Relative humidity (RH) measurements acquired in orographic wave cloud and cirrus environments are used to investigate the temperature-dependent RH required to nucleate ice crystals in the upper troposphere, RHnuc(T). High ice-supersaturations in clear air-conducive to the maintenance of aircraft contrails yet below RHnuc and therefore insufficient for cirrus formation-are not uncommon. Earlier findings are supported that RHnuc in mid-latitude, continental environments decreases from water-saturation at temperatures above -39 ¿C to 75% RH at -55 ¿C. Uncertainty in determining RHnuc below -55 ¿C results in part from size detection limitations of the microphysical instrumentation, but analysis of data from the SUCCESS experiment indicates that RHnuc below -55 ¿C is between 70 and 88%. A small amount of data acquired off-shore suggests the possibility that RHnuc may also depend on properties of the aerosols. ¿ 1998 American Geophysical Union