How important is the transition zone between clouds and aerosol?

Clouds are a macroscopic and easily observable natural phenomenon (a suspension of water droplets or ice crystals), which has been studied since the dawn of modern science. Clouds raise increasing interest in the scientific community, due to their fundamental role in the water cycle, energy balance, and climate. The atmosphere, however, contains other suspended particles that constitute the atmospheric aerosol, which also play an important role in the Earth’s radiative balance. Moreover, they have a major impact on the formation, evolution, and characteristics of clouds. Study and quantification of clouds and aerosol often requires discriminating from each other. Although in general they differ in the proportion of water (liquid or solid), composition, size, shape, etc., there are situations that lie in the border between them (hydrated aerosols, haze, dissipating clouds, etc.) This communication reviews several studies that suggest that clouds and aerosol should be treated as a continuum or that an additional phase between them must be considered. Both observational works of this transition zone and studies that deal with the radiative effect of these transition situations are commented. We conclude that it is important to carry out additional research focused on suspensions of particles with characteristics between those of typical clouds and those of pure aerosol.