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A7med
06-06-2005, 01:29 PM
Signs of Allah in clouds

Allah knows that we need water. As a result, He created a system in the sky that produces water even as we talk, read, sleep, work and do other activities. This system is visible to us through the clouds.

Clouds are amazing creations of Allah. At least once in our lifetime we look at an airplane and say "Wow, how can that big thing fly in the sky?" But did we ever look at the clouds and wonder how they remain in the sky? Let's compare the clouds and the airplanes. The airplanes have specific shapes and designs. If one of their wings break down, then the plane crashes. On the other hand, these clouds have no specific shapes or sizes. Yet, they can fly in the sky without a problem. In addition, the planes have to come down to the earth for re-fueling every once in a while. But the clouds can remain in the sky for months without coming down.

The airplanes are very heavy objects, however, the clouds are even heavier. An average cumulonimbus cloud contain enough water to fill 10,000 swimming pools! Now, that's heavy.

Moreover, clouds provide us with shade during sunny days and let's us see beautiful scenaries even in the sky. Did you ever thank Allah for these blessings?
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Nablus
06-06-2005, 01:53 PM
Moreover, clouds provide us with shade during sunny days and let's us see beautiful scenaries even in the sky. Did you ever thank Allah for these blessings?
__________________
thank Allah for these unountable blessings


May Allah guide us to the good deeds
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أحمد
06-07-2005, 03:19 PM
:sl:

:D Clouds also absorb microwave radiation and harmful rays in and out of the Earth . . . http://www.udel.edu/Geography/DeLibe..._interact.html

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
  • emr emitted from sun which passes through the atmosphere and is reflected in varying degrees by Earth's surface and atmosphere
  • detectable only during daylight

  • Sun's visible surface (photosphere) has temperature - 6000K
  • energy radiated from gamma to radio waves
  • 99% of sun's radiation fall between 0.2 - 5.6um; 80% - 0.4 - 1.5um (visible and reflected infrared, atmosphere quite transparent to incoming solar radiation
  • maximum radiation occurs 0.48um (visible)
Problem Set (#2)
  • about 1/2 of solar radiation passes through the atmosphere and absorbed in varying degrees by surface
Terrestrial radiation
  • energy emitted from the Earth and atmosphere
  • detectable both day and night

  • Earth's ambient temperature - 300K
  • Earth radiates 160,000 times less than the sun
  • essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4-25um
  • maximum emission occurs at 9.7um
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
  • strongest source of incident radiation for earth is sun
  • incoming solar radiation called Insolation
  • full moon is 2nd strongest source
When EMR strikes matter 3 interactions may occurs:
  • transmission
  • reflection
  • absorption


Proportion of energy that is transmitted, reflected or absorbed depends upon:
  • composition and physical properties of medium
  • wavelength or frequency of incident radiation
  • angle at which incident radiation strikes a surface
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
E.g.,

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:

  • absorbed and reradiated at longer wavelengths (causes air temperature to rise)

    reflected or scattered w/o change to either its velocity or wavelength transmitted in straight-line path directly through the atmosphere
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:
  • ozone carbon dioxide water vapor oxygen nitrogen



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
  • Gamma and X-ray - completely absorbed in the upper atmosphere by Oxygen and Nitrogen

    Ultraviolet (<0.2um) - absorbed by molecules of oxygen (O and O2 combine form ozone); ozone absorbs UV w/ wavelengths -0.2-0.3um in stratosphere

    0.9-2.7um - water vapor and carbon dioxide absorb in narrow bands

    thermal infrared
    • strong absorption by water vapor between 5-8um and 20um-1,000um (1cm)
    • carbon dioxide absorbs 14-20um
    • ozone 9-10um
absorbed radiation heats the lower atmosphere
  • microwave region - 3 relatively narrow absorption bands occur between 0.1 - 0.6cm (oxygen and water vapor)
beyond 0.6cm , atmospheric gases generally do not impede passage of microwave radiation
Summary
  • absorption of atmospheric gases has maximum influence in wavelengths <0.3um and minimum impact on wavelengths greater than 0.6cm important atmospheric windows exploited in remote sensing -

  • 0.3 - 1.1um UV, visible, near infrared

    1.5 - 1.8um Mid infrared

    2.0 - 2.4um Mid infrared

    3.0 - 5.0um Mid infrared

    8.0 - 14.0um Thermal Infrared

    (below ozone layer)

    10.5 - 12.5 Thermal Infrared
    (above ozone layer)

    • >0.6cm Microwave
    Atmospheric windows become less transparent when air is moist (high humidity)

    Clouds absorb most of longwave radiation emitted from Earth's surface Microwave radiation (>0.9cm) capable of penetrating clouds


Atmospheric Scattering
Scattering process disperses radiation in all directions

Important scattering agents include:

  • gaseous molecules
  • suspended particulates (aerosols)
  • clouds
3 types of atmospheric scattering are important in remote sensing
Rayleigh (molecular) scattering
  • - primarily caused by oxygen and nitrogen molecules (diameters at least 0.1 times smaller than affected wavelengths)

    - most influential at altitudes above 4.5km - amount of scattering inversely proportional to fourth power of wavelength
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
  • - blue sky - preferential scattering of blue wavelengths, clear sky appears blue in daylight; blue wavelengths reach our eyes - brilliant colors of sunrise/sunset - solar beam starts out as white light passes though long atmosphere path causing shorter wavelengths of sunlight to be scattered away leaving only longer red wavelengths that reach our eyes
Mie (nonmolecular) scattering
  • - occurs when there are sufficient particles in atmosphere w/ mean diameter 0.1 to 10 times larger than wavelength under consideration

    - caused by water vapor, tiny particles of smoke, dust, volcanic ejecta, salt crystals released from evaporation of sea spray

    - most pronounced in lower 4.5km of atmosphere - wavelength dependence varies 1/wavelength
dependent on size distribution and concentration of mie particles
Clear atmosphere is a medium for both Rayleigh and Mie scattering

Nonselective scattering
  • - occurs when lower atmosphere contains sufficient # of suspended aerosols (diameters 10 times larger than wavelengths under consideration)

    - important agents include larger equivalents of Mie particles, water droplets and ice crystals that compose clouds and fogs - scattering is independent of wavelength (near UV, visible, near infrared)
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

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
:w:
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A7med
06-07-2005, 04:02 PM
WOW!!!!!!!!!! bro NICE STUFF!!!!!!!!!!!!
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Ibn Syed
06-07-2005, 04:27 PM
:sl:
Nice posts bros.
:w:
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al_yaziya
11-18-2006, 11:49 AM
Thanks.
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