Handbook for Irrigation of Wine Grapes in South Africa

TABLE 9.2. The relationships between freeze point, minimum air temperature (T min

) and dew

point temperature (T dp

) that determine the type of dew or frost.

Temperature relationship

Type of dew or frost

0°C < T min

< T dp

Dew

T min

< 0°C < T dp

Frozen dew

< T dp

< 0°C

White frost

T min

T dp

< T min

< 0°C

Black frost

The possible incidence of frozen dew should also be considered as a warning sign, since it looks similar to frost, and sub-zero temperatures may prevail. If there is a possible frost threat, air temperature must be measured continuously at the level where the cold damage is expected until it reaches 2°C. As soon as weather conditions conform to the thresholds as indicated in Table 9.3, protection against cold damage must be applied.

TABLE 9.3. Thresholds for dry ball (T d

) and wet ball temperature (T w

), as well as wind speed

that serve as indicators for starting and stopping cold protection measures. Warning of possible cold damage Start protection when

Stop protection when

= 2°C

T d

< 1°C

T d

> 1°C

T d

and

and

< 0°C

T w

> 0°C

T w

and Wind speed < 8 km/h

The foregoing clearly implies that a number of inputs are required in order to predict the occurrence of cold damage accurately. Therefore, it would be useful to have access to hourly weather data collected on-farm, or at a nearby automatic weather station (Fig. 9.4A). Unfortunately, very few growers have 24-hour access to the real time data collected by means of these weather stations. This means that growers need to do some measurements themselves. In this regard, a wet ball-dry ball thermometer, or psychrometer, is essential to measure air temperature (= T d ), as well as T w required to calculate T dp (Fig. 9.4B). Another practical problem is that the air temperature predicted by the SAWS may differ substantially from the actual temperature at the farm, or even vineyard block level. Therefore, growers need to compare on-farm measured temperatures with the ones predicted by SAWS. It will also be useful to know how canopy temperature relates to that of the surrounding environment (Perry, 1994). An infrared thermometer produces quick and easy canopy temperatures (Fig. 9.5A). This additional information will allow growers to adjust the predicted temperature, particularly T min , for a more accurate prediction of possible cold damage. If weather station data are not readily available, wind speed

290 CHAPTER 9 – PREVENTING COLD DAMAGE IN VINEYARDS