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 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 citrus formation - are not uncommon. Earlier findings are supported that RHnuc in mid-latitude, continental environments decreases from water-saturation at temperatures above -39 degrees C to 75% RH at -55 degrees C. Uncertainty in determining RHnuc below -55 degrees 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 degrees 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.