Teide Observatory (Tenerife, Canary Islands) and Roque de los Muchachos Observatory (La Palma, Canary Islands), are first-class sites for astronomical observations. Due to the small distance between the two islands (about 150 km), their equal altitude and geographical situation, this work is applicable to both islands. The observations used here have been carried out at Teide Observatory, situated at 17.8' N and 16 29.4' W and 350 km from the northwest coast of Africa. It offers exceptional conditions due to the stable subsiding maritime airmass encountered normally above the inversion layer formed between 800 and 1200 m (McInnes & Walker 1974). These conditions are typical for isolated mountains on islands. Its altitude, about 2400 m, places the observatory above the inversion layer for a large fraction of the year. During summer occasional presence of wind-borne Saharan dust may occur but is very infrequent during the rest of the year (Sanchez 1970; Murdin 1985). The occasional presence of dust may create difficulties for some observing programmes (where very high transparency is required) but does not spoil the image quality (Brandt & Wohl 1982) and are excellent for imaging instrumentation. As a result of the excellent conditions and geographic situation, the helioseismology community has also chosen Teide Observatory as a site to take part in worldwide terrestrial network observing programmes, e.g. GONG, IRIS, BISON, TON and LOW-L.
Although several site-testing campaigns have been carried out to characterize the Canarian Observatories putting into evidence their excellent quality (Murdin 1985; Vernin & Muñoz-Tuñon 1992, 1994; Brand & Wohl 1992; Hill et al. 1994a,b; Muñoz-Tuñon et al. 1997) none of them have undertaken the study of the behaviour of the extinction coefficients at several wavelengths under different atmospheric conditions. For doing so a large database such as the one used here is necessary. Only using several years of data it is possible to cover the range from excellent "coronal'' days where extinction is very low to "absorbent'' days where Saharan dust is present in the skies above the island. Since the occasional presence of Saharan dust is unavoidable, information on its effects on astronomical observations are of great utility. A way of obtaining this information is to compare the extinction behaviour and the aerosol size distribution on days with and without dust. These points will be addressed in this paper.