Barber, K.Bellido Caceres, J.Clay, R.Dawson, B.Holmes, V.Sorokin, J.Wahrlich, P.Whelan, B.Winnick, M.2010-10-182010-10-182010Astroparticle Physics, 2010; 33(2):108-1290927-65051873-2852http://hdl.handle.net/2440/61294The air fluorescence detector of the Pierre Auger Observatory is designed to perform calorimetric measurements of extensive air showers created by cosmic rays of above 10¹⁸ eV. To correct these measurements for the effects introduced by atmospheric fluctuations, the Observatory contains a group of monitoring instruments to record atmospheric conditions across the detector site, an area exceeding 3000 km². The atmospheric data are used extensively in the reconstruction of air showers, and are particularly important for the correct determination of shower energies and the depths of shower maxima. This paper contains a summary of the molecular and aerosol conditions measured at the Pierre Auger Observatory since the start of regular operations in 2004, and includes a discussion of the impact of these measurements on air shower reconstructions. Between 10¹⁸ and 10²⁰ eV, the systematic uncertainties due to all atmospheric effects increase from 4% to 8% in measurements of shower energy, and 4 g cm^{- 2} to 8 g cm^{- 2} in measurements of the shower maximum. © 2010 Elsevier B.V. All rights reserved.enCopyright © 2010 Elsevier B.V. All rights reserved.Cosmic raysExtensive air showersAir fluorescence methodAtmosphereAerosolsLidarBi-static lidarJournal article002010021410.1016/j.astropartphys.2009.12.0050002755148000072-s2.0-7654909542033718Bellido Caceres, J. [0000-0002-0833-9194]Clay, R. [0000-0002-9040-9648]Dawson, B. [0000-0002-4271-3055]