Studies on the quantity of dew formed and on its utilisation

The results show that on the Great Hungarian Plain the number of days with dew formation during the whole of the vegetation period (Apr-Oct) was 150-1 70. A precondition for dew formation is intensive effective outgoing radiat ion (130-160.kJ.m(-2) h(-1)) from the soil surface or plant stand. A large quantity of dew is formed if the relative humidity in the evening hours is greater than 75%. The quantity of dew increases in the course of the year a s the temperature rises. Even moderate quantities of dew, formed if the rel ative humidity in the evening is 50-75%, are of importance in the water met abolism of the plants. The zone where dew is formed is just above the layer of the plant stand whi ch is coldest at night. In the course of dew formation the leaf and dewpoin t temperatures are almost identical, but both are lower than the air temper ature. The stand structure has a considerable influence on the size of the dew formation zone. The intensity of dew formation in the case of moderate heat radiation, low temperature (below 5 degrees C) and a wind speed of 0.5 m/s is 0.03 mm/h. A s the heat radiation increases and the temperature rises, the intensity inc reases. Under the climatic conditions experienced in Hungary the maximum in tensity may be as much as 0.13 mm/h. The minimum quantity of dew is 0.05 mm /night and the maximum 0.88 mm/night. These values are valid for a flat sur face. As the leaf area of the plant stand increases, the quantity of dew also ris es. This dew-increasing effect of the stand structure also depends on the p lant species. The plant stand increases the quantity of dew 1.1-1.7 times c ompared with that an a flat surface. The quantity of dew formed on a flat surface on the Great Hungarian Plain d uring the vegetation period is around 35 mm, the majority of which is forme d during late summer and early autumn.