Recent Visibility Trends in Portugal

Abstract

Good visibility is a public asset and deserves to be protected (conference announcement, 2005). Visibility is also a meteorological parameter which is provided my national weather services. However it seems that the frequency and the data quality vary very much between different stations and also between different times at the same stations. Daily averages of the visibility of 17 stations inside Portugal were used in order to investigate to investigate possible trends over the last decade. Eight stations reported values between 1994 and 2005, two stations start reporting observations in 2006 and further 7 stations provide data since the end of 1997. However one station (Sagres) stopped reports about visibility in 2001 likely due to relocation to a different place. Major problems in the analysis occurred with respect to the number of measurements for each day or to the number of days where daily averages were reported. At some station the measurement frequency were almost hourly (24 measurements a day) whereas at other stations only 3 hourly or even less frequent measurements contribute to the daily average. For most stations most at the time daily values were reported. However at some stations a data gap of about half a year occurred. Figure 1 shows a color coded trend for the 17 stations investigated. At about one third of the stations the visibility increased over the last decade and for more than one third the visibility remained unchanged. However in the area of Porto the visibility and in the southern part of Portugal the visibility decreased with time. For the station “Faro” an example for histograms for daily visibility values is given, which clearly demonstrates temporal development of the visibility. Currently the reasons are identified for sure. But it is likely that increased inductrial activities in the area of Porto and a desertification of the southern part of Portugal (various autores, 2004) are the main sources. Seasonal trends need to be analysed in order to reduce the spread of different hypothesis.