Issue link: http://winesandvines.uberflip.com/i/438260
48 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 b o o K E x C E r P t will give up heat at the same rate (provid- ing all other factors of soil density, mois- ture content, etc. are the same), and the ground will cool at the same rate. Areas that are well drained are not generally at risk for frost damage because there is no cold air accumulation. The ground temperature on a hillside is approximately equal to the ground temperature in the lower valley, but the cold air generated by the cool ground on a hillside flows off and does not build up deep enough to submerge plant tissues. The natural drainage capacity on the slopes prevents cold air accumulation and avoids the buildup of cold air. MYTH— Removing the downhill barriers such as trees and brush will improve cold air drainage. Reality—Downhill barriers can some- times block cold air that builds up below from flooding back into the vineyard and block the cold air generated from above from draining out. Cold air accumulates in areas of insuf- ficient drainage. If the basin area below the vineyard has cold air from surround- ing sources that exceeds the capacity of the basin to drain itself, cold air will build up from below. This rise in the height of the cold air mass in the lower basin will cause cold air to flood back into the vine- yard unless it is blocked from below and removed. A barrier along the lower por- tion of the vineyard may help to inhibit cold air entrance into the vineyard and be a benefit. One way to tell when there is insuf- ficient drainage and flooding from below is that the air temperature in the lower basin will be equal to or lower than the temperature in the vineyard, and the temperature in the lower basin will drop prior to the temperature in the higher vineyard. In other instances, the basin below the vineyard is well drained and the cold air that is causing frost damage is originat- ing from the vineyard area itself and/or areas above the vineyard. Cold air flow- ing downhill may not be building deep enough to cause damage until the down- hill barrier obstructs the natural drain- age causing accumulation of cold air in the vineyard which is a cold air lake. If there is adequate drainage below and cold air accumulation is caused by the barriers, the temperature in the lower basin will be higher than the tempera- ture in the vineyard. Often, both cases will exist. Under mild frost conditions there may be ade- quate drainage in the lower basin due to a smaller volume of cold air being gen- erated from the surrounding areas that flows in there, while under severe frost conditions the lower basin will be over- whelmed and accumulate cold air due to the larger volume of cold air flowing into it. When the inflow of cold air exceeds the capacity of the basin to drain itself, accumulation will occur. MYTH— Cold air is like water. Reality—While there are similarities in the way that cold air flows under strati- fied conditions as compared with the flow of water, there are also some differ- ences. Both water and katabatic cold air flows are caused by gravity and so they will both follow the path of least resis- tance. Some of the differences between air and water are: 1. Water does not change viscosity with temperature change, until it freezes or evaporates. 2. The density of air changes in direct relationship to its temperature. The colder the air, the more dense it is and the heavier the molecules. This characteris- tic guarantees that the coldest air mol- ecules will always be lower than warmer molecules under natural conditions. The heaviest water molecules are at +4º C (approximately 39º F). As water cools on top of a lake, it becomes heavier and sinks. At lower temperatures, it becomes less dense and rises again. If not for this characteristic, ice would form at the bot- tom of pool of water, not on the top. 3. The speed of air flowing downhill is determined by the slope angle, ground surface (smooth, slick surfaces produce less friction) and differences in viscosity caused by air temperature and absolute humidity. The speed of water flowing downhill is determined by slope angle and the ground surface. The temperature is not a factor. 4. Cold air running into an obstruction can build up to three X or four X the height of the obstruction before spilling over. The height that the cold air will build to is determined by the angle of the slope approaching the obstruction, viscosity of the air, and the volume of air flowing to and blocked by the obstruc- tion in relation to the available drainage capacity. The lower the angle of approach and the lower the air temperature, the larger the mass of cold air that will be blocked by the obstruction. This will result in a higher cold air mass in relation to the height of the obstruction and cold air will reach further back and higher into the vineyard. Water running up against an obstruction will build up to the height of the obstruction and then try to flow over. Water will also build higher than an obstruction to a lesser degree. If the vol- ume of the water flowing to and blocked by the obstruction exceeds the available drainage capacity over the dam or bar- rier, there will be a depth to that flow over the dam. The resulting water level in the lake behind the dam will then be higher than the dam itself, but not to the same degree as cold air accumulating behind a dam. Frost protection, spring 2008. Photo by stePhen sterling oF esterlina Vineyards & Winery