The Sun is our primary source of electro-magnetic energy. This energy moves through free space in waves that have a wide range of fre-quencies and wavelengths. Most of the energy reaching Earths lower atmosphere is at visible and near infrared wavelengths. The wavelengths of visible radiation extend from 0.4 to 0.78 microns.

A micron is equal to 0.0001 cm. Radiation occurring in the 0.4 to 0.78-micron range is visible to us only because our eyes are sensitive to these wavelengths, and our brain interprets what the eyes see.

The affect of the atmosphere on the visible spectrum differs depending on the state of the atmosphere. For example, on a cloud-free day with no haze or smoke present, the Sun appears white and the sky blue. When haze and smoke are present on a cloud-free day, the Suns appearance becomes indistinct, and the sky appears bright white. Such visible changes to the Suns energy are caused by four atmospheric processes: reflection, scattering, absorption, and emission.

REFLECTIONReflection of electromag-netic energy occurs at all wavelengths, although it is most readily perceivable in the visible spectrum. Reflection occurs when the wavelength of the radiation is smaller than the atmospheric elements it encounters. Clouds which are made up of varying percentages of water vapor and hydroscopic nuclei, are the primary reflectors of radiation in the atmosphere.

SCATTERINGScattering is dependent on the wavelength of the radiated energy and the size of the particles, gas molecules, or atoms the radiation encounters in the atmosphere. When the elements in the atmosphere are smaller in diameter than the wavelength of the radiation striking them, the radiation is scattered or deflected. Elements that are larger than the wavelength of the radiation striking them DO NOT produce scattering. There are two types of scattering: Rayleigh and Mie.

Rayieigh scattering occurs when sunlight is dispersed by molecules in the atmosphere that are much smaller than the lights wavelength. Rayleigh scattering produces our blue skies on cloud-free days.

Mie scattering occurs when sunlight is deflected by elements in the atmosphere whose sizes are near that of the wavelength of the light it is scattering. Dust, haze, smoke, and cloud droplets cause Mie scattering, and the scattering causes the sky to appear white.

Scattering, especially that caused by haze (aloft and/or at the surface) adds greatly to the problem of target acquisition through reduced risibilities.

ABSORPTIONMany atmospheric elements (mainly water vapor, carbon dioxide, and oxygen) absorb radiation propagating through the at-mosphere without scattering or reflecting it.

Absorption occurs selectively with respect to wavelength, and each atmospheric element characteristically absorbs in specific wavelength intervals called absorption bands. For example, ozone in the upper atmosphere absorbs only ultraviolet radiation; radiation at other wavelengths is not absorbed by the ozone.

In addition to band absorption, certain at-mospheric gases exhibit a characteristic described as continuum absorption. They absorb energy over a wide range of the electromagnetic spec-trum.  

Water vapor is the most important gas with respect to continuum absorption. Continuum absorption by water vapor is most significant in an atmosphere having high or ABSOLUTE HUMIDITY. The tropics has such an atmos-phere.

Cold atmospheres with high relative humidity do NOT exhibit continuum absorption, because of their low water vapor content. The ratio of the amount of radiation absorbed by a substance (for example, ozone or water vapor) to the total amount of radiation striking the substance is known as Absorptivity. Absorp-tivity varies with the wavelength of the radiation striking a body and the temperature of the absorbing body. No substance absorbs all of the radiation striking it; however, there are substances that approach perfect absorptivity; they absorb nearly all of the radiation striking them at specific wavelengths. These substances are termed "black bodies." Theoretically, black bodies are perfect absorbers.

EMISSIONEmission refers to the radiation given off by a body. Every element in the at-mosphere (gasses, clouds, and aerosols included) emits radiation. In general, atmospheric elements emit radiation at the same wavelength at which they absorb it. The energy radiated from a bodys unit surface per unit time is known as the bodys EMISSIVE POWER, while the ratio of emissive power of one body as compared to that of a black body under identical conditions (same wavelength and temperature) is known as EMISSIVITY.

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