A new cloud condensation nuclei (CCN) spectrometer capable of simulating the desired range of cloud supersaturation in a single chamber is developed. It can rapidly and continuously measure and display the spatial and temporal distributions of the CCN activity spectrum with high resolution and is suitable for airborne, field and laboratory studies. Details of the spectrometer design for producing a range of supersaturations in the sample air flow, controlled size of activated droplets for detection, and of mechanisms for continuous sampling, scanning and recording are presented. The dependency of the plateau in number concentration of the activated CCN on the sample air flow rate and the detection limit are analyzed. The droplet counting mechanism is compared with the direct photographic method and is found in reasonable agreement. The usefulness of the spectrometer is demonstrated by detecting the simulated smoke plume in the laboratory in real time. Measurements on an urban aerosol indicate a strong scavenging effect of rain and drizzle on CCN concentrations and a marked diurnal characteristic change in the slope of the CCN activation spectrum.