 120 s freq = 30 s freq = 60 s
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