NATS 1750 - The Earth and Its Atmosphere
The Evolution of Earth's Atmosphere
It all started with the Big-Bang!!
About 15 billion years ago
1 second after the Big Bang the Universe cooled to 109 K or C - protons/electrons/neutrons created
Minutes later H and He formed
About 1 billion years later - Quasars and active galaxies formed
10 billion years ago: Galaxies moved apart
and our galaxy (the Milky Way) formed
5 billion years ago: our Solar System formed
Our star, the Sun,is a 2nd generation star
Earth formed about 4.5 billion years ago
Primary atmosphere of H, He and H2O blown off by solar wind from the Sun
Secondary atmosphere formed from volcanic emissions
Volcanic emissions: H2O 85%, CO2 10% , N2 2% + SO2.
Atmosphere today: N2 78%, O2 10%, Ar 0.9%, CO2 350 ppm.
(ppm = parts per million)
Where didthe O2 come from?
Photochemical: water + light O2 + H (the H escaped to space)
More important - Photosynthesis: H2O + CO2 + light CH2O (in organism) + O2 (in air)
CH2O (in organism) using O2 (in air) can/does re-burn toH2O + CO2
Atmosphere bears imprint of life:
Other `biological' gases, CH4, N2O
Most of secondary atmosphere has gone to oceans (water) and buried carbonaterocks
Other Minor species in the Atmosphere: Ozone (O3) 10 ppm in stratosphere,
Ozone filters UV hazardous radiation
IR active (Greenhouse) gases: water, CO2, O3, CFCs, CH4, N2O.
NB: N2 and O2 are not IR active gases - do not absorb IR.
Without IR absorbing gases the `mean'/`average' temperature of the earth would be -15C.
Tarbuck, Lutgens and Tasa, (hereafter TL&T) Earth Science 13th Edition
Early evolution of Earth, p.9 -13
Birth of a Planet, p. 350-354
See alternatively:
Tarbuck & Lutgens, Earth Science11th Edition
Box 1.1 p.4-5 Earth as a System ‘Earth’s Place in the Cosmos’;
p.10-12 'Early Evolution of Earth';
Ch. 12 p. 338-339 ‘Earth’s Atmosphere Evolves’
Tarbuck & Lutgens, Earth Science10th Edition
Box I.1 p.6-7 ‘Earth’s Place in the Cosmos’;
Intro p. 2-4 'Early Evolution of Earth';
Ch. 11 p. 310-311 ‘Earth’s Atmosphere Evolves’
(9th Edition p.12-13; p296-297 Origin of Earth; p298)
Atmospheric Structure
Pressure decreases by a factor of about 2 every 6 km we goup until the tropopause
Pressure decreases constantly with height
Surface pressure = 1000 mbar which is about the weight of 1000 kg m-2
Temperature Structure, Tropo-, strato-, meso-, thermo-sphere
Troposphere - weather, T decreases with height, 6 C/km approximately - potentially turbulent or unstable air
Stratosphere - T increases with height - location of most ozone - stable air
Mesosphere - middle
Thermosphere - hot sphere - very tenuous atmosphere
TL&T 13th, p. 462-467; The Atmospheric Composition, Structure and Temperature.
p. 460-462; p 465-467
See alternatively:
T&L 11th, Height and Structure of the Atmosphere p440-443
T&L 10th, Height and Structure of the Atmosphere p409-411
(9th Ed p381-383)
Radiation and the Sun
Electromagnetic radiation is everywhere.
Light as waves
Wavelength, and frequency,
c =
speed of light, c = 3x108 m s-1
Light as photons (a bit like particles).
Energy = h
where h is a constant called Planck's constant ~6 x 10-36 J s
Spectrum of Radiation:
gamma rays, X-rays, UV, visible, IR, microwave, FM Radio (MHz) and AM Radio (KHz)
visible:
red = 0.7m = 700 nm : blue = 0.4m = 400 nm.
