The distribution of cosmogenic beryllium-7 (7Be) in the lower troposphere is described based on the analysis of long-term (1973--1979) ground level 7Be measurements. 7Be is used as a tracer of stratospheric air to suggest that the annual mean outflow of stratospheric air in the northern hemisphere can be 20 to 30% higher than in the southern hemisphere. This asymmetry is maximum during the northern hemisphere spring and reaches its peak in April. Over North America, the 7Be concentrations show a high positive correlation (linear coefficients of 0.8 to 0.9) with the occurrence of tropospheric low-pressure troughs over several latitude belts, thus supporting the postulate that tropopause folding events, associated with low-pressure troughs, are the dominant stratospheric/tropospheric (S/T) exchange mechanism. It is demonstrated that S/T air exchange shows a distinct seasonal variation that is highly dependent on the latitude. At midlatitudes (40--50¿N) over North America, the S/T air exchange is maximum in summer; at low altitudes (20--30¿N), it is maximum in winter and early spring. The study suggest a mechanistic model that can be used to characterize S/T air exchange at any geographical location using routinely available meterological information.