The Twomey effect describes how additional cloud condensation nuclei (CCN), possibly from anthropogenic pollution, may increase the amount of solar radiation reflected by clouds.
This is an indirect effect (or radiative forcing) by such particles, as distinguished from direct effects (forcing) due to enhanced scattering or absorbing radiation by such particles not in clouds.
Cloud droplets normally form on aerosol particles that serve as CCN.
Increasing the number density of CCN can lead to formation of more cloud droplets with a smaller size.
The decrease in global mean absorption of solar radiation due to increases in CCN concentrations exerts a cooling influence on climate; the global average magnitude of this effect over the industrial era is estimated as between −0.3 and −1.8 W/m2.
[1] Assume a uniform cloud that extends infinitely in the horizontal plane, also assume that the particle size distribution peaks near an average value of
Taking our assumptions into account we can combine the previous two equations to yield To derive the effect of changing
we can write therefore This illustrates the Twomey Effect mathematically, that is, for a constant liquid water content,