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The Twomey effect describes how additional
cloud condensation nuclei
Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...
(CCN), possibly from
anthropogenic
Anthropogenic ("human" + "generating") is an adjective that may refer to:
* Anthropogeny, the study of the origins of humanity
Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows:
* Human im ...
pollution
Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
, may increase the amount of
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
reflected by
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may co ...
s. This is an indirect effect (or
radiative forcing
Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured by watts / metre2. It is a scientific concept used to quantify and compare the external ...
) by such particles, as distinguished from direct effects (forcing) due to enhanced
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
or absorbing radiation by such particles not in clouds.
Cloud droplets normally form on
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
particles that serve as CCN. Increasing the number concentration of CCN can lead to formation of more cloud droplets, which, in turn, have
smaller size.
The increase in number concentration increases the
optical depth
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through ...
of the cloud, which results in increase in the
cloud albedo
Cloud albedo is a measure of the albedo or reflectivity of a cloud. Clouds regulate the amount of solar radiation absorbed by a planet and its solar surface irradiance. Generally, increased cloud cover correlates to a higher albedo and a lower ab ...
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 −1.8 W/m².
[IPCC 4th Assessment Report, 2005]
Derivation
Assume a uniform cloud that extends infinitely in the horizontal plane, also assume that the particle size distribution peaks near an average value of
.
The formula for the optical depth of a cloud:
:
Where
is the optical depth,
is cloud thickness,
is the average particle size, and
is the total particle density.
The formula for the liquid water content of a cloud is:
:
Where
is the density of water.
Taking our assumptions into account we can combine the two to derive this expression:
:
If we assume liquid water content (
) is equal for the cloud before and after altering the particle density we obtain:
:
Now we assume total particle density
is increased by a factor of 2 and we can solve for how
changes when
is doubled.
:
We can now take our equation that relates
to
to solve for the change in optical depth when the particle size is reduced.
:
In more general terms, the Twomey effect states that for a fixed liquid water content
and cloud depth, the optical thickness can be represented by:
:
This brings us to the conclusion that increasing the total particle density also increases the optical depth, illustrating the Twomey Effect mathematically.
See also
*
Albrecht effect The Albrecht effect describes how cloud condensation nuclei (CCN), possibly from anthropogenic pollution, may increase cloud lifetime and hence increase the amount of solar radiation reflected from clouds. Because it does not directly interact with ...
*
Sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ar ...
*
Aerosols and soot
*
Contrail
Contrails (; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails ar ...
References
Bibliography
* Hartmann, Dennis L. Global Physical Climatology. Amsterdam: Elsevier, 2016. Print.
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Cloud and fog physics
Climate forcing
Atmospheric radiation
Air pollution
Particulates