The Atmosphere

LayersComposition

Composition of “air” - What’s in it?

Stable Components:

N2 78%

O2 21%

CO2 < 1%

100%

Variable Components:

H2O Vapor – highly variable (0 to 4%)

O3 - photochemical reactions

Layers of the Atmosphere

Troposhpere

• Where we live

• Weather

• 90% of total mass of

atmosphere

Stratosphere

• Contains O3

• Absorbs ________??

Mesosphere

• Coldest layer

• Meteor dust act as cloud nuclei

Thermosphere

• Warmest layer

Ionosphere

• Absorbs cosmic rays, gamma rays, x-

rays, shortest UV

Effect of Atmosphere on EMR

All solar emr passes through space to reach top of

atmosphere, but not all reaches Earth's surface.

Atmosphere scatters, absorbs and reflects a portion of

in-coming solar radiation.

Earth scatters, absorbs and reflects solar radiation that

gets transmitted through the atmosphere.

Finally - atmosphere scatters, absorbs and reflects the

electromagnetic radiation that is reflected off the Earth's

surface back toward the sensor.

Atm. Gases;

– Critical to earth's energy balance through

absorption and emission.

– Determines solar radiation reaching surface

"windows"

– atm. effects are minimal

– allows ground-based measurements of

celestial objects, and satellite-based

measurements of earth's surface/atm.

Signal reaching the sensor may include reflection off

Earth's surface that contains information, but it also

includes in-coming and reflected EMR that has been

scattered by the atmosphere.

This can result in a loss of detail in the resulting

images, making interpretation more difficult.

Challenges of Remote Sensing

4% of in-coming solar radiation is reflected back from

Earth's surface.

5% re-radiated after absorption as thermal IR.

These two components are the focus of most terrestrial

remote sensing.

Challenges of Remote Sensing

Only selected wavelengths are able to penetrate Earth's

atmosphere and be reflected back to sensor.

Thus, only some wavelengths are available for analysis

and some objects of interest may not have unique

spectral signatures within the set of available

wavelengths.

Composition of atmosphere is important in

understanding the role it plays in remote sensing and in

interactions with electromagnetic radiation.

– largely a mixture of gases

– some with fairly constant concentrations

– others are variable in space and time.

– In addition - suspended particles (e.g. aerosol, smoke, ash etc.)

and hydrometeors (e.g. cloud droplets, raindrops, snow, ice

crystals, etc).

About 99% of the mass lies below an altitude of 30km.

Table 1: composition of atmosphere below 100km.

Main gases which absorb radiation.

ultraviolet (UV), visible, infrared (IR) and microwave wavelengths.

main spectral regions ("windows") for which atmospheric absorption is small, are listed at the bottom of the table.