TY - JOUR T1 - Microclimates on Spring Frost Nights in Napa Valley Vlneyards JF - American Journal of Enology and Viticulture JO - Am J Enol Vitic. SP - 81 LP - 87 DO - 10.5344/ajev.1961.12.2.81 VL - 12 IS - 2 AU - Herbert B. Schultz Y1 - 1961/01/01 UR - http://www.ajevonline.org/content/12/2/81.abstract N2 - Three years' recordings of night climate during spring frost conditions were evaluated in Napa Valley vineyards of California. On clear nights, shoot temperature was 4°F lower than air temperature. Therefore, 27° was set as threshold vine temperature, for frost damage, since the long established threshold air temperature was 31°F, which could also be confirmed on some frost nights in 1959 and 1960. Covering with 4-ft wide black polyethylene strips kept night temperatures of the vines 3°F higher than in unprotected vines, insufficient protection against stronger frosts, which occur in one out of four years in Napa Valley.At 30-ft height the air was 2°F to 5°F warmer than in the 4-ft standard shelter, which means a difference of 6°F to 9°F between 30 ft and the vines. This appears to be a satisfactory base for operation of wind machines, which, however, would need some support on very severe nights. the use of heaters alone, the most reliable method, encounters the difficulty of fighting sudden sharp temperature drops at sunrise time, during which lighting might not proceed fast enough. ER -