The frequency and duration of dew occurrence over a year

Abstract

In this paper, a new dew model is presented which is easy to apply and requires only a very limited amount of synoptic data, viz., cloud cover, relative humidity, air temperature and wind velocity. In the dew model, the contribution of the fluxes of infrared radiation, sensible and latent heat to the energy balance of a dew plate are parametarised according to detailed micrometeorological measurements of Holtslag and De Bruin. The model is tested with measurements of dew on the glass surface of an automatic sequential dew sampler. Results of model calculations and measurements of dew occurrence show a fairly close correspondence. The frequency and duration of dew deposition over a year, for 3 places in the Netherlands, have been calculated with the model, using only synoptic data. The numbers of dewy nights and dewy hours were calculated for the year 1987. This resulted in about 220 dewy nights and 1600 dewy hours. Taking the hours of rain into account, it appears that about 75% of the time a surface is wet during the nocturnal hours because of dew or rain. This fact may be important for the quantification of the dry deposition of easily soluble air pollutants and for the effect of air pollutants on vegetation.