Handbook for Irrigation of Wine Grapes in South Africa

TABLE 9.4. The Beaufort wind scale used to estimate wind speed on land. Beaufort no. Description Wind speed (km/h) Conditions 0 Calm < 1 Smoke rises vertically. 1 Light air

1 - 5 Direction shown by smoke drift, but not by wind vanes. 6 - 11 Wind felt on face; leaves rustle; wind vane moved by wind. 12 - 19 Leaves and small twigs in constant motion; light flags extended. 20 - 28 Raises dust and loose paper; small branches moved. 29 - 38 Small trees in leaf begin to sway; crested wavelets form on inland waters. 39 - 49 Large branches in motion; whistling 50 - 61 Whole trees in motion; inconvenience felt when walking against the wind. 62 - 74 Twigs break off trees; generally impedes progress. 75 - 88 Slight structural damage (chimney pots and slates removed). 89 - 102 Seldom experienced inland; trees uprooted; considerable structural damage. 103 - 117 Very rarely experienced; accompanied by widespread damage. Figure 9.6 heard in telegraph wires; umbrellas used with difficulty.

2 Light breeze

3 Gentle breeze

4 Moderate breeze

5 Fresh breeze

6 Strong breeze

7 High wind, moderate to near gale

8 Gale, fresh gale

9 Strong/severe gale

10 Storm, whole gale

Figure 9.6

11 Violent storm

≥ 118 Devastation.

12 Hurricane force

A

B

FIGURE 9.6. Examples of (A) calm conditions where wind speed is less than 1 km/h and (B) a moderate breeze of 20 to 28 km/h raising dust and moving small branches near Vredendal in the Lower Olifants River region.

Figure 9.7

Figure 9.7

292 CHAPTER 9 – PREVENTING COLD DAMAGE IN VINEYARDS