Observations of the air concentration (and in some cases of the vertical flux) of artificial radioactivity near the earth's surface at a network of stations in Europe and elsewhere have been examined in order to determine their relation with the major features of the atmospheric flow patterns. The examination is confined to cases when the origin of the radioactivity is to be found in the stratosphere and particular attention has been raid to the mechanisms and time scales of the motions which are responsible for transferring activity from the stratosphere to the earth's surface. Both concentration and vertical flux have a characteristic distribution in relation to normal distance to jet streams within 1000 to 1500 km of the sampling stations at 9 hours before the 24-hour sampling periods - both quantities rise and fall "across" the jet, with a maximum some 200 to 300 km to the right of the jet looking downstream. Deviations of concentrations from characteristic values for place in relation to the jet or for seasons are examined statistically and synoptically in relation to jet structure, humidity at 700 nib and 500 mb, cumulonimbus activity, and lower troposphere stability. Abnormally high radioactivity is commonly associated with jet entrance and middle regions of cyclonic curvature, often but not always with dry air at 700 mb and 500 mb, and occasionally with cumulonimbus. Three special periods of large winter/spring surges of radioactivity have been investigated in relation to stratosphere and troposphere activity (sudden war.lings and coolings in the former). Evidence is found for transport from the lower stratosphere to the earth's surface in a day or two, probably in association with a direct transverse circulation around the jet at entrance and middle regions.