Energy interactions in the atmosphere

- The composition of the atmosphere influences both the incoming solar radiation and the outgoing terrestrial radiation

- The radiance (the energy reflected by the surface) received at a satellite is a result of electromagnetic radiation that undergoes several processes which are wavelength dependent

Energy interactions in the atmosphere

Energy interactions in the atmosphere

• Scattering• Refraction • Absorption

Scattering- The redirection of EM energy by particles

suspended in the atmosphere or large molecules of atmospheric gases

- Scatter differs from reflection in that the direction associated with scattering is unpredictable, whereas the direction of reflection is predictable

- Type of scattering is a function of:Type of scattering is a function of:- the the wavelengthwavelength of the incident radiant energy, of the incident radiant energy,

andand– the the sizesize of the gas molecule, dust particle, and/or of the gas molecule, dust particle, and/or

water vapor droplet encountered.water vapor droplet encountered.

Scattering

• Types of scattering

– Rayleigh scattering– Mie scattering– Nonselective scattering

Atmospheric Layers and Constituents

Major subdivisions of the atmosphere and the types of molecules and aerosols found in each layer.

Atmospheric scattering

Rayleigh scattering- It occurs when atmospheric particles' diameters are

much smaller than the wavelength of the radiation d<<

- It is common in the high atmosphere (3-8 km)

- Rayleigh scattering is proportional to the inverse of the wavelength raised to the fourth power: shorter wavelengths are scattered more than longer wavelengths

- At daytime, the sun rays travel the shortest distance through the atmosphere- Blue sky

- At sunrise and sunset, the sun travel a longer distance through the Earth’s atmosphere before they reach the surface- The sky appears orange or red.

- Tends to dominate under most atmospheric conditions

Rayleigh scattering

Mie scattering• Particles' diameters are equivalent

to the wavelength d ≈ - Water vapor and dust are major

causes of Mie scattering- Mie scattering tends to influence

longer wavelengths. - It is common in lower atmosphere

where large particles are more abundant, and dominates under overcast could conditions.

Nonselective scattering

• Particles are much larger than the wavelength d>>

- Water droplets (5-100 μm) and larger dust particles

- Non-selective scattering is independent of wavelength

- All wavelength are scattered equally (A could appears white)

- It scatters all visible and near to mid IR wavelengths.

Effects of scattering

• It causes haze in remotely sensed images• It decreases the spatial detail on the images• It also decreases the contrast of the images

Absorption• Absorption is the process by which radiant energy is absorbed and converted

into other forms of energy

• The atmosphere prevents, or strongly attenuates, transmission of radiation through the atmosphere

• An absorption band is a range of wavelengths (or frequencies) in the electromagnetic spectrum within which radiant energy is absorbed by substances such as water (H2O), carbon dioxide (CO2), oxygen (O2), ozone (O3), and nitrous oxide (N2O).

• Three gases:

- Ozone (O3): absorbs ultraviolet radiation high in atmosphere

- Carbon-dioxide (CO2): absorbs mid and far infrared (13-17.5 μm) in lower atmosphere

- Water vapor (H2O): absorbs mid-far infrared (5.5-7.0, >27 μm) in lower atmosphere

Absorption

Transmission, reflection, scattering, and absorption

Atmospheric windows (transmission bands )

-The wavelength ranges in which the atmosphere is particularly transmissive

Atmospheric windows

Atmospheric Windows • The windows:

  UV & visible:  0.30-0.75 m   Near infrared:  0.77-0.91 m   Mid infrared:   1.55-1.75m,  2.05-2.4 m   Far infrared:   3.50-4.10 m,  8.00- 9.20 m, 

10.2-12.4 m   Microwave:      7.50-11.5 mm,  20.0+mm

• X-Rays and UV are very strongly absorbed and Gamma Rays and IR are somewhat less strongly absorbed.

