INTRODUCTION:The Concept of Remote Sensing

If you have heard the term "remote sensing" before you may have asked, "what does it mean?" It's a rather simple, familiar activity that we all do as a matter of daily life, but

that gets complicated when we increase the scale at which we observe. As you view the screen of your computer monitor, you are actively engaged in remote sensing.

A physical quantity (light) emanates from that screen, whose imaging electronics provides a source of radiation. The radiated light passes over a distance, and thus is "remote" to some extent, until it encounters and is captured by a sensor (your eyes). Each eye sends a signal to a processor (your brain) which records the data and interprets this into information. Several of the human senses gather their awareness of the external world almost entirely by perceiving a variety of signals, either emitted or reflected, actively or passively, from objects that transmit this information in waves or pulses. Thus, one hears disturbances in the atmosphere carried as sound waves, experiences sensations such as heat (either through direct contact or as radiant energy), reacts to chemical signals from food through taste and smell, is cognizant of certain material properties such as roughness through touch (not remote), and recognizes shapes, colors, and relative positions of exterior objects and classes of materials by means of seeing visible light issuing from them. In the previous sentence, all sensations that are not received through direct contact are remotely sensed.

Remote Sensing in the most generally accepted meaning refers to instrument-based techniques employed in the acquisition and measurement of spatially organized (most commonly, geographically distributed) data/information on some property(ies) (spectral; spatial; physical) of an array of target points (pixels) within the sensed scene that correspond to features, objects, and materials, doing this by applying one or more recording devices not in physical, intimate contact with the item(s) under surveillance (thus at a finite distance from the observed target, in which the spatial arrangement is preserved); techniques involve amassing knowledge pertinent to the sensed scene (target) by utilizing electromagnetic radiation, force fields, or acoustic energy sensed by recording cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, sound detectors, seismographs, magnetometers, gravimeters, scintillometers, and other instruments.

This is a rather lengthy and all-inclusive definition. Perhaps two more simplified definitions are in order: The first, more general, includes in the term this idea: Remote Sensing involves gathering data and information about the physical "world" by detecting and measuring signals composed of radiation, particles, and fields emanating from objects located beyond the immediate vicinity of the sensor device(s). The second is more restricted but is pertinent definition is: In its common or normal usage (by tacit implication), Remote Sensing is a technology for sampling electromagnetic radiation to acquire and interpret non-contiguous geospatial data from which to extract information about features, objects, and classes on the Earth's land surface, oceans, and atmosphere (and, where applicable, on the exteriors of other bodies in the solar system, or, in the broadest framework, celestial bodies such as stars and galaxies).

Any beam of photons from some source passing through medium 1 (usually air) that impinges upon an object or target (medium 2) will experience one or more reactions that are summarized in this diagram:

The primary source of energy that illuminates natural targets is the Sun. Solar irradiation (also called insolation) arrives at Earth at wavelengths which are determined by the photospheric temperature of the sun (peaking near 5600 °C). The main wavelength interval is between 200 and 3400 nm (0.2 and 3.4 µm), with the maximum power input close to 480 nm (0.48 µm), which is in the visible green region. As solar rays arrive at the Earth, the atmosphere absorbs or backscatters a fraction of them and transmits the remainder.

Upon striking the land and ocean surface (and objects thereon), and atmospheric targets, such as air, moisture, and clouds, the incoming radiation (irradiance) partitions into three modes of energy-interaction response:(1) Transmittance (τ) - some fraction (up to 100%) of the radiation penetrates into certain surface materials such as water and if the material is transparent and thin in one dimension, normally passes through, generally with some diminution.(2) Absorptance (α) - some radiation is absorbed through electron or molecular reactions within the medium ; a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target;(3) Reflectance (ρ) - some radiation (commonly 100%) reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays.

There are two general types of reflecting surfaces that interact with EMR: specular (smooth) and diffuse (rough). 

Atmospheric Effects

What happens to solar radiation as it travels through the atmosphere?

Energy Essentials • Incoming solar rad’n (insolation) is the primary energy

source for the atmosphere

• land, oceans, clouds, atmospheric gases and dust intercept insolation

• Energy Pathways and Principles – Input: shortwave energy from the Sun– Output: longwave energy from Earth’s surface

• atmosphere and Earth’s surface heated by solar energy: unevenly distributed by latitude and season

Atmospheric Influences on Insolation

1. Absorption2. Reflection and Scattering3. Transmission

Absorption

• absorption – transfers energy from radiation to absorber, absorber warms

• gases, particulate matter, droplets absorb energy

• radiation absorbed function of wavelength (not equal)

• e.g. UV vs. visible light

• near infrared rad’n absorbed by CO2 and H20v

Reflection and Scattering• reflection – redirection of energy w/o

absorption • all objects reflect visible light effectiveness

varies• albedo – % of visible light reflected

• There are two types of reflection (solid surface:– Specular: light is reflected with equal

intensity (e.g. mirror)– Diffuse reflection OR scattering: light is

reflected in multiple directions, weakly (e.g. snow)

• Rad’n reaching Earth’s surface can be either:– Diffuse rad’n (scattered)– Direct rad’n (unscattered)

• IMPORTANT: – Scattered energy is re-directed

NOT absorbed– size of scattering agent relative to

wavelength determines

type of scattering

3 Types of Scattering: 1. Raleigh

2. Mie

3. Non-Selective

A discussion of each type follows…

1) Rayleigh scattering• involves gases, smaller than insolation

wavelength• scatters light in all directions • most effective at short wavelengths

(violet, blue)…hence, blue sky• explains reddish-orange sunsets when

light travels through thick slice of atmosphere

2. Mie scattering– involves aerosols, larger than gas molecules– forward scatter– equally effective across visible spectrum– explains hazy, gray days– accentuates sunset/rise

(e.g., in polluted areas)

3) Non-selective scattering– water droplets in clouds (larger than PM, gas

molecules)– Act like lenses; scatter all wavelengths equally– Why clouds appear grey or white– Explains rainbows

when viewing rain

in the distance (each

wavelength bent a

different amount)

Transmission

• percentage of energy passing through atmosphere and reaching surface

• Amount reaching Earth surface is function of atmospheric absorption, scatter, and reflection

• Clear v. hazy, cloudy days

Fate of Solar Radiation

what happens to it ???

