Handbook for Irrigation of Wine Grapes in South Africa

Chapter 9

Figure 9.1 Figure 9.1

PHOTO: P. SNYMAN.

A

B

PHOTO: P. SNYMAN.

FIGURE 9.1. Examples of frost damage to (A) newly sprouted grapevine shoots and (B) an entire vineyard in the Breede River valley.

9.2 TYPES OF COLD DAMAGE Based on the way the cold atmospheric conditions develop, vineyards can either be damaged by a windborne (advective) freeze or radiation frost. A windborne freeze occurs when wind exceeding 8 km/h blows a sub-zero air mass from one area into another area (Perry, 1994). The thickness of the cold air may vary between 100 and 1 500 m (Fig. 9.2A). Since air temperature deceases with altitude, an air mass with a temperature above 0°C can cool down to sub-zero temperatures if strong wind blows the air mass onto higher ground (Fig. 9.2B). Unfortunately, it is very difficult to change the environment under such conditions. Consequently, there is not much that growers can do to protect vineyards against a windborne freeze (Perry, 1994). Figure 9.2

A

100 to 1 500 m

Cold air mass (below freeze point)

Strong wind > 8 km/h

Cold air mass (below freeze point) Below freeze point Zone B

Strong wind > 8 km/h

Above freeze point Zone B

Zone A

Zone A

100 to 1 500 m

B

Air mass cools down (below freeze point) Below freeze point

Above freeze point

S t r o n g w i n d

Cold air mass (above freeze point)

Zone B

Zone B

Strong wind > 8 km/h

Zone A

Zone A

FIGURE 9.2. Schematic illustration of a windborne, or advective, freeze formation.

IRRIGATION OF WINE GRAPES 285