The ambient temperatures and humidities required for contrail formation and persistence are determined from in situ measurements during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) experiment. Ambient temperatures and water vapor concentrations were measured with the meteorological measurement system, a laser hygrometer, and a cryogenic hygrometer (all onboard the DC-8). The threshold temperatures are compared with theoretical estimates based on simple models of plume evolution. Observed contrail onset temperatures for contrail formation are shown to be 0--2 K below the liquid-saturation threshold temperature, implying that saturation with respect to liquid water must be reached at some point in the plume evolution. Visible contrails observed during SUCCESS persisted longer than a few minutes only when substantial ambient supersaturations with respect to ice existed over large regions. On some occasions, contrails formed at relatively high temperatures (≥-50 ¿C) due to very high ambient supersaturations with respect to ice (of the order of 150%). These warm contrails usually formed in the presence of diffuse cirrus. Water vapor from sublimated ice crystals that entered the engine was probably necessary for contrail formation in some of these cases. At temperatures above about -50 ¿C, contrails can only form if the ambient air is supersaturated with respect to ice, so these contrails should persist and grow. ¿ 1998 American Geophysical Union