Frost Protection - UTL Repository

More documents

Recommendations

Info

] 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 [ an expected frost night, the wind machines (and heaters) should be started earlier to attempt to dry the fruit before ice can form on the fruit. Wind machines are not recommended when the wind is blowing at more than about 2.5 m s -1 (8 km h -1 ) or when there is supercooled fog. When the wind is more than 2.5 m s -1 , it is unlikely that an inversion is present and it is possible that the fan blades could experience wind damage. A simple method to estimate the wind speed is to hang plastic ribbon from a tree branch or street sign. Police crime scene tape is an example of the type of plastic ribbon that could be used. Such ribbon can be purchased from farm or engineering supply stores. If the wind is greater than 2.5 m s -1 , the bottom of the hanging ribbon should be blowing back and forth to about 30 cm from horizontal. In a supercooled fog, water droplets can freeze on the fan and severe wind machine damage can occur if the ice cases one blade to break off but not the other. Vertical flow wind machines The use of vertical flow fans to pull down the warm air aloft has been investigated; however, these fans generally work poorly because mechanical turbulence mixing with the trees reduces the area affected by the ventilation. Also, the high wind speed near the base of the tower can damage horticultural and ornamental crops. Wind machines that blow vertically upwards are commercially available and there has been some testing of the machines. The idea is that the fan will pull in cold dense air near the ground and blow it upwards where it can mix with warmer air aloft. In theory, cold air is removed near the surface and the warmer air aloft drops downward hence lowering the inversion. Limited testing has shown that this method has a temporary positive effect on temperatures near the fan; however, the extent of influence and duration of the effect is still unknown. To our knowledge, the method has only been used in small valleys where cold air ejected upwards is likely to fall back towards the surface. In a location where prevailing winds aloft might move the air horizontally away from the crop, more protection could result. However, there is no known research evidence at this time. Helicopters Helicopters move warm air from aloft in an inversion to the surface. If there is little or no inversion, helicopters are ineffective. Due to the large standby and operational costs, the use of helicopters for frost protection is limited to high value crops or emergencies (e.g. when the normal method breaks down). 160
ACTIVE PROTECTION METHODS Authors differ on their estimates of the protection area for helicopters. The area covered by a single helicopter depends on the helicopter size and weight and on the weather conditions. Estimated coverage area varies between 22 and 44 hectares (Evans, 2000; Powell and Himelrick, 2000). Passes are needed every 30 to 60 minutes, with more passes under severe conditions. Waiting too long between passes allows the plants to supercool and the agitation from a passing helicopter can cause heterogeneous nucleation and lead to severe damage. Temperature sensors are often mounted on the outside of the helicopter and the pilots fly at a height where they observe the highest temperature reading. The optimal height is commonly between 20 and 30 m. Common flight speeds are 25 to 40 km h -1 (Powell and Himelrick, 2000) or 8 to 16 km h -1 (Evans, 2000). Higher velocities have not improved protection. Temperature increases between 3.0 °C and 4.5 °C are common for a hovering helicopter. Pilots often load helicopter spray tanks with water to increase the weight and increase thrust. Under severe frosts with a high inversion, one helicopter can fly above another to enhance the downward heat transfer. Thermostat controlled lights at the top of the canopy are used to help pilots see where passes are needed. As the helicopter passes over the crop, the temperature rises and the lights go out. Cooling to the thermostat temperature causes the lights come on. This helps the pilot to find and fly over cold spots. Alternatively, a ground crew should monitor temperature in the crop and communicate with the pilot where flights are needed. On the sides of hills, heat transfer propagates down-slope