Theory of Scattering in Atmosphere Atmosphere

Earth is enveloped by a vast amount of air called the atmosphere. It is relative thin compared to the size of the earth and fades away with increasing distance from the earth's surface. The earth's atmosphere is categorized into 4 layers- troposphere, stratosphere, mesosphere and thermosphere. Troposphere where all the weather takes place and stratosphere, the two closest layers to earth's surface constitute more than 99 % of the atmosphere. The troposphere and stratosphere extend up to 12 km and 53 km respectively from the earth's surface. The earth's atmosphere is composed of many gases. Nitrogen from decay of biological products constitutes 98 % and Oxygen from photosynthesis constitutes 21 % of the atmosphere. Gravity holds the atmosphere close to the earth's surface and explains why the density of the atmosphere decreases with altitude. The density and pressure of the atmosphere vary with altitude and depends on solar heating and geomagnetic activity. The simplest is an exponential fall off model where pressure and density decrease exponentially with altitude. In 1976, US Standard Atmosphere Model was adopted by COESA (Committee on Extension to the Standard Atmosphere) which describes the earth's atmosphere as composed of 7 layers up to 86 km and pressure, temperature and density are specified for each layer [McCartney1976]. The density is calculated using a perfect gas relationship. In addition to the various gases, atmosphere also contains water vapor, dust particles, etc. The molecules and particles absorb energy at discrete wavelengths, which are determined by their internal properties. For example, molecular oxygen and ozone absorbs light in the ultra violet spectrum. Water vapor, methane, nitrous oxide, ozone, and CO2 absorb light in the infrared range. In addition to absorption, molecules and particles also scatter energy out from its original direction. Sun’s white light is scattered once (primary scattering) or multiple times (secondary scattering) into the viewing ray as shown in Figure 1. The scattered light is received at the earth’s surface from all directions as diffuse skylight or daylight. secondary primary Atmosphere Eart