This study examines how jet aircraft contrails develop precipitation trails, using data collected on 12 May, 1996 during SUCCESS. The DC-8 sampled the precontrail conditions, produced a contrail largely in clear air at -52 ¿C, and sampled the contrail and developing trails for over an hour. The environment was highly ice-supersaturated, reaching nearly water saturation in some locations. Inside the contrail core, almost all ice particles remained small (~1 to 10 &mgr;m) due to high crystal concentrations (~101 to 102 cm-3) which reduced the vapor density to saturation. Mixing of moist environmental air and vapor-depleted contrail air produced localized regions of supersaturation along the contrail periphery, where crystals grew to several hundred microns at about 0.1 &mgr;m s-1. These particles could then fall from the contrail into the vapor-rich, undepleted, supersaturated environment below. As heavier crystals left the contrail, others moved into the regions of ice supersaturation. Precipitation trails developed as this process continued over time. ¿ 1998 American Geophysical Union