Issue link: http://winesandvines.uberflip.com/i/438260
p r a c t i c a l w i n e r y & v i n e ya r d J a n U a r y 2 0 1 5 49 b o o K E x C E r P t MYTH — More and hotter heat is better than less heat. Reality—Applied heat that is too hot will rise quickly to great heights above the vineyard and have no effect in the growing zone. As cold air molecules near the heat source are warmed, the warmer mole- cules become lighter and less dense than the surrounding colder air molecules. This causes the warmer molecules to be thermally buoyant and to rise. The greater the difference in tempera- ture between the heated molecules and the surrounding colder natural mole- cules, the more buoyant the warmer molecules will become. These molecules will quickly lose heat as they rise, and at some point above the ground, they will be the same temperature and density as the surrounding molecules. The greater the difference in temperature between the warm molecules and the cold molecules, the greater the tendency to rise, resulting in faster and higher ascendency of the warm molecules. The height above the ground where the heated molecules no longer rise is referred to as the "ceiling" and this is where the molecules "stack" under the ceiling. Since molecules of the same tem- perature will not push away each other, molecules of approximately the same temperature will stack under each other until the temperature in the entire area below the ceiling is equalized. The ceiling height can be controlled by modifying the amount of heat or heat concentration coming from the heat source. A more indirect heat — or radiated heat— will impart the same level of heat energy (BTU's) over a larger area while turning the heaters down and burning less fuel will provide less BTU's. Either approach will provide less temperature difference between the heated molecules and the natural molecules resulting in less thermal buoyancy. This will result in a lower ceiling or "stacking" height. In most cases the ideal ceiling height is just slightly above the canopy. Using more heaters, each with less fuel consumption will have the same effect as fewer heaters that have better radiation capability. When the molecules at the ceiling begin to cool they start to fall until they are again heated by the heat source and the process repeats. This continuing cir- culation of molecules being re-heated, rising and falling is called "convection." No real protection occurs until the con- vection and stacking process is complete. If a molecule is heated with a direct concentrated and non-radiating heat source such as a bonfire, then the heated molecules will rise to a much higher level and may not have the time to stack or cool and circulate back down into vineyard. In this case, the grower will only have succeeded in heating the atmosphere above the vineyard making no impact on the temperature inside the growing zone. The limit of the temperature rise inside a vineyard is the temperature at the ceil- ing. For example, if the temperature at 15 feet above the ground is 3º F higher than at ground level, and this is where the ceiling is set, then the maximum rise in temperature inside the vineyard will be 3º F. In order to achieve a higher tempera- ture rise, then the ceiling must be raised which would require more fuel and more time for the stacking/convection process. The thermal gradient (the temperature rise at a given height increment) spreads out as the height from the ground increases. For example, there may be a 3º F rise from the ground to 15 feet, but the next 15 feet may raise only 1.5º F, and the next 15 feet above that may raise only 0.5º F. Fuel consumption rises exponen- tially to the rise in temperature required in the growing zone. In all cases using heaters, control of the ceiling height to accomplish specific goals is essential and more heat is not always better. MYTH — Heating water will improve the effect of under-vine irrigation. Reality — Heating water prior to apply- ing it for under-vine irrigation provides little or no benefit. Under-vine irrigation releases heat generated from the cooling and freezing of the water that is emitted or flooded over the ground. The heat that is released from the water is transferred into the surrounding air molecules, which then become thermally buoyant and rise until they are the same temperature as the air around them. At this point they will stack under each other creating a ceiling and begin convection. The principle way that under-vine water protects crops is the same as the principle of heaters. The main differ- ence is the intensity of the protection. Molecules heated by gas-burning heat- ers can rise high above the vineyard canopy while molecules heated by water will tend to rise only a few feet from the ground. The temperature rise in the vineyard in both methods is limited to the temperature at the ceiling height. To accomplish heat release from the water in the most effective way, the water must convert to ice before it soaks into the ground. Heat released due to the water-changing state from a liquid to a solid is called the latent heat of fusion. The following formulas illustrate the potentials: Water state changes that will release heat and warm the air — 1 pound (approximately 1 pint) of water is cooled by 1º F = 1 BTU heat released. 1 pound of water changes state from liquid to solid (ice forms) = 144 BTU's heat released. 1 pound of water is condensed from a gas state (as when dew forms) = 1,077 BTU's heat released. Water state changes that will use heat and cool the air — 1 pound (approximately 1 pint) of water is warmed by 1º F = 1 BTU heat used. 1 pound of water changes state from solid to a liquid (ice melting) = 144 BTU's heat used. 1 pound of water is evaporated from a liquid to a gas (evaporation) = 1,077 BTU's heat used. Achieving a high conversion rate from liquid to solid, and condensation from water vapor to water are the most effi- cient ways of releasing the stored heat in water. Heating the water will slow the time it takes for the water to reach freez- ing temperature. This delay will cause a lesser conversion rate to ice before the water soaks into the ground. When the water applied does not freeze, but instead soaks into the ground, the major source of heat and frost protection is lost. MYTH — Heaters placed along the high side of a vineyard perimeter will cause heat to flow down and drift into the vineyard. Reality — Heaters around the high perimeter create a thermal barrier and block cold air flowing downhill from entering the vineyard from the higher areas outside. A thermal "wall" is created by plac- ing the heaters at a close distance to each other along a perimeter boundary, usually along the uphill side, or the side facing a prevailing cold air drift. The heat from the heaters will not flow downward