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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 47 b o o K E x C E r P t Myths of Frost Protection Excerpt from: COLD AIR Accumulation and the Grower's Guide to Frost Protection Below is an excerpt from Cold Air Accumulation and the Grower's Guide to Frost Protection, Chapter 4, available from Westbow Press, 85 pages, $31.95 (Paperback), $3.99 (Ebook). MYTH—Cold air that causes damage "falls" from the sky and settles in the low spots. Reality— Cold air that causes frost dam- age is cooled by the ground and flows along the ground downhill into accu- mulation areas due to gravity. The cold air is the result of the ground losing heat through long wave radiation during nights that have no wind and no clouds. The ground will lose heat at a faster rate than the air and as the ground loses heat and cools, it in turn cools the air closest to the ground. Colder air molecules are heavier and denser than warmer ones and will stay closer to the ground. The lower, colder molecules are affected by gravity. Cold air will begin to move downhill until insufficient drainage conditions such as physical obstructions or low slope impedes the flow and causes the cold air to accumulate. When the cold air builds up deep enough to submerge plant tissue frost damage may occur. Cold air masses that come in over the ground due to a weather system and are below the critical temperature for the crop to be protected are rare. Even though there are the characteristics of radiation frost-clear skies and no wind, this is a regional frost event that is the result of a cold mass of air associated with a winter storm or advection freeze. When these types of events happen more frequently than is economically viable, then the grower must consider that the crop he is trying to grow is not suitable for the region and make adjust- ments. MYTH —Blowing wind over vines will prevent frost. 1 Reality—Blowing air or otherwise dis- turbing plant tissue while it is in a super cooled state will promote frost damage by stopping the super cooling process causing ice crystallization in and around the cells. In order for wind to be effective, there must be warmer air from the higher layers pushed down and mixed with the colder air to increase the overall air temperature, or heat must be added to the blowing wind. It is the rise in tem- perature produced by mixing warmer air molecules with the colder air molecules close to the ground that protects against frost, not the movement of air. MYTH — Smoke protects against frost. 2 Reality— Smoke does nothing to stop or slow heat loss from the ground. On the contrary, smoke does prevent heat from reaching the ground when the sun comes up in the morning. Smoke is not only useless in protecting against frost, it is a detriment. The ground loses heat through long wave radiation which is approximately 10 to 15 microns in diameter. Smoke par- ticles can be less than 1 micron in diam- eter. Smoke does nothing to stop, absorb, or reflect radiation losses from the soil. It is invisible to the long wave radiation and has no impact on ground cooling or the resulting cooling and stratification. When the sun comes up in the morn- ing, the heat radiated back to the ground from the sun is short wave radiation. Short wave radiation is approximately 1 micron in diameter and is effectively blocked by smoke. This effect will delay the warming process. Smoke can be useful when using heat- ers for frost protection to indicate the height of the stacking of the molecules. When using heaters for frost protec- tion, the stacking point of the warmed molecules is a ceiling. When heaters warm the air, the difference in tempera- ture between the heated air molecules and the natural surrounding molecules will cause the warmer molecules to rise. This effect is called "heat buoyancy." The warmer air molecules will rise, and quickly lose heat, to a height where they are "neutrally buoyant," or the same tem- perature as the air around them. For example, since a molecule of 32º F will not push away another molecule of 32º F (the same temperature and den- sity), the upward rising molecule will stop when it is blocked by a natural molecule of the same density- a process called "stack- ing." This point at which the heated mol- ecules stop rising is the "ceiling." The process of heating molecules and stacking them at a given height creates a thermal boundary line. The newly stacked molecules will then begin to cool and move downward again in a convec- tion manner until they are reheated by the heat source and rise again. One use for smoke particles is that they will move horizontally along the demarcation line (thermal boundary line) and give a visual representation of the height of the "ceiling." Smoke particles will rise with the ris- ing warm air molecules and will move horizontally along the thermal bound- ary line giving a visual representation of the stack height of the heated molecules. The temperature from the heaters can be adjusted up or down as necessary to achieve the proper ceiling height. MYTH—Frost on the ground indicates a frost problem. Reality—Ice on the ground is not an indication of frost damage on the plants. It is the deepening mass of lethal cold air that causes frost damage, not the tem- perature of the ground. The ground in all areas of the vineyard Steve Hammersmith BY » CHAPTER FOUR «