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Lecture 2: Properties of Radiation Chapter 2 & 3 Petty Properties of Radiation • What is radiation? • How it behaves at the most fundamental physical level? • What conventions are used to classify it according to wavelength and other properties? • How do we define the characteristics (e.g. intensity) that appear in quantitative descriptions of radiation and its interaction with the atmosphere Properties of Radiation • The Nature of Electromagnetic Radiation - Electric and Magnetic fields (detectable at some distance from their source) F1=F2=Kc*q1q2 r2 Properties of Radiation • The Nature of Electromagnetic Radiation - A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, the basis of operation for electrical generators, induction motors, and transformers). - Similarly, a changing electric field generates a magnetic field. - Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity—the electromagnetic field Properties of Radiation • The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths. • Electromagnetic energy passes through space at the speed of light in the form of sinusoidal waves. The wavelength is the distance from wavecrest to wavecrest (see the figure below) Properties of Radiation Electromagnetic Waves -EM wave propagate as rays will spread the wave’s energy over a larger area and weaken as it gets further away. - EM waves follow principle of superposition. - EM waves are “transverse” waves. http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html WAVE NATURE OF LIGHT l Wavelength Blue: l = 400 nm Light is an electromagnetic wave. Different wavelengths in the visible spectrum are seen by the eye as different colors. Red: l = 700 nm Laser-Professionals.com Properties of Radiation • Electromagnetic Waves Basic concepts and definitions Electromagnetic radiation: * Energy propagated in the form of an advancing electric and magnetic field disturbance. * Travels in wave form at the speed of light (c). * Wavelength ( l ) is the physical distance between adjacent maxima or minima in the electric or magnetic field. unit: m * Wave number ( k ) is the number of wavelengths in a unit distance, i.e., k 1/ l unit: 1/cm * Frequency ( ) is the number of successive maxima or minima passing a fixed point in a unit of time, c / l unit: cycle-per-second (cps) or 1/s Frequency and wavelength Speed of light Frequency (Hz) v= c l Wavelength 1 hertz (Hz) = one cycle per second c = 3.0 x 108 ms-1 Weather Radar, 3GHz wavelength?? Frequency Frequency Decomposition What if electromagnetic disturbance is not a steady oscillating signal? - Frequency Decomposition • Eq. (2.2): any EM fluctuation can be thought of as a composite of a number of different “pure” periodic fluctuation Broadband vs. Monochromatic Broadband vs. Monochromatic ELECTROMAGNETIC SPECTRUM Blue Green Yellow Red Visible Gamma Ray X-ray Ultraviolet Short Wavelength Infrared Radio Microwaves Long Wavelength Lasers operate in the ultraviolet, visible, and infrared. Laser-Professionals.com Radio Major Spectral Bands --Visible Band Relevant to remote sensing • As a proportion of total solar irradiance: • Total energy from 0 – 0.75μm 54% – all energy up to infra-red • Total energy from 0.39μm – 0.75μm 43% – visible light only • Total energy from 0 – 4μm 99% – all “shortwave” • Total energy from 4-infinity 1% – all “longwave” • Total energy from 13μm-infinity 0.03% – major 15μm CO2 band and above • Terminology: • >0.75μm is infra-red (slightly different conventions exist about the maximum value for visible light, but nothing substantial) • 0-4μm is “shortwave” – a climate science convention referring to solar radiation • 4μm-infinity is “longwave“- a climate science convention referring to terrestrial radiation Answer: Radiation properties • Quantum description • Wave description Quantum Properties of Radiation STIMULATED EMISSION Incident Photon Excited Atom Incident Photon Stimulated Photon same wavelength same direction in phase Laser-Professionals.com • When energy is absorbed by an atom, some of the electrons in that atom move into larger, higher energy orbits. When energy is released by the atom, the electrons move to smaller orbits. The lowest energy state is called the ground state. This is when all the electrons are as close to the nucleus as possible. Higher energy states are called excited states. Excited atomic states are not stable. Excited atoms tend to release energy in the form of photons and drop to lower energy states. • Ordinary light is produced by spontaneous emission as excited atoms drop to lower energy levels