II. Global Energy Balance. A. Electromagnetic Radiation: self-propagating electric and magnetic...

II. Global Energy Balance

II. Global Energy BalanceA. Electromagnetic Radiation: self-propagating electric and magnetic waves. Or …. Radiation transmitted through the vacuum of space without a medium. Moves at the speed of light.

1. wavelength: distance between two successive peaks or troughs.

II. Global Energy BalanceA. Electromagnetic Radiation: self-propagating electric and magnetic waves. Or …. Radiation transmitted through the vacuum of space without a medium. Moves at the speed of light.

1. wavelength: distance between two successive peaks or troughs.

2. Although we describe electromagnetic (EM) radiation as a wave, it also sometimes behaves like a stream of particles; one particle = photon.

3. Rules of thumb:Shorter wavelength EM has higher temperature/energyLonger wavelength EM has less energy/lower temp.

II. Global Energy BalanceA. Electromagnetic RadiationB. Electromagnetic Spectrum

1. Radiation comes in a vast range of wavelengths


1. Radiation comes in a vast range of wavelengths.

2. The electromagnetic wavelength determines its properties.

3. Hot bodies radiate at shorter wavelengths than do cooler bodies.

4. About 50% of solar EM radiation is in the visible spectrum, 40% at longer wavelengths (mostly infra-red [IR]), and 10% shorter (ultra-violet [UV]).

Sun

Earth (288 K)

UV IRVisible

5800 K


1. Radiation comes in a vast range of wavelengths.2. The electromagnetic wavelength determines its

properties.3. Hot bodies radiate at shorter wavelengths than do cooler

bodies.4. About 50% of solar EM radiation is in the visible

spectrum, 40% at longer wavelengths (mostly infra-red [IR]), and 10% shorter (ultra-violet [UV]).

Infrared is longer, so less energetic than visible light; Ultra-violet is shorter wavelength, so more energetic. This is one reason UV light is so damaging.


new terms: flux, blackbody

Flux: the amount of energy or material that passes through a given area over a specific time period.


new terms: flux, blackbody

Blackbody is an object that emits/absorbs electromagnetic radiation with 100% efficiency at all wavelength.

Earth and Sun approximate blackbodies.

Sun is about 5500 °C (5800 K), and has a peak EM radiation at about the middle of visible spectrum, whereas Earth is cooler (15 °C) and emits peak energy in the IR range (sensible heat).

** Because Earth’s EM wavelength is about 20 times that of the Sun, we call Earth’s radiation “long wave” and Sun’s radiation “short wave”.

II. Global Energy BalanceA. Electromagnetic RadiationB. Electromagnetic SpectrumC. Planetary Energy Balance: this is based primarily on:

1. flux of solar radiation (predicted by temperature of the Sun, and Earth’s distance from the Sun).

2. proportion of solar radiation reflected (planetary albedo).

3. from these, we calculate Earth’s average planetary temperature to be –18 °C. The actual average temperature is +15 °C (60 °F).

4. The Greenhouse Effect.

II. Global Energy BalanceD. Structure and composition of the atmosphere

1. Composition


1. Composition2. State of the atmosphere:

a. Temperature (C, K, F)b. atmospheric pressure

1 atm = 760 mm Hg = 29.92 inches



a. Temperature (C, K, F)b. atmospheric pressurec. humidity

relative humidity: fraction of water vapor in a parcel of air compared to its maximum capacity

absolute humidty: the actual volume of water vapor in a given mass of air.

Stratosphere

Troposphere

Mesosphere

Thermosphere

Inherently unstable

Inherently stable

Tropopause



a. Temperature (C, K, F)b. atmospheric pressurec. humidity

3. Vertical structurea. inversionsb. adiabatic lapse rate

Rule of thumb in the Troposphere:Temperature decreases 6 °C for every 1000 m elevation gain.

II. Global Energy BalanceE. Heating the Atmosphere

1. The fate of solar radiationReflection (albedo), absorbtion, scatter, selective

scatter2. Heat transport mechanisms

a) Conductionb) Radiationc) Convection

RadiationConduction

Convection

Radiation

II. Global Energy BalanceE. Heating the Atmosphere

2. Heat transport mechanismsa) Conductionb) Radiationc) Convection

d) Latent heat vs Sensible heatSensible Heat is measured directly by temperature(the speed at which the molecules are moving)

Latent Heat is the heat energy gained or released inthe transition from one phase to another (gas - liquid - solid)

RadiationConduction

Convection

Radiation

Latent Heat (water vapor)

II. Global Energy BalanceF. Greenhouse Effect

1. Unlike solids (blackbodies), gases are not blackbodies. Many gases are selective absorbers and emitters.

2. Greenhouse Gases (GHG): gases that are transparent to short wave solar radiation, but opaque a some wavelengths of Earth’s longerwave radiation.

3. Important GHG

Name Formula Pre-industrialconcentration

CurrentConcentration

Life-span

GreenhousePotential

Water Vapor H2Ov Var iable Var iable weeks Mostimportant

Carbon dioxide CO2 280 ppm 375 ppm 100 yr 1

Methane CH4 700 ppb 1800 ppb 15 yr 21

Nitrous oxides NOx

N2O275 ppb 315 ppb 100 yr 200

CFCsChloro-fluoro-carbonsCCF-12 CCl2F2

0 0.5 ppb 100 yr 15,000

Sulfur hexafluoride SF6 0 0.03 ppb 3000 yr 24,000


4. How do GHG absorb EM radiation?

Molecules (like CO2, H2Ov) rotate and vibrate. EMradiation of specific wavelengths are absorbed andincrease either the rotational speed or the vibrationamplitude of these molecules.


Molecular rotation

5. Why do some GHG have a greater “Greenhouse Potential” than others? Because there are “windows” in Earth’s greenhouse, EM wavelengths where there is little or no absorbance. GHG that absorn in these windows are more effective, molecule for molecule, than more H2O or CO2

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