The principle of this method is based on 3 factors:

• When the water freezes, its latent heat is released. This latent heat keeps the temperature of the plant from dropping below freezing point.
• A mixture of ice and water exposed to below freezing point remains at 0º C until all the water is frozen.
• Most plants do not suffer frost damage until the temperature drops slightly below 0º C, because the freezing point of the plant tissue liquid is below that of water.

This method of protection is continued, until the temperature of the surrounding air has risen above 0º C and all ice formations on the plants have melted.

Key considerations

Successful protection of crops from frost damage using sprinklers depends on three crucial factors:

a. Sprinkler rotation speed

Research has shown that a more consistent plant temperature is maintained with a faster rotating sprinkler. To be considered fast enough, a sprinkler should complete one full revolution in less than 60 seconds. Thirty to forty seconds is ideal.

b. Water application rate

The water volume in relation to area application rate has been found to be one of the most important considerations when designing for frost protection. The application rate is calculated after considering factors such as air temperature, wind speed and humidity levels. (See Table for minimum precipitation rates).

Winds affect evaporation levels as well as application uniformity. Wind conditions result in the need for a higher water application rate, to provide the same degree of protection provided when there is no wind.

c. Uniformity

Effective frost protection also depends on how uniformly the sprinkler distributes the water. Extreme care should be exercised in evaluating sprinkler spacing, operating pressures and wind conditions. The choice of sprinkler should be made carefully. An accepted standard would be where the sprinkler produces a CU equal to or higher than 84% with its DU being equal to or higher than 75%.

For targeted frost protection such as the NaanDan Flipper concept, the guideline is not the CU, but full coverage with a defined minimum precipitation rate.