Geog 474
Energy Interactions with the Atmosphere and at the Surface

Solar and Terrestrial Radiation
Most remote sensing instruments are designed to detect solar radiation and terrestrial radiation

Solar radiation

Problem Set (#2)
Terrestrial radiation
Problem Set (#3)
Wavelengths covering most of Earth's energy output are several times longer than those covering most of the solar output.  Therefore, refer to following as:
terrestrial radiation - longwave radiation

solar radiation - shortwave radiation

Radiation-Matter Interactions

Incident radiation - energy impinges upon matter When EMR strikes matter 3 interactions may occurs:

Proportion of energy that is transmitted, reflected or absorbed depends upon:

Transmission - process by which incident radiation passes through matter w/o measurable attenuation (transparent to radiation); cause change in velocity and wavelength but not frequency

Specular reflection - process whereby incident radiation "bounces off" the surface of substance in a single, predictable direction; caused by surfaces smooth relative to wavelengths of incident radiation; no change in velocity or wavelength

Scattering (diffuse reflection) - occurs when incident radiation is dispersed or spread out unpredictably in many different directions; occurs when surfaces rough relative to wavelengths of incident radiation; no change in velocity or wavelength

Absorption - process by which incident radiation is taken in by the medium (e.g., surface, atmospheric particulates, atmospheric layer); medium opaque to incident radiation

Interrelationships between energy interactions expressed as:
   Equation (1)

Problem Set (#7)

Opaque materials transmit no incident radiation

Transparent material have little or no absorption and scattering


clear glass - high transmission, low reflection and absorption

fresh snow - high reflectance, low transmission and absorption

fresh asphalt - high absorption, minimum transmission and reflection

EMR-Atmosphere Interactions

EMR travels through space w/o modification

Diversion and depletions occurs as solar and terrestrial radiation interact with earth's atmosphere

Interference is wavelength selective - meaning at certain wavelengths emr passes freely through atmosphere, whereas restricted at other wavelengths

atmospheric windows (transmision bands) - areas of ems where specific wavelengths pass relatively unimpeded through atmosphere

absorption bands (atmospheric blinds) - areas where specific wavelengths are totally or partially blocked

Objective to study earth's surface - different remote sensing instruments designed to operate w/i windows where cloudless atmosphere will transmit sufficient radiation for detection

Objective to study atmosphere constituents - operate in atmospheric windows and absorption bands

EMR interacts w/ atmosphere in # of ways:
Radiation Balance

Incoming solar radiation = Outgoing longwave radiation

100 = 35 (reflected - albedo) + 65 (terrestrial emitted)

Problem Set #5

Atmospheric Absorption and Transmission

Most significant absorbers of EMR:


Absorption-transmission characteristics of cloud-free atmosphere shows gases responsible for EMR absorption as function of wavelength

16% of shortwave solar radiation absorbed directly by atmospheric gases

2% by clouds

Atmospheric gases - selective absorbers w/ reference to wavelength

absorbed radiation heats the lower atmosphere
beyond 0.6cm , atmospheric gases generally do not impede passage of microwave radiation

Atmospheric Scattering

Scattering process disperses radiation in all directions

Important scattering agents include:

3 types of atmospheric scattering are important in remote sensing
Rayleigh (molecular) scattering
Equation (2)

E.g. UV at 0.3um scattered 16x as readily as red 0.6um

Blue 0.4um scattered about 5x as readily as red
Mie (nonmolecular) scattering
dependent on size distribution and concentration of mie particles
Clear atmosphere is a medium for both Rayleigh and Mie scattering

Nonselective scattering

clouds appear brilliant white - colorless water droplet and ice crystals scatter all wavelengths equally well w/i visible

Skylight and Haze

Clear sky is source of illumination because its gases preferentially scatter shorter wavelengths of sunlight
diffuse radiation (skylight, sky radiation)
EMR - Surface Interactions
Natural and man-made (cultural) features of Earth's surface interact with solar radiation differently

On average, 50% of incident shortwave radiation on TOA reaches and interacts with Earth's surface features

50% incident @ surface = 4% reflected directly + 46% absorbed


- proportion of absorbed shortwave radiation is reradiated or emitted back to atmosphere as longwave terrestrial radiation (5%)

- most heat emitted at wavelengths falling within thermal infrared atmospheric windows; contains information about different temperature properties of Earth's surface features

Albedo - average amount of incident radiation reflected by an object/feature

Equation (3)

Albedo of Earth's - atmosphere system (50% cloud cover) - 30%
- meaning 30% of insolation is reflected, 70% absorbed

- earth made visible from space only by its albedo

Earth's brightest features - clouds, snow and ice surfaces; darkest - water bodies

Percent reflected energy from Earth's surface objects/features

Albedo also helps explain how warm an object becomes when exposed to sunlight

-objects w/ high albedo are good reflectors but poor absorbers (dictates slow and small temperature increases)

- objects w/ low albedo are poor reflectors but good absorbers (dictates rapid and large jumps in temperature when exposed to sunlight)

E.g., walking barefoot on black asphalt versus grass

wearing light or dark clothing on summer day
Spectral Signatures
Every natural and synthetic object reflects and emits emr over a range of wavelengths in its own characteristic way according to is chemical composition and physical state

spectral signatures - distinctive reflectance and emittance properties of objects/features and their conditions

- w/i some limited wavelength region, particular object/feature or condition exhibit a diagnostic spectral response pattern that differs from other objects

- remote sensing depends upon operation in wavelength regions where detectable differences in reflected and emitted radiation occur; features and their different conditions show enough variation to allow for individual identification

Typical spectral signature of vegetation, soil and water

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Last revised on September 23, 1999 by Tracy DeLiberty.