Interactions atmosphere

The energy used for remote sensing passes through the atmosphere before reaching the Earth’s surface. The energy interacts with gases (water vapour, ozone, carbon dioxide) and particles (ash, smoke, dust). The gases and particles can absorb or scatter the energy, in which case the energy will not be transmitted through to the Earth’s surface. 

Scattering in the atmosphere occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. How much scattering takes place depends on several factors including the wavelength of the radiation, the abundance of particles or gases, and the distance the radiation travels through the atmosphere.

Rayleigh scattering occurs when particles are very small compared to the wavelength of the radiation. These could be particles such as small specks of dust or nitrogen and oxygen molecules. Rayleigh scattering causes shorter wavelengths of energy to be scattered much more than longer wavelengths. Rayleigh scattering is the dominant scattering mechanism in the upper atmosphere. The fact that the sky appears “blue” during the day is because of this phenomenon. As sunlight passes through the atmosphere, the shorter wavelengths (i.e. blue) of the visible spectrum are scattered more than the other (longer) visible wavelengths.

Absorption is the other main mechanism at work when electromagnetic radiation interacts with the atmosphere. Instead of scattering the energy, the molecules absorb or capture the energy, so it does not reach the Earth’s surface. In that case, we can’t use remote sensing for these wavelengths because there is very little for us to measure. Areas of the spectrum that we can use, where energy is not absorbed and will reach the Earth’s surface, are called atmospheric windows. Sensors are made to take advantage of these windows.