The Earth's rotation rate, and the associated length-of-day (LOD), show fluctuations at different timescales. At the decadal timescale, this is due mainly to the interaction between the mantle and the liquid core, while the variations at shorter time scales are attributed to interaction with the external fluid layers, in particular the atmosphere, with the oceans and hydrology contributing at about the 1% level. In this paper, we investigate the effect of changes of atmospheric mass on the length-of-day modeling, which are associated with evaporation/precipitation and mass closure problems in the atmospheric analyses. We show that an important part of the residuals between observed LOD and the ocean and atmosphere induced LOD variations can be explained by this mass exchange.