after reaching the surface. Therefore, flying over the upslope side of a crop usually provides more protection because the effects are felt downwind as well. Flights are stopped when the air temperature upwind from the crop has risen above the critical damage temperature. SPRINKLERS Using sprinklers for frost protection has the advantage over other methods that water application is generally less expensive. The energy consumption is considerably less than that used in frost protection with heaters (Gerber and Martsolf, 1979) and, therefore, operational costs are low compared to heaters and even wind machines. Labour is mainly needed to ensure that the system does not stop and the heads do not ice up during the night. In addition to frost protection, one can use the sprinklers for irrigation, enhancing fruit colour by over-plant evaporative cooling, reducing sun injury by over-plant irrigation, delaying bloom prior to bud break, fertilizer application and a combination these 161
Page 1 and 2:
- F A O E N A N D V I MO N I T O R
Page 3 and 4:
FOREWORD Agrometeorology deals with
Page 5 and 6:
ACKNOWLEDGEMENTS We want to thank D
Page 7 and 8:
35 35 35 36 36 37 37 37 37 38 38 39
Page 9 and 10:
174 175 176 176 177 178 179 180 180
Page 11 and 12:
LIST OF PRINCIPAL SYMBOLS Roman Alp
Page 13 and 14:
T n °C Observed minimum temperatur
Page 15 and 16:
CHAPTER 1 INTRODUCTION OVERVIEW Whe
Page 17 and 18:
INTRODUCTION Freeze and frost defin
Page 19 and 20:
INTRODUCTION FIGURE 1.2 Mean air an
Page 21 and 22:
INTRODUCTION FIGURE 1.4 Air (T a )
Page 23 and 24:
INTRODUCTION TABLE 1.2 Categories a
Page 25 and 26:
INTRODUCTION example, because bloom
Page 27 and 28:
INTRODUCTION south in Florida in re
Page 29 and 30:
INTRODUCTION assistance might be in
Page 31 and 32:
CHAPTER 2 RECOMMENDED METHODS OF FR
Page 33 and 34:
RECOMMENDED METHODS OF FROST PROTEC
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53:
Page 56 and 57:
] F R O S T P R O T E C T I O N : F
Page 58 and 59:
Page 60 and 61:
Page 62 and 63:
Page 64 and 65:
Page 66 and 67:
Page 68 and 69:
Page 70 and 71:
Page 72 and 73:
Page 74 and 75:
Page 76 and 77:
Page 78 and 79:
Page 81 and 82:
CHAPTER 4 FROST DAMAGE: PHYSIOLOGY
Page 83 and 84:
FROST DAMAGE: PHYSIOLOGY AND CRITIC
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
CHAPTER 5 FROST FORECASTING AND MON
Page 107 and 108:
FROST FORECASTING AND MONITORING di
Page 109 and 110:
FROST FORECASTING AND MONITORING A
Page 111 and 112:
FROST FORECASTING AND MONITORING Fo
Page 113 and 114:
FROST FORECASTING AND MONITORING FI
Page 115 and 116:
FROST FORECASTING AND MONITORING WE
Page 117 and 118:
FROST FORECASTING AND MONITORING FI
Page 119 and 120:
FROST FORECASTING AND MONITORING De
Page 121 and 122:
FROST FORECASTING AND MONITORING de
Page 123 and 124: FROST FORECASTING AND MONITORING FI
Page 125 and 126: FROST FORECASTING AND MONITORING st
Page 127 and 128: CHAPTER 6 PASSIVE PROTECTION METHOD
Page 129 and 130: PASSIVE PROTECTION METHODS enhance
Page 131 and 132: PASSIVE PROTECTION METHODS use an
Page 133 and 134: PASSIVE PROTECTION METHODS FIGURE 6
Page 135 and 136: PASSIVE PROTECTION METHODS After st
Page 137 and 138: PASSIVE PROTECTION METHODS of miner
Page 139 and 140: PASSIVE PROTECTION METHODS FIGURE 6
Page 141 and 142: PASSIVE PROTECTION METHODS CANOPY T
Page 143 and 144: PASSIVE PROTECTION METHODS damage h
Page 145 and 146: PASSIVE PROTECTION METHODS of culm
Page 147 and 148: PASSIVE PROTECTION METHODS AVOIDING
Page 149 and 150: PASSIVE PROTECTION METHODS TABLE 6.
Page 151 and 152: PASSIVE PROTECTION METHODS Organic
Page 153 and 154: PASSIVE PROTECTION METHODS Fucik (1
Page 155: PASSIVE PROTECTION METHODS Many com
Page 158 and 159: ] F R O S T P R O T E C T I O N : F
Page 192 and 193: ] Once started, the sprinklers shou
Page 203 and 204: CHAPTER 8 APPROPRIATE TECHNOLOGIES
Page 205 and 206: APPROPRIATE TECHNOLOGIES frost prot
Page 207 and 208: APPROPRIATE TECHNOLOGIES fashion. O
Page 209 and 210: APPROPRIATE TECHNOLOGIES CROP COUNT
Page 211 and 212: APPROPRIATE TECHNOLOGIES CROP COUNT
Page 213: APPROPRIATE TECHNOLOGIES other crop
Page 224 and 225:
Page 226 and 227:
Page 228 and 229:
Page 231:
APPENDIX 2 CONSTANTS Specific heat
Page 234 and 235:
Page 236 and 237:
Page 239:
- FAO ENVIRONMENT AND NATURAL RESOU
show all

Frost Protection - UTL Repository

Create successful ePaper yourself

Delete template?

Save as template?