This paper presents a 1-year climatology of airstreams, based on a large number of three-dimensional air parcel trajectories started daily from points on a grid covering the Northern Hemisphere troposphere and lowermost stratosphere. Different types of airstreams (warm conveyor belts, dry intrusions, stratosphere-to-troposphere flows, and troposphere-to-stratosphere flows) are identified, and their geographical distribution and associated mass fluxes are examined. Warm conveyor belt air parcels, which are deposited in the uppermost troposphere and even the lowermost stratosphere, originate most frequently in the boundary layer over the warm water pools at the eastern seaboards of North America and Asia, close to the worldwide highest anthropogenic emissions. Therefore their chemical characteristics likely range from very clean to highly polluted. Because warm conveyor belts rarely start over Europe, European trace gas emissions have fewer chances of reaching the upper troposphere and lowermost stratosphere with warm conveyor belts than emissions from other densely populated regions. On the other hand, the upper troposphere over Europe frequently receives outflow from warm conveyor belts originating over the North American seaboard. Dry intrusion activity is highest in winter and lowest in summer and shows maxima over western North America and eastern Asia. Deep intrusions from the stratosphere into the lower troposphere are almost nonexistent during the summer. Mass fluxes associated with each of the airstream types are reported and are an alternative to Eulerian mass flux estimates. All types of airstreams with strong vertical motions are highly incoherent. Rather than depositing compact air parcels, they stir air masses into a new environment and create filamentary tracer structures. ¿ 2001 American Geophysical Union