UPPER-TROPOSPHERIC RELATIVE-HUMIDITY OBSERVATIONS AND IMPLICATIONS FOR CIRRUS ICE NUCLEATION

Relative humidity (RH) measurements acquired in orographic wave cloud and citrus environments are used to investigate the temperature depend ent RH required to nucleate ice crystals in the upper troposphere, RHn uc(T). High ice-supersaturations in clear air - conducive to the maint enance of aircraft contrails yet below RHnuc and therefore insufficien t for citrus formation - are not uncommon. Earlier findings are suppor ted that RHnuc in mid-latitude, continental environments decreases fro m water-saturation at temperatures above -39 degrees C to 75% RH at -5 5 degrees C. Uncertainty in determining RHnuc below -55 degrees C resu lts in part from size detection limitations of the microphysical instr umentation, but analysis of data from the SUCCESS experiment indicates that RHnuc below -55 degrees C is between 70 and 88%. A small amount of data, acquired off-shore suggests the possibility that RHnuc may al so depend on properties of the aerosols.