This work deals with the retrieval of the aerosol optical thickness (AOT) needed to carry out the atmospheric correction of remote sensing data measured in Barrax (Spain) on 4 June 1999 in the framework of 1999 Digital Airbone Imaging Spectrometer Experiment (DAISEX'99). The AOT was estimated through three approaches based on: spectral extinction of direct solar irradiance at ground level, airborne nephelometer measurements at different altitudes, and backscatter lidar in the lower troposphere. We found extremely low AOT values due to a cold Atlantic front that swept across the Iberian Peninsula from west to east producing light rain over the test area on 2 June 1999. The results were solar irradiance extinction: 0.085±0.018; nephelometer: 0.063±0.020; lidar: 0.083±0.030. Nephelometer- and lidar-derived values account for both extinction and absorption, assuming a single scattering albedo value of 0.90. Errors values include measurements and retrieval uncertainties as well as statistical variability.