![Canada Centre for Remote Sensing Canada Centre for Remote Sensing](/web/20061103052156im_/http://www.ccrs.nrcan.gc.ca/esst_images/ccrs_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Canada Centre for Remote Sensing > Fundamentals of remote sensing
Introduction Interactions with the Atmosphere
Before radiation used for remote sensing reaches the Earth's
surface it has to travel through some distance of the Earth's atmosphere. Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are
caused by the mechanisms of scattering and absorption.
Scattering 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. There are three (3) types of
scattering which take place.
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. At sunrise and sunset the light has to travel
farther through the atmosphere than at midday and the scattering of the
shorter wavelengths is more complete; this leaves a greater proportion of
the longer wavelengths to penetrate the atmosphere.
Mie scattering occurs when the particles are just about the same size
as the wavelength of the radiation. Dust, pollen, smoke and water vapour
are common causes of Mie scattering which tends to affect longer
wavelengths than those affected by Rayleigh scattering. Mie scattering
occurs mostly in the lower portions of the atmosphere where larger
particles are more abundant, and dominates when cloud conditions are
overcast.
The final scattering mechanism of importance is called nonselective scattering. This occurs when the
particles are much larger than the wavelength of the radiation. Water
droplets and large dust particles can cause this type of scattering.
Nonselective scattering gets its name from the fact that all wavelengths
are scattered about equally. This type of scattering causes fog and clouds
to appear white to our eyes because blue, green, and red light are all
scattered in approximately equal quantities (blue+green+red light = white
light).
Absorption is the other main
mechanism at work when electromagnetic radiation interacts with the
atmosphere. In contrast to scattering, this phenomenon causes molecules in
the atmosphere to absorb energy at various wavelengths. Ozone, carbon
dioxide, and water vapour are the three main atmospheric constituents
which absorb radiation.
Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight.
You may have heard carbon dioxide referred to as a greenhouse gas. This is because it tends to absorb radiation strongly in the far infrared portion of the spectrum - that area associated with thermal heating - which serves to trap this heat inside the atmosphere. Water vapour in the atmosphere absorbs much of the incoming longwave infrared and shortwave microwave radiation (between 22µm and 1m). The presence of water vapour in the lower atmosphere varies greatly from location to location and at different times of the year. For example, the air mass above a desert would have very little water vapour to absorb energy, while the tropics would have high concentrations of water vapour (i.e. high humidity).
Because these gases absorb electromagnetic energy in very specific regions of the spectrum, they influence where (in the spectrum) we can "look" for remote sensing purposes. Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called
atmospheric windows. By comparing the characteristics of the two most common energy/radiation sources (the sun and the earth) with the atmospheric windows available to us, we can define those wavelengths that we can use most effectively for remote sensing. The visible portion of the spectrum, to which our eyes are most sensitive, corresponds to both an atmospheric window and the peak energy level of the sun. Note also that heat energy emitted by the Earth corresponds to a window around 10 µm in the thermal IR portion of the spectrum, while the large window at wavelengths beyond 1 mm is associated with the microwave region.
Now that we understand how electromagnetic energy makes its journey from its source to the surface (and it is a difficult journey, as you can see) we will next examine what happens to that radiation when it does arrive at the Earth's surface.
"...sorry, no pot of gold at the end of this rainbow..."
![Water droplets](/web/20061103052156im_/http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/images/rain.gif)
...water droplets act as tiny, individual prisms. When sunlight passes through them, the constituent wavelengths are bent in
varying amounts according to wavelength. Individual colours in the sunlight are made visible and a rainbow is the result, with shorter wavelengths (violet, blue) in the inner part of the arc, and longer wavelengths (orange, red) along the outer arc.
...if scattering of radiation in the atmosphere did not take place, then shadows would appear as jet black instead of being various degrees of darkness. Scattering causes the atmosphere to have its own brightness (from the light scattered by particles in the path of sunlight) which helps to illuminate the objects in the shadows.
1. Most remote sensing systems avoid detecting and recording wavelengths in the ultraviolet and blue portions of the spectrum. Explain why this would be the case.
The answer is ...
2. What do you think would be some of the best atmospheric conditions for remote sensing in the visible portion of the spectrum?
The answer is ...
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1. Detecting and recording the ultraviolet and blue wavelengths of radiation is difficult
because of scattering and absorption in the atmosphere. Ozone gas in the upper atmosphere
absorbs most of the ultraviolet radiation of wavelengths shorter than about 0.25 µm.
This is actually a positive thing for us and most other living things, because of the harmful
nature of ultraviolet radiation below these wavelengths. Rayleigh scattering, which affects
the shorter wavelengths more severely than longer wavelengths, causes the remaining UV radiation
and the shorter visible wavelengths (i.e. blue) to be scattered much more than longer wavelengths,
so that very little of this energy is able to reach and interact with the Earth's surface. In fact,
blue light is scattered about 4 times as much as red light, while UV light is scattered 16 times as much as red light!
2. Around noon on a sunny, dry day with no clouds and no pollution would be very good for remote sensing in the visible wavelengths. At noon the sun would be at its most directly overhead point, which would reduce the distance the radiation has to travel and therefore the effects of scattering, to a minimum. Cloud-free conditions would ensure that there will be uniform illumination and that there will be no shadows from clouds. Dry, pollutant-free conditions would minimize the scattering and absorption that would take place due to water droplets and other particles in the atmosphere.
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