[en] Tropospheric concentrations of radioactive debris from nuclear weapons tests continued to decrease during the period from 1983 to 1985. For the first time since this measurement program was begun, no clear springtime peak was discernible in the concentrations of /sup 90/Sr, /sup 137/Cs, and /sup 239,240/Pu in the surface air of the Northern Hemisphere. The data for quality assurance samples measured as part of the program indicate that the low ambient concentrations during this period introduced significant uncertainties into the fission product data for routine monthly samples. Meteorological factors probably produce the large observed week to week variations in the concentrations of /sup 7/Be and /sup 210/Pb at sites that are sampled on a weekly basis. Four such factors are responsible for much of the seasonal variation of /sup 7/Be concentrations. These are variations in the rate of exchange of air between the stratosphere and the troposphere, of vertical transport within the troposphere, of horizontal mixing between high- and mid-latitudes, and of washout by rainfall