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In this topic you will learn about the interactions that take place between the different forms of radiation you learned about in previous chapters with the Earth’s atmosphere. The layer surrounding our planet has tremendous impact on the signal that we are measuring. While some sensors are more less immune to this kind of influence (you will learn about this later), others produce uninterpretable signals.

A protective layer – the atmosphere

Before solar radiation reaches the Earth’s surface, the atmosphere will influence it and different ways. Hence, the strength of different types of EM radiation is severely changed through the course of the atmosphere as it consists of molecular nitrogen, oxygen, water vapour and particles (aerosols) such as dust. The impact of such particles can be massive. In February 2021, great amounts of sand from the Sahara desert were relocated to Europe due to seasonal winds, heavily impacting the signal that passive sensors measure.

Airborne dust particles observed by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA-20 spacecraft on 18th February 2021 (NASA 2021).
ESA’s Sentinel-5P locating the Saharan dust plume moving westward in Sumer 2020 (ESA 2020).

The changes of the radiation can vary with wavelength, condition of the atmosphere and the position of the sun. So how does the atmosphere interact with the radiation? Absorption is caused, for example, by the presence of water vapour in the Earth’s protective layer. Scattering and absorption in the atmosphere cause a transmission loss of the solar radiation before it reaches the Earth’s surface. In parts of the EM spectrum the atmosphere is hardly or not at all transparent, thus these parts are not suitable for certain wavelengths used in RS. Those parts of the spectrum where the atmospheric transmittance is high, which are specifically useful for RS applications and they are called “atmospheric windows”.

Scattering in the atmosphere

Through the course, that the radiation takes to pass the Earth’s atmosphere a variation of physical processes take place that alter the EM waves. Processes of scattering and (partial) absorption define the appearance of the Earth and its surrounding layer.

Why is the sky blue?

Blue Sky caused by Rayleigh scattering

The colour of the sky as we see it is usually a shade of blue. This can be attributed to the location of blue light in the visible part of the EM spectrum. Due to its very short wavelength it is most likely to be scattered from the smallest particles in the atmosphere. Especially in upper layer of the atmosphere this process called ‘Rayleight scattering‘ is common. While longer wavelengths simply pass through the particles, shorter wavelengths exhibit a greater probabilty of being scattered. The molecules involved in the scattering process are usually in the size of 1/10 of a wavelength.

Why are clouds white?

Cloud droplets scatter all visible wavelengths in various directions

When the particle size equals or exceeds the wavelength another process comes into play. The Mie scattering is not strongly depending on the wavelength and results in a white appearance (e.g. around clouds). This is caused by the fact that all visible wavelengths are scattered relatively equally.

Where do rainbows come from?

Rainbows occur due to geometric scattering effects

Nonselective scattering is the third import process happening in the atmosphere and occurs when the particles are significantly larger than the wavelength. In this scenario, mostly water droplets act as lenses that scatter and bend all wavelengths. This effect can results in beautiful rainbows.