Of the energy that reaches the Earth from the Sun, a small amount is absorbed by the atmosphere, a larger amount (about 30%) is reflected back to space by clouds and the Earth's surface, and most of it is absorbed at the planet surface and then released as heat.
Energy is transferred between the Earth's surface and the atmosphere in a variety of ways, including radiation, conduction, and convection. The graphic below uses a campstove to summarize the various mechanisms of heat transfer. If you were standing next to the campstove, you would be warmed by the radiation emitted by the gas flame. A portion of the radiant energy generated by the gas flame is absorbed by the frying pan and the pot of water. By the process of conduction, this energy is transferred through the pot and pan. If you reached for the metal handle of the frying pan without using a potholder, you would burn your fingers! As the temperature of the water at the bottom of the pot increases, this layer of water moves upward and is replaced by cool water descending from above. Thus convection currents that redistribute the newly acquired energy throughout the pot are established.
As in this simple example using a campstove, the heating of the Earth's atmosphere involves radiation, conduction, and convection, all occurring simultaneously. A basic tenet of meteorology is that the Sun warms the ground and the ground warms the air. This activity focuses on radiation, the process by which the Sun warms the ground. Energy from the Sun is the driving force behind weather and climate.
What do trees, snow, cars, horses, rocks, centipedes, oceans, the atmosphere, and you have in common? Each one is a source of radiation to some degree. Most of this radiation is invisible to humans but that does not make it any less real.
Radiation is the transfer of energy by electromagnetic wave motion. The transfer of energy from the Sun across nearly empty space is accomplished primarily by radiation. Radiation occurs without the involvement of a physical substance as the medium. The Sun emits many forms of electromagnetic radiation in varying quantities.
About 43% of the total radiant energy emitted from the Sun is in the visible parts of the spectrum. The bulk of the remainder lies in the near-infrared (49%) and ultraviolet section (7%). Less than 1% of solar radiation is emitted as x-rays, gamma waves, and radio waves.
A perfect radiating body emits energy in all possible wavelengths, but the wave energies are not emitted equally in all wavelengths; a spectrum will show a distinct maximum in energy at a particular wavelength depending upon the temperature of the radiating body. As the temperature increases, the maximum radiation occurs at shorter and shorter wavelengths. The hotter the radiating body, the shorter the wavelength of maximum radiation. For example, a very hot metal rod will emit visible radiation and produce a white glow. On cooling, it will emit more of its energy in longer wavelengths and will glow a reddish color. Eventually no light will be given off, but if you place your hand near the rod, the infrared radiation will be detectable as heat.
The amount of energy absorbed by an object depends upon the following:
- The object's absorptivity, which, in the visible range of wavelengths, is a function of its color
- The intensity of the radiation striking the object
Every surface on Earth absorbs and reflects energy at varying degrees, based on its color and texture. Darker-colored objects absorb more visible radiation, whereas lighter-colored objects reflect more visible radiation.
Albedo is the amount of energy reflected by a surface without being absorbed. An object with high albedo reflects the majority of the radiation while an object with low albedo reflects only a small amount of incoming radiation but absorbs the majority of the radiation.
Think about being outside and how different materials reflect or absorb the incoming radiation. White-colored surfaces such white paint reflect most of the radiation and therefore the albedo value is around 1. Dark-colored surfaces such as gravel and asphalt absorb most of the radiation and have low albedo values closer to 0. This also explains why darker-colored surfaces have higher temperatures. For example, in summer black asphalt roads can be scorching hot. As cities grow, and asphalt, concrete, and dark roofs replace the vegetation, these urban surfaces absorb—rather than reflect—the Sun's heat, causing surface temperatures and urban air temperatures to rise. This is known as the urban heat island effect.
The city of Phoenix, Arizona has made an effort to paint more roofs white to combat the urban heat island effect. The aerial view of Phoenix illustrates the different land surfaces such as water (river located at bottom of image), soil and rock (brown), vegetation (green), buildings with white roofs, roads and an airport (lower right).
Credit: Google Earth