Teitel, M., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan 50250, Israel
Peiper, U.M., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan 50250, Israel
Zvieli, Y.

Shading screens stretched horizontally above the ground, were found effective in reducing the risk of frost damage. The screens reduce the net amount of long-wave radiation from the ground to the sky during the night and thus keep the temperature of the plants under the screens at a higher temperature than ambient. A model for calculating the reduction in long-wave radiation exchange between the ground and the sky, due to the presence of a screen, was devised and verified by experiments. The model suggests that three parameters affect net radiation under the screen, shading percentage of the screen, radiometric properties of the screen and the ratio between screen area and the ground area beneath it. Of several types of screens that were tested, an aluminized screen was found to be the most effective in reducing frost damage. A simple model for calculating leaf temperature is offered and used for calculating the temperature of an upper leaf. The experimental data and calculations show that during the night the temperature of the leaves is lower than the air temperature and therefore frost protective devices should be controlled according to leaf temperature and not air temperature.