Solar Radiation
Solar radiation drives atmospheric circulation. Since solar radiation represents almost all the energy available to the earth, accounting for solar radiation and how it interacts with the atmosphere and the earth's surface is fundamental to understanding the earth's energy budget.
Solar radiation reaches the earth's surface either by being transmitted directly through the atmosphere ("direct solar radiation"), or by being scattered or reflected to the surface ("diffuse sky radiation"). About 50 percent of solar (or shortwave) radiation is reflected back into space, while the remaining shortwave radiation at the top of the atmosphere is absorbed by the earth's surface and re-radiated as thermal infrared (or longwave) radiation.
The intensity of solar radiation striking a horizontal surface is measured by a pyranometer. The instrument consists of a sensor enclosed in a transparent hemisphere that records the total amount of shortwave incoming solar radiation. That is, pyranometers measure "global" or "total" radiation: the sum of direct solar and diffuse sky radiation. Incoming (or "downwelling") longwave radiation is measured with a pyrgeometer. Outgoing ("upwelling") longwave radiation is measured in various ways, such as with pyrgeometers or with sensors that measure the temperature of the surface.
The net all-wave radiation at a given point (Rn) is calculated by the equation:
where is incoming solar radiation,
is surface albedo,
is downwelling longwave radiation (thermal infrared radiation emitted from cloud bases and atmospheric gases), and
is upwelling longwave radiation (thermal infrared radiation emanating from the earth's surface). All radiation fluxes are expressed as energy per unit area (generally watts per square meter, or W/m2). Accurate estimates of albedo are especially important as albedo places a fundamental limit on the amount of solar radiation that can be absorbed by the surface. For example, albedo strongly determines the rate of melt of sea ice. Over longer periods of time, changes in components of the radiation balance can be manifested in climate change.
Factors Modifying the Role Solar Radiation Plays in the Earth's Energy Budget
The most important factors influencing how much shortwave radiation reaches the earth's surface and how much is absorbed are time and day of year, cloud cover, and albedo.
Time of Day and Year
The intensity of solar radiation varies significantly over the course of a year ranging from no solar radiation during the polar winter to a maximum of 350 to 400 watts per square meter (W/m2) in the summer. Over the course of a day, the sun's angle above the horizon (solar altitude) influences the intensity of solar radiation: the noon sun is more intense than the rising or setting sun. The maximum altitude of the sun depends on time of year and latitude. Of course, during the polar winter the sun is below the horizon for 24 hours, and there is no solar radiation, while at midsummer the sun changes little in altitude over the course of a day.
Cloud Cover
Clouds reflect some incoming radiation back to space, thereby reducing the amount of radiation that reaches the earth's surface. However, clouds also re-radiate infrared energy back toward the earth's surface, thereby moderating the temperature of the lower atmosphere. Globally, clouds have a cooling effect on the earth-atmosphere system, because of their high albedos. In polar regions however, clouds seem to have a net warming effect as the reduction in solar radiation is outweighed by the effect of clouds in increasing longwave radiation to the surface.
Albedo
Incoming solar radiation that strikes the earth's surface is partially reflected and partially absorbed, in proportion to surface reflectivity (albedo). Darker surfaces have a lower albedo and absorb more solar energy than do lighter surfaces. The albedo of a surface is also a function of the incidence angle of solar radiation (that is, the amount of solar energy a surface absorbs will depend on the solar altitude).
Newly fallen snow has an albedo of approximately 0.90, meaning that it reflects about 90 percent of incoming radiation. In contrast, melting snow has an average albedo of 0.50, meaning that it absorbs 50 percent and reflects 50 percent of the incoming radiation. Because a darker surface absorbs more solar radiation, snow covered by dust (dirty snow) melts faster than clean snow. The albedo of sea ice varies with ice age, but when snow covered is on the order of 0.70.
Open water absorbs the most radiation of all arctic surfaces. With an albedo of about 0.08, it reflects only 8 percent of the incoming radiation. However, the variation of albedo with solar altitude is especially pronounced for the surfaces of oceans and lakes. The albedo of a water surface increases with decreasing solar altitude and approaches a mirror-like 100 percent near sunrise and sunset, or when the sun is low in the arctic sky.
Important changes in surface albedo can occur seasonally. Over land, heavy winter snow cover increases surface albedo considerably. In middle and high latitudes, significant increases in surface albedo accompany the winter formation of lake and sea ice.
A description of the seasonal cycle of solar radiation based on gridded global radiation fields has been drawn from the data section of the Atlas.