Extensive measurements of cloud condensation nuclei (CCN) spectra (1 to 0.02% supersaturation; S) at altitudes up to 12 km are presented. These were similar to many marine boundary layer measurements when normalized to standard sea level pressure. Extremely low CCN concentrations (3000 cm-3). This was suggestive of cloud scavenging of CCN and production of small particles. Measurements in the exhaust of a B-757 jet aircraft showed very high CCN/CN ratios and enhancements of CCN concentrations for all S. This suggests a greater impact of jet aircraft on clouds and climate than previous measurements. ¿ 1998 American Geophysical Union