Abstract
The state of knowledge of the particles upon which liquid droplets condense to form atmospheric water clouds is presented. The realization of cloud condensation nuclei (CCN) as a distinct aerosol subset originated with the cloud microphysical measurements and theoretical insights of Patrick Squires 40 years ago. He helped originate and continue the development of CCN counters and made significant CCN measurements for more than 25 years. Recognition of the importance of CCN has expanded from warm-rain efficiency to aerosol scavenging, cloud radiative properties and other topics. In spite of a promising beginning and much encouragement over the years, CCN knowledge has increased minimally. Significant uncertainties about global climate change cannot be reduced without expansion of the knowledge base of CCN.
