Twomey effect

The Twomey effect describes how cloud condensation nuclei (CCN), possibly from anthropogenic pollution, may increase the amount of solar radiation reflected by clouds. This is an indirect effect.

Aerosol particles can act as CCN's creating more droplets which have a smaller size distribution. The reduction in size distribution increases the Optical depth of the cloud. This increases the cloud albedo as clouds appear whiter and larger, leading to a cooling of between -0.3 and -1.8 Wm⁻². For example, on satellite imagery we observe trails of white clouds from ships crossing the oceans due to this effect.

Assume a uniform cloud that extends infinitely in the horizontal plane, also assume that the particle size distribution peaks near an average value of ${\displaystyle {\bar {r}}}$.

The formula for the optical depth of a cloud:

${\displaystyle \tau =2\pi h{\bar {r}}^{2}N}$

Where ${\displaystyle \tau }$ is the optical depth, ${\displaystyle h}$ is cloud thickness, ${\displaystyle {\bar {r}}}$ is the average particle size, and ${\displaystyle N}$ is the total particle density.

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