• The atmospheric windows are important for RS sensor design

Energy Interactions with Earth Surface Features

• All EM energy reaches earth's surface must be reflected, absorbed, or transmitted

• The proportion of each depends on:– the spectral reflectance properties of the surface

materials– the surface smoothness relative to the radiation

wavelength– wavelength– angle of illumination

Energy Interactions with Earth Surface Features

Energy Interactions with Earth Surface Features

- Light ray is redirected as it strikes a nontransparent surface

- Albedo - Spectral reflectance R (): the average amount of incident radiation reflected by an object at some wavelength interval

R () = ER () / EI () x 100 Where ER() = reflected radiant energy EI () = incident radiant energy

Specular versus diffuse reflectance- Specular reflectors are flat surfaces that manifest

mirrolike reflections. The angle of reflection equals the angle of incident.

- Diffuse (or Lambertian) reflectors are rough surfaces that reflect uniformly in all the directions- If the surface is rough, the reflected rays go in many

directions, depending on the orientation of the smaller reflecting surfaces

- Diffuse contain spectral information on the color of the reflecting surface, whereas specular reflections do not.

- In remote sensing we are often interested in measuring the diffuse reflectance of objects.

Specular versus diffuse reflectance

Transmission

• Radiation passes through a substance without significant attenuation

• Transmittance (t):

                  transmitted radiation            t =  ---------------------------                    incident radiation

Absorption

Reflection + Transmission + Absorption = 100%

Emission

Spectral Characteristics of Features

Identification of Surface Materials Based on Spectral Reflectance

Spectral reflectance curves for vegetation, soil, and water

Vegetation

• Contains water, cellulose (tissues and fibres), lignin (non-carbohydrate constituent of wood), nitrogen, chlorophyll (“green” pigments) and anthocyanin (water-soluble pigments).

• Depending on how ‘active’ (i.e. kinds of chlorophyll) a green vegetation is, the combination of transmittance, absorbance and reflectance vary in different bands of the spectrum.

Physiological Factors• Leaf structure• Reflectance, transmittance, and absorptance spectra• Leaf maturation• Mesophyll arrangements (internal structural differences)

Spectra of vegetation

• Chlorophyll absorbs blue and red, reflects green • Vegetation has a high reflection and transmission at

NIR wavelength range• Reflection or absorption at MIR range, the water

absorption bands

Chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths. (This is why healthy vegetation appears green.)

The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths.

Measuring and monitoring the near-IR reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation may be.

Absorption is dominant process in visibleScattering is dominant process in near infraredWater absorption is increasingly important with increasing wavelength in the infrared.

Spectra of vegetation

Spectra of soil• What are the important properties of a soil in

an RS image -Soil texture (proportion of sand/silt/clay) -Soil moisture content -Organic matter content -Mineral contents, including iron-oxide and

carbonates -Surface roughness

Dry soil spectrum

20

60

100

Perc

ent R

efle

ctan

ce

0.5 0.7 1.1 1.30

Wavelength ( m)

80

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

Silt

Sand

10

30

50

70

90

•Coarse soil (dry) has relatively high reflectance•Increasing reflectance with increasing wavelength through the visible, near and mid infrared portions of the spectrum

Soil moisture and texture

• Soil moisture decreases reflectance• Clays hold more water more ‘tightly’ than

sand.• Thus, clay spectra display more prominent

water absorption bands than sand spectra

Factors: Spectral Reflectance

Bio Physical Controls of Soil Reflectance

Moisture content. The near-surface moisture content of soil is the most important reflectance factor due to its dynamic nature and large overall impact on soil reflectance. As shown in Figure 2.25, there is an inverse relationship between edaphic moisture content and soil spectral reflectance. Note the persistence of the water absorption bands (1.45 and l.92 micrometers) even in the air-dried sample. This results from water films being held tightly onto the relatively large proportion of very fine silt and clay particles in this particular soil. Also notable is the strong hydroxyl absorption band at 2200 nm which many clay-rich soils will exhibit. Comparing soils from different natural drainage classes, the better drained soils are more reflective (Figure 2.26).

Soil moisture and texture

20

60Pe

rcen

t Ref

lect

ance

0.5 0.7 1.1 1.30

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

22 – 32%

10

30

50

Sand

20

60

0.5 0.7 1.1 1.30

Wavelength (m)

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

35 – 40% 10

30

50 2 – 6%

0 – 4% moisture content

5 – 12%

Clay

a.

b.