• annual variation in insolation is ~ 7% (remember perihelion/aphelion

• the insolation reaching top atmosphere can be• transmitted• absorbed (atmosphere and surface)• scattered/reflected back to space

• assume 100 units (100%) of insolation reach top of atmosphere

• Now What?

Earth’s Solar (“Shortwave”) Radiation Balance:

100% sunlight in at top of atmosphere

19 + 45 + 25 + 6 + 5 = 100%

Q: Surface absorbs 45% - why doesn’t it become extremely hot?....

Answer – Other types of energy transfer also occur:

1. Longwave Radiation Transfer (mainly between earth & atmos.) • Sun heats earth surface, earth emits radiation (I=σT4)• Called “Longwave” because much longer wavelength than

radiation emitted by sun due to cooler temp of earth

2. “Sensible” Heat Transfer (due to temperature gradients)

3. “Latent” Heat Transfer (evaporation, melting)

A discussion of each follows…

1. Longwave Radiation Balance

So net surface loss due to longwave = 16%

But there was an excess of 45% of solar radiation. Thus, we still have 45% -16% = 29% excess radiation at the surface…how do we get rid of it?

Answer: “Sensible” and “Latent” heat transfers.

1. Conduction– This is how excess heat in ground is

transferred to the atmosphere via an extremely thin layer of air in contact with the surface

2. Convection– Once the heat is transferred from the

surface to the air via conduction, convection takes over from here via “sensible” and “latent” heat transfers

First, recall 2 other methods of energy transfer in addition to

radiation:

Free Convection(just like boiling water)

Forced Convection(due to wind)

Sensible Heat

• Heat energy which is readily detected

• Magnitude is related to an object’s specific heat– The amount of energy needed to change the temperature of

an object a particular amount in J/kg/K

• Related to mass– Higher mass requires more energy for heating

• Sensible heat transfer occurs from warmer to cooler areas (i.e., from ground upward)

• Globally, about 8 units of energy are transferred to the atmosphere as sensible heat

Latent Heat

• Energy required to induce changes of state in a substance

• In atmospheric processes, invariably involves water

• When water is present, latent heat of evaporation redirects some energy which would be used for sensible heat– Wet environments are cooler relative to their insolation amounts

• Latent heat of evaporation is stored in water vapor– Released as latent heat of condensation when that change of

state is induced

• Latent heat transfer occurs from regions of wetter-to-drier

• Globally, 21 units of energy are transferred to the atmosphere as latent heat

Understanding Latent heat:

• phase change requires +/- of energy

• latent heat stored in H-bonds

• latent heat of evaporation• latent of condensation

• Energy absorbed S L Vreleased V L S

• Example - sweating

Latent Heat

Summary : Total Energy Balance

SW + LW +SH +LH = 0 for surface, atmos, top-of-atmos

Atmospheric Window

• gases in atmosphere not effective at absorbing radiation between 8 – 11 μm

Radiative properties of Atmospheric gases:Terrestrial RadiationSolar Radiation

UVVIS

IR

Radiative properties of gases continued

• When all gases are combined:– Atmos. fairly transparent to VIS (Shortwave)– Atmos. fairly opaque to IR (Longwave)

• Exception atmos. window from ~8-12 μm

• Increase/decrease concentration and changes capacity radiation a gas can absorb (kind of like using a thicker blanket makes you warmer)

• The wildcard: clouds! These absorb just about all longwave, including in atmos. window– Will there be more clouds in a warming climate?

Net Radiation and Temperature

• Earth’s radiation balance is a function of an incoming and outgoing radiation equilibrium (SW + LW = Net)

• If parameters were changed, a new equilibrium would be achieved

• Balances occur on an annual global scale and diurnally over local spatial scales

Daily/Seasonal Radiation Patterns

• insolation peak vs. temperature• daily lag• seasonal lag• Lag is function of type of

surface, wetness, wind, etc• Temperature increases when input > output• Temperature decreases when input < output

• tropic-to-tropic – energy surplus• poles – energy deficits• ~ 38o N/S – balance

• imbalance of net radiation at surface Equator/Tropics vs. high latitudes

• drives global circulation• agents: wind, ocean currents,

weather systems

Latitudinal Variations in Net Radiation

Greenhouse Effect

• relates to the trapping of terrestrial rad’n by gases in the atmosphere• major GH gases: CO2, H20(v), CH4 • imprecise analogy• the atmosphere/greenhouses are:

• transparent to insolation (incoming)• opaque to longwave (outgoing)

• greenhouses reduce the loss of energy by limiting convection (atm does not)• increase in GHG, increases counter-rad’n

Concepts• Fate of solar radiation (absorption, scattering,

transmission

• Earth’s energy balance (SW + LW + SH + LH)

• General radiative properties of gases and greenhouse effect