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 increase in number density increases the optical depth of the cloud, which results in an increase in the cloud albedo making clouds appear whiter. Satellite imagery often shows trails of cloud, or of enhanced brightness of cloud, behind ocean-going ships due to this effect. 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 1.8W/m2.

Derivation

.

The formula for the optical depth of a cloud is

is the number density of cloud droplets.

The formula for the liquid water content of a cloud is

is the density of water.

Taking our assumptions into account we can combine the previous two equations to yield

constant, from the last equation we can write

we can write

therefore

, increases the optical depth of the cloud.

See also

• Albrecht effect

• Sulfate

• Aerosols and soot

• Contrail

References

Bibliography

• Hartmann, Dennis L. Global Physical Climatology. Amsterdam: Elsevier, 2016. Print.