MET 60: Chapter: 4 (W&H) and 2 (Stull)

Radiative TransferDr. Craig Clements

San José State University

There are three modes of energy There are three modes of energy transmission in the atmosphere.transmission in the atmosphere.

Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion.

Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy.

Radiation: the propagation of electromagnetic waves through space.

Energy transmissionEnergy transmission

ConductioConductionn

ConvectionConvection

Electromagnetic radiation

Radiation is the transfer of energy by rapid oscillations of electromagnetic fields.

The most important general characteristic is its wavelength (), ______________________________.

Radiation travels through space at the speed of light (3 x 108 m s-1) or 670,616,630 MPH.

Defined as the crest-to-crest distanceDefined as the crest-to-crest distance

The Spectrum of Radiation

/1

Electromatic radiation may be viewed as an ensemble of waves propagating at the speed of light (c*=2.998 x 108 m/s through vacuum).

As for any wave with a known speed of propagation, frequency , wavelength λ, and wave number, ν (i.e., the number of waves per unit length in the direction of propagation) are interdependent. Wave number is the reciprocal of wavelength

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The electromagnetic radiation in a specific direction passing through a unit area (normal to direction considered) is called:

monochromatic radiance (or spectral intensity or monochromatic radiance) λ

The integral of the monochromatic intensity over some finite range of electromagnetic spectrum is called the intensity or radiance, I [W m-2]

dIdII

2

1

2

1

The “Spectrum”

Wavelength, λ

I λ

λ2λ1

Spectrum of Monochromatic Intensity(theoretical)

Radiation What emits radiation?

– All objects with a temperature greater than 0°K emit some type of radiation (energy)

Examples:

Radiation laws:

– Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law)

– Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

Wien’s Law:

E = σT4

λ = w / Tλ = maximum wavelength (μm)w = constant = 0.2897 (μm K)T= temperature of the object (K)

Stefan-Boltzmann Law:

E = radiation emitted (W m-2)σ = Stefan-Boltzmann constant= 5.67 x 10-8 (W m-2 K-4)T= temperature of the object (K)

Review questions

Considering the previous discussion– Which object would emit more (intensity) radiation:

Earth or Sun?

– If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength?

– What wavelength radiation are you emitting right now?

Sun

Earth

infrared

Solar Radiation (Sunlight)

Sunlight is primarily made up of the following:– Visible Light (44%)– Infrared Radiation (48%)– Ultraviolet Radiation (7%)

Unit: 1 m =

0.000001 m

Terrestrial or Longwave Radiation

Planets mainly emit infrared radiation Radiation emitted by

planets occurs mainly at wavelengths _____

than those contained in solar radiation

Solar Radiation (“Shortwave”)

Terrestrial Radiation (“Longwave”)

longer

Solar vs. Terrestrial Radiation

The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation;

Thus …

Energy from the Sun

Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get?

Sun’s energy is either– Scattered (reflected away) or– Absorbed

Scattering happens by bouncing off – Particles in the atmosphere– Earth’s surface

Absorption happens when certain gases absorb the energy– The reality is the only certain gases absorb certain

wavelengths.

Absorption of radiation

Absorption of shortwave radiation by atmospheric gas molecules is fairly weak;

– most absorption of shortwave radiation occurs at the Earth’s surface.

Most gases do not interact strongly with longwave radiation, however

– Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiationIncoming solar radiation

Each ‘beam’ of incoming sunlight can be either:– Reflected back to space:

Clouds Atmosphere Surface

– Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface.

AlbedoAlbedo

Recap

______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation

__________________ radiation comes from the Earth and is composed of infrared radiation

Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength.

Shortwave or solar

Longwave, terrestrial or infrared

Review - sensors that measure radiation

A _________________ measures solar radiation.

A__________________ measures infrared radiation (terrestrial) that comes from the Earth.

Pyranometer

Pyrgeometer