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 [ experienced grower would attempt to grow citrus further to the west where potential frost damage is considerably higher due to regional scale cold air drainage. A third reason for growing citrus in this area is that the fog often clears in the afternoon, so sunlight can strike the soil and plants to store some heat during the day. This would not be the case on the west side of the valley because a mountain range to the west blocks radiation during the late afternoon and evening. On the east side of the valley, the slope of the land is generally facing to the west, so the receipt of energy per unit area from the sun is better on the east than the west side of the valley in the afternoon. The first step in selecting a site for a new planting is to talk to local people about what crops and varieties are appropriate for the area. Local growers and extension advisors often have a good feeling for which locations might be problematic. One should avoid planting in areas where low, ground fogs form first. Low ground fogs are radiation fogs, and, like radiation frosts, they tend to form in the coldest spots. This should not be confused with high inversion fogs that form well above the surface, or steam fogs that come in from the ocean or large bodies of water. Areas with high inversion or steam fogs are actually less prone to frost damage. The next step in identifying a good planting site is to look for climatic data to characterize the probability and risk of frost damage. A good source of climate data is the FAO CLICOM dataset, which can be accessed through the FAO Web site (http://www.fao.org/waicent/faoinfo/agricult/agl/aglw/climwat.stm). In locations where climate data are limited or unavailable, it is worthwhile to conduct a minimum temperature survey of the planting site during at least one frost season before risking losses to frost damage. Ideally, one would record air temperature each day with a continuously recording sensor mounted inside of a Stevenson screen (Figure 6.1) standard weather shelter. One advantage from using a Stevenson screen is that the temperatures are then comparable with climate records from weather services that typically use Stevenson screens to shield instruments. If available, it is also desirable to measure relative humidity and wind speed and direction. In recent decades, electronic temperature and humidity sensors are more commonly used and a Gill radiation shield (Figure 6.2) rather than a Stevenson screen is often preferable. Because they are inexpensive and easy to construct, fruit-frost shelters are often used for nighttime temperature measurements during frost events (Figure 6.3). Regardless of the sensor shield, the temperature sensors are typically mounted at between 1.25 and 2.0 m height above soil level. The chosen height should be the same as that used by your local weather service. Some meteorologists and growers 116
PASSIVE PROTECTION METHODS use an “actinothermal index”, which is simply an unshielded thermometer mounted on a wooden support (Durand, 1965; Perraudin, 1965; Schereiber, 1965). The thermometer is mounted at 0.1 m height for short crops and 0.5 m height for taller crops. Because the thermometers are unshielded, the temperature is purported to be close to that of a plant branch or twig. To evaluate the suitability of a site, collecting nocturnal data during 10 to 20 clear, cold nights should provide sufficient information to assess the potential for frost damage (Bouchet, 1965). FIGURE 6.1 Stevenson screen weather shelter Photo: J P de Melo-Abreu (ISA) 117
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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CHAPTER 8 APPROPRIATE TECHNOLOGIES
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APPROPRIATE TECHNOLOGIES frost prot
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APPROPRIATE TECHNOLOGIES fashion. O
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES other crop
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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