We analyze the citation distributions of all papers published in Physical Review journals between 1985 and 2009. The average number of citations received by papers published in a given year and in a given field is computed. Large variations are found, showing that it is not fair to compare citation numbers across fields and years. However, when a rescaling procedure by the average is used, it is possible to compare impartially articles across years and fields. We make the rescaling factors available for use by the readers. We also show that rescaling citation numbers by the number of publication authors has strong effects and should therefore be taken into account when assessing the bibliometric performance of researchers.