Chapter 18 OPTICAL AND INFRARED PROPERTIES OF THE ATMOSPHERE

This chapter deals with the absorption, scattering, emis1920s. Early Standard Atmosphere Models represented mean sion and refractive properties of the natural atmosphere from conditions of temperature, density, and pressure as a functhe ultraviolet through the microwave region. Nonlinear tion of altitude. During the 1950s and 1960s extensions and propagation processes (such as thermal blooming) are not supplemental atmospheres were derived that describe in adcovered. dition the seasonal and latitudinal variability of atmospheric The natural atmosphere includes the range of those avstructure (see Chapters 14 and 15). erage conditions which are not directly affected by limited For the purpose of modeling the optical properties of local processes or sources, such as gases or particulates from the atmosphere, specifically as described in Sections 18.4.1 a fire or industrial plant, a dust cloud from vehicular traffic (FASCODE) and 18.4.2 (LOWTRAN), the 1962 U.S. Stanetc. The natural atmosphere does include the effects of moldard Atmosphere and supplemental model atmospheres were ecules, aerosol (haze) particles, clouds, fogs, rain, and snow. used as models. Although updated Standard AtmoThe vertical extent of the atmosphere is from the surface spheres have been published as recently as 1976, differences up to 100 km altitude. A brief discussion is included on the are limited to altitudes above 50 km where the adopted earth's surface reflectance properties since they also affect radiance models were not well characterized. the atmospheric optical properties. Pressure, temperature, water vapor density, and ozone The chapter includes descriptions of the basic physical density profiles as a function of altitude are provided to relationships of optical propagation as well as the current describe these molecular atmospheric models. These prostate of experimental knowledge and modeling of the atfiles, taken from McClatchey et al. [1972], correspond to mospheric propagation medium. An extensive annotated the U.S. Standard Atmosphere, 1962 and five supplemenbibliography covering much of the material of this chapter tary models [Cole et al., 1965 or U.S. Standard Atmosphere was compiled by Wiscombe [1983] in Reviews of GeoSupplements 19661: Tropical (15°N), Midlatitude Summer physics and Space Physics. (45°N, July), Midlatitude Winter (45°N, January), Subarctic Summer (60°N, July), and Subarctic Winter (60°N, January). These profiles are identical to the ones used in the 18.1 ATMOSPHERIC GASES LOWTRAN code [Kneizys et al., 1980]. The water vapor and ozone latitude profiles added to the 1962 U.S. Standard Atmosphere by McClatchey et al. [1972] were obtained 18.1.1 Atmospheric Molecules, Models from Sissenwine et al. [1968] and Hering and Borden [1964] of the Atmospheric Composition respectively, and correspond to mean annual values. The water vapor densities for the 1962 U.S. Standard AtmoThe propagation of optical and infrared radiation through sphere correspond to relative humidities of approximately the atmosphere depends on the composition and variability 50% for altitudes up to 10 kin, whereas the relative humidity of the atmosphere. Systematic variations in the density, values for the other supplementary models tend to decrease pressure, temperature, water vapor, and ozone as a function with altitude from approximately 80% at sea level to apof latitude and season have been known for many years. proximately 30% at 10 km altitude. Above 12 km, the water The development of model atmospheres goes back to the vapor density profiles depicted here have been replaced by