This research provides evidence for a reduction of air visibility and decrease in water evaporation in Venezuela, as measured and reported by the weather stations' observers. 36 Venezuela Air Force (VAF) weather stations, the most complete and efficiently maintained meteorological network in the country, were first examined for both the length of their records and missing data for the selected period 1951-1995. From the 36 stations, the best 25 (the ones with longest records and fewest missing data) were chosen and retained for further analysis. These stations' records were first analyzed for homogeneity, following the techniques of Alexandersson (1995) as well as Szentimrey's (1998): Multiple Analysis of series for Homogeneity. Results of homogeneity for parameter air visibility were quite good (no adjustments were needed) which is relatively easy to understand since measuring this parameter does not rely on any instrumental device but on visual appreciations and observations of landmarks located in the surroundings of the stations, although some other eventual source of inhomogeneities might have been introduced with the regular changes of the stations' observers (since not all have the same visual capacity). The homogeneity results of evaporation were also acceptable (only 4 records needed to be adjusted). From the 25 weather stations, 21 present noticeable decrease and diminishing of horizontal air visibility. Almost all those 21 stations are installed nearby the airports which serve the largest and most populated Venezuelan cities. Air visibility reduction averages in most stations more than 20% during the last 15-20 years of the period. Recent studies on evaporation using the same stations network during same period 1951-1995 show also similar and important decrease since 1980 up to now (Quintana-Gomez, 1998); 2/3 of the stations group exhibit this decreasing trend for evaporation.
The downward trends of both air visibility and evaporation is closely related with increasing smog, aerosols, dust and other pollutants floating in the air, mostly observed during the dry season of Venezuela (December - March). Strong correlation exists between these two parameters, and also between each with DTR (Diurnal Temperature Range) reduction and increasing cloudiness on the area, as observed and evaluated . Since most of the stations are located close to Venezuela's largest cities, representing in much those cities' climatology, it should be possible to devise an index or indicator of air pollution for these cities, based on air visibility and water evaporation. This is one of the most important findings of the research which fits quite well for purpose of applied climatology, climate change detection and atmospheric chemistry.