UV energies at various altitudes.

Plots of the Sun's energy, a quantity referred to as solar flux. It refers to the amount of solar energy in watts falling perpendicularly on a surface one square centimeter, and the units are watts per cm² per nm. The plots represent the Sun's energy (or flux) at four different altitudes in the atmosphere: the surface, 20 km, 30 km, and the "top" of the atmosphere (over 100 km). Because of the strong absorption of UV radiation by ozone in the stratosphere, the intensity decreases at lower altitudes in the atmosphere. In addition, while the energy of an individual photon is greater if it has a shorter wavelength, there are fewer photons at the shorter wavelengths, so the Sun's total energy output is less at the shorter wavelengths. Because of ozone, it is virtually impossible for solar ultraviolet to penetrate to Earth's surface. For radiation with a wavelength of 290 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere.

To appreciate how important this ultraviolet radiation screening is, we can consider a characteristic of radiation damage called an action spectrum. An action spectrum gives us a measure of the relative effectiveness of radiation in generating a certain biological response over a range of wavelengths. This response might be erythema (sunburn), changes in plant growth, or changes in molecular DNA. The blue line superimposed on the figure shows the action spectrum for DNA. It represents the probability of DNA damage by UV radiation at various wavelengths. Fortunately, where DNA is easily damaged (where there is a high probability), ozone strongly absorbs UV. At the longer wavelengths where ozone absorbs weakly, DNA damage is less likely. The red line shows the calculated UV spectrum at Earth's surface if there was a 10% decrease in ozone. In response to this decrease in protective ozone, the amount of DNA damaging UV increases, in this case, by about 22%. Considering that DNA damage can lead to maladies like skin cancer, it is clear that this absorption of the Sun's ultraviolet radiation by ozone is critical for our well being.

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NASA. Studying Earth's Environment From Space. June 2000. (April 14, 2005) http://www.ccpo.odu.edu/SEES/index.html.

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• (del) (cur) 08:01, 14 April 2005 . . en:User:SEWilco SEWilco ( en:User_talk:SEWilco Talk) . . 504x345 (40696 bytes) (UV energies at various altitudes. == Source == { { PD-USGov-NASA } } en:NASA NASA. Studying Earth's Environment From Space. en:June_2000 June 2000. ( en:April_14 April 14, en:2005 2005) http://www.ccpo.odu.edu/SEES/index.html.)