The Science
Dust and other aerosol particles in the atmosphere can join with water vapor to become cloud droplets. The formation of cloud droplets depends on the size and chemical composition of the aerosol particles and the humidity in the atmosphere, especially when above 100 percent. Scientists’ previous formulation of this problem assumed clouds formed only when the humidity rose above 100 percent, a condition scientists call supersaturation. This new research examined the effects of turbulence in the air. The research found that if the humidity falls below 100 percent, the primary driver of cloud formation transitions from a process dominated by supersaturation to a process dominated by small changes in humidity from turbulence. Those small changes mean that air that was previously thought too dry to form clouds will do so if turbulent fluctuations intermittently kick humidity above 100 percent.
The Impact
The results reveal new complexity in the formation of cloud droplets from aerosol particles. When examining cloud formation, scientists must consider both fluctuations in relative humidity and average humidity. The findings will help climate researchers build more accurate computer models of cloud formation. This is an important factor in modeling the Earth’s atmosphere and climate.
Summary
Aerosol particles and water vapor become cloud droplets when saturation exceeds a critical value, which depends on the aerosol particle size and chemical composition. The traditional formulation of this problem considers only average saturation. Using experiments and theory, researchers examined the effects of fluctuations produced by turbulence. They found a clear transition from a regime in which the average saturation dominates to one in which the fluctuations determine cloud properties. The theory developed to interpret these measurements indicates a transition from an average- to a fluctuation-dominated regime, based on the relative values of the average and standard deviation of the environmental saturation and the critical saturation at which aerosol particles activate, meaning they become droplets.
Funding
This work was supported by the National Science Foundation and by the Department of Energy Office of Science.
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