Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 105

The solar radiation spectrum incident on the outermost part of the earth's atmosphere is fairly close to the theoretical blackbody radiation curve for an object at 5780 K, with peak emission around 500 nm in the visible region and significant emission throughout the UV region. Oxygen (O₂) strongly absorbs UV at wavelengths shorter than 200 nm, with weaker absorption out to 245 nm, and ozone (O₃) in the stratosphere absorbs UV over the range 230–300 nm. Due to atmospheric absorption by O₂ and O₃, practically no solar UV of wavelength less than 290 nm reaches the earth at sea level. Selected wavelength bands in the visible and IR regions are attenuated by O₂, O₃, water vapor, and carbon dioxide (11). The spectral distribution of direct sunlight reaching the earth's surface is further altered by Rayleigh scattering, which takes some light out of the direct path from the sun and disperses it around the upper atmosphere. Short wavelengths, including UV and blue light, are more strongly scattered than longer wavelengths. The blue color of the sky comes from this selective scattering of short‐wavelength sunlight. Change in the path length of the sun's rays through the atmosphere throughout the day, as well as variation in path length and ozone content by latitude and season, lead to temporal and geographical variation in the solar spectrum at the earth's surface.