Increasing cloud droplet number at fixed liquid-water content and cloud thickness reduces mean droplet size and increases total extinction area, raising shortwave optical depth and cloud albedo. This idealized response produces negative shortwave radiative forcing. Changes in cloud amount, water content and dynamics are separate adjustments.[ref]
The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey. Fixed-water-content droplet-number and albedo response
For at fixed liquid-water content and thickness, and , the derivative is . For a fixed fractional number increase, the initial response is greatest at . The exact finite-change expression is with . Uncertain activation of additional particles and changes in water content limit direct application to real clouds.
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The Twomey effect refers to a phenomenon in atmospheric science and environmental studies where an increase in the number of cloud condensation nuclei (CCN) leads to the formation of smaller cloud droplets. This effect results in clouds that are more reflective (more effective at scattering sunlight) and can influence climate and weather patterns. The concept is named after Professor Alan Twomey, who proposed it in a 1974 paper.