Cloud albedo is a measure of the
albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
or
reflectivity
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
of a
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 ...
. 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
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
and a lower absorption of
solar energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
. Cloud albedo strongly influences the
Earth's energy budget
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but m ...
, accounting for approximately half of Earth's albedo.
Cloud albedo depends on the total mass of water, the size and shape of the droplets or particles and their distribution in space.
Thick clouds (such as
stratocumulus
A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and th ...
) reflect a large amount of incoming solar radiation, translating to a high albedo. Thin clouds (such as
cirrus
Cirrus may refer to:
Science
*Cirrus (biology), any of various thin, thread-like structures on the body of an animal
*Cirrus (botany), a tendril
* Infrared cirrus, in astronomy, filamentary structures seen in infrared light
*Cirrus cloud, a typ ...
) tend to transmit more solar radiation and, therefore, have a low albedo. Changes in cloud albedo caused by variations in cloud properties have a significant effect on
global climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorological ...
.
Cloud condensation nuclei and cloud albedo
On a microscopic scale, clouds are formed through the
condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
of water on
cloud condensation nuclei, such as
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 ...
and
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. The size, concentration, structure, and chemical composition of these particles influence cloud albedo.
[Lohmann, U.; Feichter, J. (2005)]
"Global indirect aerosol effects: a review"
''Atmospheric Chemistry and Physics''. 5: 715–737. For example,
black carbon
Chemically, black carbon (BC) is a component of fine particulate matter (PM ≤ 2.5 µm in aerodynamic diameter). Black carbon consists of pure carbon in several linked forms. It is formed through the incomplete combustion of fossil fue ...
aerosol particles absorb more solar radiation and
sulfate aerosol reflects more solar radiation. Smaller particles form smaller cloud droplets, which tend to decrease precipitation efficiency of a cloud, increasing cloud albedo.
Additionally, more cloud condensation nuclei increases the size of a cloud and the amount of reflected solar radiation.
Causes of cloud albedo variation
Cloud albedo on a planet varies from less than 10% to more than 90% and depends on drop sizes, liquid
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
or ice content, thickness of the cloud, solar zenith angle, etc.
Liquid Water Path
A cloud's
liquid water path Liquid water path - in units of g/m2 is a measure of the total amount of liquid water present between two points in the atmosphere.
LWP is an important quantity in understanding radiative transfer in the atmosphere. It is defined as the integral o ...
varies with changing cloud droplet size, which may alter the behavior of clouds and their albedo.
The variations of the albedo of typical clouds in the atmosphere are dominated by the column amount of liquid water and ice in the cloud.
The smaller the drops and the greater the liquid water content, the greater the cloud albedo, if all other factors are constant.
The Twomey Effect (Aerosol Indirect Effect)
The
Twomey Effect
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, ...
is increased cloud albedo due to cloud nuclei from pollution. Increasing
aerosol concentration and aerosol density leads to higher cloud droplet concentration, smaller cloud droplets, and higher cloud albedo.
In macrophysically identical clouds, a cloud with few larger drops will have a lower albedo than a cloud with more smaller drops. The smaller cloud particles similarly increase cloud albedo by reducing precipitation and prolonging the lifetime of a cloud. This subsequently increases cloud albedo as solar radiation is reflected over a longer period of time. The
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 ...
is the related concept of increased cloud lifetime from cloud nuclei.
Zenith Angle
Cloud albedo increases with the total water content or depth of the cloud and the
solar zenith angle
The solar zenith angle is the zenith angle of the sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between the ...
.
The variation of albedo with zenith angle is most rapid when the sun is near the horizon, and least when the sun is overhead. Absorption of solar radiation by plane-parallel clouds decreases with increasing zenith angle because radiation that is reflected to space at the higher zenith angles penetrates less deeply into the cloud and is therefore less likely to be absorbed.
Influence on global climate
Cloud albedo indirectly affects global climate through solar radiation
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 ...
and
absorption in Earth's radiation budget.
Variations in cloud albedo cause atmospheric instability that influences the
hydrological cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
, weather patterns, and
atmospheric circulation
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but t ...
.
These effects are parameterized by
cloud radiative forcing, a measure of short-wave and long-wave radiation in relation to
cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
. The
Earth Radiation Budget Experiment
The Earth Radiation Budget Satellite (ERBS) is a NASA scientific research satellite within NASA's ERBE (Earth Radiation Budget Experiment) Research Program - a three-satellite mission, designed to investigate the Earth radiation budget. It also ...
demonstrated that small variations in cloud coverage, structure, altitude, droplet size, and phase have significant effects on the climate. A five percent increase in short-wave reflection from clouds would counteract the greenhouse effect of the past two-hundred years.
Cloud Albedo-Climate Feedback Loops
There are a variety of positive and negative
cloud albedo-climate feedback loops in cloud and climate models. An exampled of a negative cloud-climate feedback loop is that as a planet warms, cloudiness increases, which increases a planet's albedo. An increase in albedo reduces absorbed solar radiation and leads to cooling. A counteracting positive feedback loop considers the rising of the high cloud layer, reduction in the vertical distribution of cloudiness, and decreased albedo.
[{{Cite journal, last1=Wetherald, first1=R. T., last2=Manabe, first2=S., date=1988, title=Cloud Feedback Processes in a General Circulation Model, url=https://journals.ametsoc.org/view/journals/atsc/45/8/1520-0469_1988_045_1397_cfpiag_2_0_co_2.xml, journal=Journal of the Atmospheric Sciences, language=EN, volume=45, issue=8, pages=1397–1416, doi=10.1175/1520-0469(1988)045<1397:CFPIAG>2.0.CO;2 , bibcode=1988JAtS...45.1397W , issn=0022-4928]
Air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
can result in variation in cloud condensation nuclei, creating a feedback loop that influences atmospheric temperature, relative humility, and cloud formation depending on cloud and regional characteristics. For example, increased
sulfate aerosols
The term sulfate aerosols is used for a suspension of fine solid particles of a sulfate or tiny droplets of a solution of a sulfate or of sulfuric acid (hydrogen sulfate). They are produced by chemical reactions in the atmosphere from gaseous prec ...
can reduce precipitation efficiency, resulting in a positive feedback loop in which decreased precipitation efficiency increases aerosol atmospheric longevity.
On the other hand, a negative feedback loop can be established in mixed-phase clouds in which
black carbon
Chemically, black carbon (BC) is a component of fine particulate matter (PM ≤ 2.5 µm in aerodynamic diameter). Black carbon consists of pure carbon in several linked forms. It is formed through the incomplete combustion of fossil fue ...
aerosol can increase ice phase precipitation formation and reduce aerosol concentrations.
References
Atmospheric radiation
Clouds
Satellite meteorology