and release photons spontaneously. The result is light that is a mixture of many different wavelengths, is emitted in all directions, and has random phase relationships. • Laser light is produced by stimulated emission when excited atoms are struck by photons in the laser beam. This stimulates the excited atoms to emit their photons before they are emitted randomly by spontaneous emission. The result is that each stimulated photon is identical to the stimulating photon. This means that all photons produced by stimulated emission have the same wavelength, travel in the same direction, and are in phase. Thus the stimulated emission process leads to the unique properties of laser light. Flux and Intensity Flux and Intensity Flux Intensity • - Spherical Polar Coordinate Fig. 2.3 Solid angle • The ratio of the area of the sphere intercepted by the cone to the square of the radius – / r2 – Units: Steradians (sr) • What is the area cut out of a sphere by one steradian? • What is the solid angle representing all directions at a point? HW2: A meteorological satellite circles the earth at a height h above the earth’s surface. Let the radius of the aeearth be and show that the solid angle under which the earth is seen by the satellite sensor is 2 [1 (2ae h h 2 )1/ 2 /( ae h)] Solid angle and definition of steradian Solid angle and definition of steradian Chapter 3 Electromagnetic Spectrum Blackbody radiation • Examine relationships between temperature, wavelength and energy emitted • Blackbody: A “perfect” emitter and absorber of radiation... does not exist Measuring energy • Radiant energy: Total energy emitted in all directions (J) • Radiant flux: Total energy radiated in all directions per unit time (W = J/s) • Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area in a unit time (W m-2) • Radiance: Irradiance within a given angle of observation (W m-2 sr-1) • Spectral radiance: Radiance for range in l Radiance Normal to surface Toward satellite Solid angle, measured in steradians (1 sphere = 4 sr = 12.57 sr) Stefan-Boltzmann Law M BB = T 4 Total irradiance Stefan-Boltzmann constant emitted by a blackbody (sometimes indicated as E*) The amount of radiation emitted by a blackbody is proportional to the fourth power of its temperature Sun is 16 times hotter than Earth but gives off 160,000 times as much radiation Planck’s Function • Blackbody doesn't emit equal amounts of radiation at all wavelengths • Most of the energy is radiated within a relatively narrow band of wavelengths. • The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function: Planck’s function First radiation constant Wavelength of radiation c1l-5 B l (T) = exp (c2 / lT ) -1 Absolute temperature Second radiation constant Irridance: Blackbody radiative flux for a single wavelength at temperature T (W m-2 m-1) Total amount of radiation emitted by a blackbody is a function of its temperature c1 = 1.19x10-16 W m-2 sr-1 c2 = 1.44x10-2 m K Planck curve Wein’s Displacement Law lmT = 2897.9 m K Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature Earth @ 300K: ~10 m Sun @ 6000K: ~0.5 m Intensity and Wavelength of Emitted Radiation : Earth and Sun Solar Spectrum window Atmosphere Window Rayleigh-Jeans Approximation Bl (T) = (c1 / c2) l -4 T When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengths At microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4 Bl (T) TB = (c1 / c2) l -4 Brightness temperature (TB) is often used for microwave and infrared satellite data, where it is called equivalent blackbody temperature. The brightness temperature is equal to the actual temperature times the emissivity. Emissivity and Kirchoff’s Law l l l Actual irradiance by a non-blackbody at wavelength l Emittance: Often referred to as emissivity Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature l l absorbed/ l incident Absorptivity (rl , reflectivity; tl , transmissivity) Solar Constant • The intensity of radiation from the Sun received at the top of the atmosphere • Changes in solar constant may result in climatic variations • http://www.space.com/scienceastronomy/0 71217-solar-cycle-24.html CLOUD RADIATIVE FORCING Clouds can either warm or cool the climate depending on the cloud type Cooling • By reflecting solar radiation back to space • Particularly low clouds • Global average short wave cooling is - 48 W m-2 Warming • By acting as a greenhouse absorber and emitter of • long wave radiation • Particularly thin cirrus clouds • Global average long wave warming is + 28 W m-2 Net Effect • Global cooling of about - 20 W m-2 • But what will the cloud feedback be with global