The &bgr;-activity in polar snow reflects the combined effects of fallout from and radio-active decay in the stratospheric reservoir of nuclear debris, which has become virtually uniform since the last bomb tests in 1965. This makes it possible to estimate the stratospheric residence time from the decrease in the &bgr; activity of snow with age (depth). The residence times thus determined for Antarctica and Greenland average 1.63 years and are in good agreement with published estimates from atmospheric measurements.