Measurement and modelling of dew in island, coastal and alpine areas

We compare the characteristics of dew at nearly the same latitude (42° 45° N) for the Mediterranean island of Corsica (Ajaccio, France) and two continental locations (Bordeaux, France, Atlantic coastal area; Grenoble, France, Alpine valley). Dew amount was measured on a horizontal reference plate made of Polymethylmethacrylate (Plexiglas) and placed at 1 m above the ground. Data are correlated with plate and air temperature, air relative humidity, wind speed and cloud cover during the period 14 Aug. 1999 to 15 Jan. 2003. General features as well as particularities of the sampling sites are discussed. The average daily dew yield is higher for the island station at Ajaccio (0.07 mm) than the Bordeaux coastal area (0.046 mm) or the Grenoble valley (0.036 mm). However, the accumulated dew yield was highest for the coastal station (9.8 mm/year) as compared to the island (8.4 mm/year), and much larger than in the alpine valley (4 mm/year). The difference between cumulated and average dew yield stems from the greater number of dew days in the coastal area (58 %), vs 33 % for the island and 30 % in the valley. The higher wind speeds at the island station (average wind during dew is 2 m/s) and lower relative humidity explains the smaller number of dew days. The dew rate seasonal variation is negligible in Bordeaux and exhibits during summer a maximum in Ajaccio and a minimum in Grenoble. A computer model that includes simple meteorological data (air temperature and relative humidity, wind speed, cloud cover) is used to determine the thermal balance and fit to dew mass evolution. Two parameters that account for heat and mass exchange can be adjusted. It was found that, within the uncertainties, these two numbers are the same for the three measurement sites, thus allowing dew formation on plates to be evaluated from only simple meteorological measurements. This model does have limitations, however, which are discussed.