The solar radiation, which approximately corresponds to the radiation from a black body of temperature 6000 K, meets the Earth-atmosphere system and interacts with it, producing temperatures which at the Earth's surface typically vary in the range 220-320 K. The average (over time and geographical location) temperature of the Earth's surface is presently 288 K.
As a first approach to understanding the processes involved, one may look at the radiation flux passing through unit horizontal areas placed either at the top of the atmosphere or at the Earth's surface. The net flux is the sum (with proper signs) of the fluxes passing the area from above and from below. The flux direction towards the centre of the Earth will be taken as positive, consistent with reckoning the fluxes at the Sun as positive, if they transport energy away from the solar centre.
Since the spectral distributions of black-body radiation (see section 2.A) at 6000 and 300 K, respectively, do not substantially overlap, most of the radiation fluxes involved can be adequately discussed in terms of two broad categories, called short-wavelength (sw) and long-wavelength (Iw) or thermal radiation.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.