In addition to being absorbed or transmitted, electromagnetic radiation can also be reflected or scattered by particles in the atmosphere. Scattering is the redirection of electromagnetic energy by suspended particles in the atmosphere. The type and amount of scattering that occurs depends on the size of the particles and the wavelength of the energy. There are three main types of scattering that impact incoming solar radiation:
Rayleigh scatter occurs when radiation (light) interacts with molecules and particles in the atmosphere that are smaller in diameter than the wavelength of the incoming radiation. Shorter wavelengths are more readily scattered that longer wavelengths. Light at shorter wavelengths (like blue and violet visible light) are scattered by small particles that include NO2 and O2. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than longer wavelength red light. Rayleigh scatter is responsible for the blue color of the sky. Rayleigh scatter is can also produce haze in images. In aerial photography special filters are used to filter out the scattering of blue light to reduce haze. In digital images there are different techniques used to minimize the impacts of Rayleigh scatter.
At sunrise and sunset the incoming sunlight travels a longer distance (path length) through the atmosphere. The longer path leads to scatter of the short (blue) wavelengths that is so complete that we only see the longer wavelengths of light, the red and orange. In the absence of particles and scattering the sky would appear black.
Image Source: Principles of Remote Sensing (Tempfli et al.)
Haze in Shanghai, China due to air pollution
Mie scatter occurs when the wavelength of the electromagnetic radiation is a similar size to the atmospheric particles. Mie scatter generally influences radiation from the near UV through the mid- infrared parts of the spectrum. Mie scatter mostly occurs in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast. Pollen, dust and smog are major cause of mie scatter. Mie scatter produces general haze in images.
Non-selective scattering occurs when the diameter of the particles in the atmosphere are much larger than the wavelength of radiation. Non-selective scatter is primarily caused by water droplets in the atmosphere. Non-selective scattering scatters all radiation evenly through out the visible and infrared portions of the spectrum - hence the term non-selective. In the visible wavelengths light is scattered evenly, hence fog and clouds appear white. Since clouds scatter all wavelengths of light, this means that clouds block most energy from reaching the Earth's surface. This can make interpreting and analyzing remote sensed imagery difficult in areas prone to cloud and fog cover. Clouds also cast shadows that change the illumination and relative reflectance of surface features. This can be a major limitation in remote sensing imagery.
Cloud covered Landsat 8 imagery of the North Coast (left) and cloud shadows in aerial imagery (right)
Impact of Atmospheric Interaction
In remote sensing it is important to understand the impact the atmosphere has on electromagnetic radiation. First, it is crucial to understand atmospheric windows and to be able to identify whether or not a sensor can “see” through atmosphere in a given portion of the spectrum. Because the atmosphere absorbs and scatters the incoming radiation we often want to correct for these interactions. This process is known as "atmospheric correction" and is a common image processing technique.
For Fun: Martian Sunset
The atmosphere on Mars is different from that of the Earth and has a significant amount of fine particulate in the atmosphere. This causes different light scattering than what we see here on Earth. The dust in the Martian atmosphere allows blue light to penetrate the atmosphere more efficiently than longer-wavelength light. That causes the blue colors in the mixed light coming from the sun to stay closer to sun's part of the sky, compared to the wider scattering of yellow and red colors. The effect is most pronounced near sunset, when light from the sun passes through a longer path in the atmosphere than it does at mid-day. This produces a blue tinted sunset on Mars.
Sunset on Mars
Image Source: NASA/JPL