Atmospheric Absorption & Transmission
The Earth is constantly bombarded with electromagnetic radiation but the atmosphere protects us from the exposure to higher energy waves that can be harmful to life – i.e. X-Ray and Gamma Rays. The Earth's atmosphere reflects, absorbs and scatters electromagnetic radiation. This has significant implications to remote sensing as most radiation detected by passive remote sensors passes through the atmosphere where it interacts with molecules and particles in the atmosphere. In portions of the electromagnetic spectrum significant amounts of energy are absorbed by the Earth's atmosphere, with very little of the energy reaching the Earth's surface. All radiation detected by remote sensors passes through the atmosphere for some distance, this distance is known as path length . The path length can very, it could be a short path (i.e. hand-held camera) or a very long path (space photography).
A portion of the incoming solar radiation is absorbed by gases in the Earth's atmosphere. These gases absorb electromagnetic energy at certain wavelengths, therefore in some portions of the electromagnetic spectrum very little energy is absorbed (for example the visible) while in other portions like the Ultraviolet, nearly all energy is absorbed. The portions of the spectrum that are absorbed by atmospheric gases are known as absorption bands.
The primary gases that are responsible the atmospheric absorption of energy are water vapor, carbon dioxide, and ozone.
- Water Vapor (H2O): Very strong absorber in 5.5-7.0 μm range and > 27 μm. Water vapor in the atmosphere is also variable in time and space.
- Carbon Dioxide (CO2) : Primarily absorbs in the mid and far (thermal infrared) portions of the spectrum
- Ozone (O3) : Absorbs strongly in the UV portion of the spectrum (short wavelengths) and is responsible for protecting us from damaging radiation that causes skin cancer.
In contrast to the absorption, transmission is when electromagnetic energy is able to pass through the atmosphere and reach the surface. Visible light, largely passes (is transmitted) through the atmosphere. Some types of electromagnetic radiation easily pass through the atmosphere, while other types do not. The ability of the atmosphere to allow radiation to pass through it is referred to as its transmissivity, and varies with the wavelength/type of the radiation. The gases that comprise our atmosphere absorb radiation in certain wavelengths while allowing radiation with differing wavelengths to pass through. In contrast to the absorption bands, there are areas of the electromagnetic spectrum where the atmosphere is transparent (with minimal to no absorption of radiation) to specific wavelengths. These regions are known as "atmospheric windows" since they allow the radiation to easily pass through the atmosphere to Earth's surface.
Most remote sensing instruments on uses on aircraft or satellite platforms operate in one or more of these windows