Studies on Dew

Abstract

The present paper deals with the characters of dew under winter-dry climate in southwest region of Taiwan (represented by Pingtung area). The dew amount was measured by means of weighing method with acrylic plate, and the time of dew formation start and dissipation were recorded by an improved Taylor type dew duration recorder. Records were analyzed for quantity and duration of dew and the dew amount were compared with precipitation and class A-pan evaporation. From November 1977 to May 1978 and October 1978 to May 1979, the observed records of dew were summarized as follows:(1) From November to May, the total number of dewy days were 149 in 1977 and 155 in 1978, while in the same periods the total dew amount were 25.9mm in 1977 and 26.2mm in 1978. The monthly average frequency of dewy day was 71%.(2) The maximum dew amount records were 4.2mg cm-2 hr-1, 43mg cm-2 day-1 and 6.57mm cm-2 month-1. The maximum value of duration was 13 hours and the average of that was 8.2 hours.(3) The daily dew amount can be calculated from the following equation with meteorological data, Y=9.3304X1+0.2358X2-0.1348X3+0.4167X4-20.3038, where, Y is the dew amount (mg cm-2 day-1), X1 the nocturnal cooling rate of air temperature in screen (°Chr-1), X2 the nocturnal relative humidity in screen (%), X3 the nocturnal wind speed at 1.5m height (cm s-1) and X4 the total nocturnal net radiation (ly).The proportion of calculated dew amount from regression equation was 79% to observed dew amount.(4) During the rainless month in winter season the dew amount was more than precipitation. The monthly average of dew amount was amount to 7% of precipitation and 4% of pan evaporation in normal year. The daily proportion of dew amount to pan evaporation which more than 10% was about 17% in total dewy days during the winter season.