Handbook for Irrigation of Wine Grapes in South Africa

Radiation frost occurs during nights when clean, dry air and calm winds of less than 8 km/h prevail, allowing a temperature inversion to develop. The latter occurs when continued radiation causes a significant drop in air temperature near the surface, whereas the temperature in the upper layers increases (Perry, 1994). Such a temperature gradient is the inverse of daytime conditions when the upper air layers are cooler. Therefore, it is referred to as an “inversion”. The thickness of an inversion may vary from 9 to 60 m above the soil surface (Fig. 9.3). There are two types of radiation frost. White frost (hoar frost) occurs when the moisture in the atmosphere freezes to form small ice crystals on grapevine leaves and shoots. Consequently, it is called white frost. The layer of ice causes plant cells to die back upon exposure to sub-zero temperatures over a certain period. During a black frost, few or no ice crystals form since the air in the lower atmosphere is too dry (Perry, 1994). Under such conditions, the T dp is low, and condensation cannot take place. This causes the air temperature to fall way below freezing point. Although no ice crystals form during a black frost, the low temperatures cause ice formation within plant tissues. The ice ruptures the cells, causing the dead leaves and shoots to turn black. Hence, it is referred to as black frost. Since the T dp is lower than the minimum air temperature, a black frost generally causes more damage to vineyards than a white frost. Figure 9.3

Clear skies

Warm air

Inversion (9 to 60 m)

Calm, wind < 8 km/h

Heat flow

Cold air

Temperature falls to 0°C or lower

FIGURE 9.3. Schematic illustration of radiation frost formation.

286 CHAPTER 9 – PREVENTING COLD DAMAGE IN VINEYARDS