Perc

ent R

efle

ctan

ce

SandSandSand

ClayClayClay

Organic Matter Content. Mineral soils, as distinct from organic soils, are dominantly mineral material with less than 20 percent organic carbon by weight. As shown in Figure 2.27, for mineral soils, as the organic matter content increases, soil reflectance decreases. As shown in Figure 2.28, some researchers have demonstrated a workable relationship between remotely sensed soil reflectance and organic carbon content.

The reflectance of organic soils, on the other hand, is controlled primarily by state of decomposition of the plant material (Figure 2.29). Peat (fibric material) is composed of plant remains which have under gone only minimal decomposition. This type of organic soil is usually dark brown to reddish-brown. The highly decomposed sapric material (muck) is generally black. Organic soils of intermediate decomposition are classed as hemic soils.

Particle Size Distribution. The larger-diameter particle sizes (e.g. medium sand, coarse sand, etc.) exhibit pronounced interstitiel voids. This increased surficial micro-roughness, compared to the fine particle sizes, presents many more “light traps” to any irradiance. Assuming the other soil factors are equal, the finer particle sizes will exhibit greater soil reflectances (Figure 2.30). With moisture content equilibrated, and the organic matter content naturally similar, the multi-sample data presented in Figure 2.31 illustrate the relationship between soil texture and spectral reflectance.

Iron Oxide

Recall that iron oxide causes a charge transfer absorption in the UV, blue and green wavelengths, and a crystal field absorption in the NIR (850 to 900 nm). Also, scattering in the red is higher than soils without iron oxide, leading to a red color.

Surface Roughness

• Smooth surface appears black. • Smooth soil surfaces tend to be clayey or silty,

often are moist and may contain strong absorbers such as organic content and iron oxide.

• Rough surface scatters EMR and thus appears bright.

Iron Oxide Content. Iron oxide (Fe2 O3) is one of the primary causes of the red colors in many soils. Iron oxide content and organic matter content are the two most important soil properties affecting the spectral reflectence characteristics of eroded soils, particularly in the 500 to 1200 nm region (Weismeiier et al., 1984). The data presented in Figure 2.32 illustrate the relationship between iron oxide content and soil spectral reflectance. Chemically removing the extractable iron oxides from a soil sample results in increased reflectance especially at wave-lengths less than 1100 nm. A broad absorption feature, centered at 900 nm and attributed to iron oxide, is obvious in this graph.

BioPhysical Controls of Water Reflectance

Energy Partitioning. There are three types of possible reflectance from a water body-surface (specular) reflectance, bottom reflectance, and volume reflectance (Figure 2.33). Of these, only volume reflectancec contains information relating to water quality. For deep (> 2m), clear water bodies, volume reflectance is very low (6-8 percent) and is confined to the visible wavelengths (Figure 2.34). Transmittance in these cases is very high especially in the blue-green part of the spectrum, but diminishes rapidly in the near-infrared wavelength and Absorptance, on the other hand, is notably low in the shorter visible wavelengths, but increases abruptly in the near-infrared sector. Shallow water (<2m deep) transmits significant amounts of NIR radiation (Figure 2.35). As depth increases the peak transmittance wavelength for clear water decreases and finally stabilize at about 480nm.

Volume Reflectance. Clear water reflects very little solar irradiance, but turbid water is capable of reflecting significant amounts of sunlight (Figure 2.36). It is notable that the peak-reflectance point shifts to longer wavelengths as turbidity increases (Figure 2.37). As shown in Figures 2.38 and 2.39, as the chlorophyll content of a water body increases (resulting from an increase in algae, phytoplankton, etc.) its blue-light reflectance decreases while its green light reflectence increases. The “hingepoint” in this relationship, over four orders of magnitude of concentrate on differences, remains relatively stable at 510-520 nm. Also noteworthy, is the asymptotic reflectance change in the blue wavelengths as chlorophyll concentration increases compared to the reflectance differences in the longer wavelengths.