Solar Radiation Flow in the Earths Surface Atmosphere System

As it propagates through the atmosphere, solar radiation undergoes scattering and absorption by gases, aerosol, and clouds. The fraction of solar radiation that survives and reaches the surface is partly reflected back to the atmosphere. The remainder is absorbed by the surface. The scattered (or diffused) radiation can undergo many acts of scattering and reflection until it is either reflected back to the space or absorbed by the Earth's surface-atmosphere system. Absorption of solar radiation, a process by which the energy transported by electromagnetic waves is converted to other forms, is the sole significant source ofheat that ultimately supports the climate and life on our planet. Unlike absorption, scattering is a process that conserves the radiant energy but redirects the energy from the incident wave in all directions.

Atmospheric gases scatter solar radiation at all wavelengths but they absorb only in selected wavelength bands. In the upper atmosphere, molecular oxygen (O2) and ozone (O3) absorb almost completely the solar UV radiation with wavelengths less than about 0.3 mm. In the troposphere, water vapor (H2O) is the chief absorbing gas in the visible band. Water vapor also absorbs in the near-IR band along with carbon dioxide (CO2). The main gases that can absorb solar radiation are present only in small amounts compared to nitrogen (N2) and oxygen (O2) that are responsible for >99% of the total mass of the atmosphere. Molecular (Rayleigh) scattering controlled

byN 2 and O2 is the largest near the surface and decreases with altitude as air density decreases. The characteristic feature of molecular scattering is the inverse proportionality to the fourth power of the wavelength. This causes more blue light to be scattered than green, yellow, and red, so the sky appears blue on clear days.

Scattering and absorption by particles (aerosols and cloud drops) depend on their size and composition. In particular, the amount of scattered energy and its directional distribution strongly depend on the ratio of the incident wavelength and particle size. The larger the ratio, the larger the amount of radiation scattered in the forward direction. Because of larger sizes of cloud drops, scattering by clouds is much greater than molecular (Rayleigh) scattering and scattering by aerosol particles. For this reason the presence of clouds is a main factor controlling the amount of solar radiation scattered back to space. Clouds appear white because, unlike molecules, they scatter all visible wavelengths equally. The cloud albedo increases with cloud water path (a total mass of cloud water in a vertical column per unit surface area). Clouds also absorb some solar radiation in the near-IR. Both cloud albedo and absorption of solar radiation are sensitive to sizes of cloud particles. For the fixed cloud water path, clouds consisting of smaller drops tend to have larger albedos. Clouds strongly vary in space and time, but on average they cover c. 62 % of the entire planet.

Aerosols, liquid, solid, or mixed-phase particles suspended in the air, all can scatter solar radiation. Whether they can absorb solar radiation or not depends on their chemical compositions. In the troposphere, the common aerosol types are sulfates, nitrates, carbonaceous (organic and black carbon), mineral dust, and sea salt. Of those, black carbon and mineral dust absorb solar radiation. Some organic aerosols can absorb in the UV, but this absorbed energy is too small to be important in the global radiation balance. In the stratosphere, aerosol particles originating mainly from volcanic eruptions do not absorb sunlight. The amount of solar radiation scattered and absorbed by aerosols also depends on aerosol particle concentration. Both concentration and aerosol composition vary greatly with time and location. Thus aerosol scattering adds to the amount ofsolar energy that is reflected back to space, whereas absorption of solar radiation by black carbon and dust particles when present contributes to the radiant energy that stays in the system. Collectively, atmospheric gases, aerosols, and clouds absorb only about 20% of solar radiation.

The Earth's surface also absorbs some of the solar radiation that survived passing through the atmosphere. The remainder is reflected back to the atmosphere. The fraction of solar radiation reflected by a surface is called surface albedo. Surfaces with low albedo reflect a small amount of sunlight; those with high albedo reflect a large amount. The larger the albedo, the lower the amount ofthe

Table 2 Albedo of various surfaces


Albedo (%)



Bare soil




Fresh snow


Old snow


Sand, desert


Deciduous forest


Coniferous forest


solar energy absorbed by the surface. Different types of vegetated and bare surfaces have different albedos. Although it is a function of wavelength, in the context of the energy balance the surface albedo averaged over the solar spectrum is of interest. Table 2 gives some examples for common natural surfaces. The water surfaces have the lowest values while ice- and snow-covered surfaces have the highest. Forests typically have lower albedo than deserts. The albedo of vegetated surfaces varies temporally (e.g., seasonal changes) more than that of bare soils.

Ultimately, the reflection from the surface and the atmosphere controls the amount of the solar energy returning back to space, which can be expressed in terms of the albedo of the Earth as a whole, called planetary albedo. The Earth's planetary albedo is about 0.3, that is to say that about 30% of solar radiation is reflected and the remainder 70% is absorbed by the Earth's surface-atmosphere system.

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