(1 m = 1.0-6 metre and 1 nm = 1.0 -9 metre)
TL&T 13th,Spectroscopy, p. 668-669; Structure of the Sun, 678-684
See alternatively:
‘Radiation’ 11th T&L p449-450; 10th p. 418-419, (9th p. 390-391)
‘Study of Light’11th T&L p640-643; 10th p. 604-606,(9th p. 580-582)
Radiation Laws:
(1st) All bodies emit radiation - N.B. this means even in the dark
(2nd) Stefan Boltzmann Law:
Amount of EM Radiation emitted by body is
E (Joules m-2 s-1 ) = T4
(3rd) Wien's Displacement Law:
max x T = 3000 m K
Explains why sunlight peaks at close to 0.5 m (because T of the Sun is about 6000K)
Earthshine (I.R. radiation from Earth)peaks at 10 m (because T of Earth is about 300K)
(4th) Kirchoff's Law:
Good absorbers are good emitters - Poor absorbers are poor emitters
The Sun
The Sun is an average Milky Way star
Radius 700,000 km - about 100 times radius of Earth 6370 km
Therefore 1,000,000 times bigger volume than Earth
Suns structure:
CORE (10,000,000 K), radiation-zone, convection-zone,
PHOTOSPHERE (6,000 K)
CHROMOSPHERE (4,500 K)
CORONA (1,000,000 K).
PHOTOSPHERE radiates with temperature close to 6000 K with peak at about 0.5m.
We see the photosphere which is only ~300 km thick.
Composition 90% H, 10% He.
Solar wind, prominences, granules, sunspots.
Rotation period = 27 days at Sun's equator, 33 days nearer poles
sunspots (cooler region of chromosphere) , 11 year solar cycle also reflected in occurrences of Solar Flares
Enhancement of Solar Wind during flares increases the frequency of occurrence of Northern Lights - Aurora Borealis - when electrons and protons bombard
the Earth's upper atmosphere at about 120 km above surface and produce light in the atmosphere.
Sun's energy generated in the CORE where T= 10,000,000 K
Nuclear reactions there change 4 H nuclei into 1 He nucleus with loss of mass
This mass is converted to energy according to Einstein's E=mc2
Every second 600 million tons of H are converted into 596 million tons of He
The missing 4 million tons of mass is converted to energy to produce
3.6 x 1020 megawatts, i.e. 3.6 x 1020 Megajoules of energy each second
However Sun has about 1.4 x 1027 tons of H to destroy so it could last about another 74 billion years !!
Solar Constant = 1380 W/m2 , i.e 1.38 kW/m2 ,
This is the energy passing through 1 metre2 ( i.e 1 square metre) every second at Earth's distance from the Sun which is ~150,000,000 km. [Note " ~" means approximately]
If Earth was 300,000,000 km from Sun (i.e. twice as far away) the Solar Constant would be 4 times smaller, i.e. 1380/4 W/m2 = 345 W/m2
If Earth was 600,000,000 km from Sun (i.e. four times further away) the Solar Constant would be 16 times smaller.
TL&T 13th,Structure of the Sun, p. 678-684
See alternatively:
‘Radiation’ 11th T&L p449-450; 10th p. 418-419, (9th p. 390-391)
‘Study of Light’11th T&L p640-643; 10th p. 604-606,(9th p. 580-582)
Balance of Radiation Energy in from the Sun and out from Earth
The expected equilibrium temperature of Earth may be obtained using the 2nd Radiation Law for the energy radiated by each square metre of Earth
i.e. E (Joules m-2 s-1 ) = T4
and equating the Total energy in from the Sun with the Total energy emitted by the Earth to space as IR radiation
i.e., the Total energy received from Sun = (Solar Constant ) x cross sectional area of planet (i.e. R2) = Total IR out from Earth
But Total IR out = IR out per unit area (watts m-2 ) x surface area of planet (4R2)
This gives IR out per unit area as 345 (watts m-2 ), then we use the Stefan-Boltzmann Law, E (Joules m-2 s-1 ) = Teq4, to work out the temperature.
This tells us that the Temperature at equilibrium, Teq, must be
Teq = (E/)¼
Using the full Solar Constant this gives an expectedequilibrium temperature for the Earth of Teq= 279K or 6C which is close, but not close enough, to what we have on Earth today 15C.
To see why we are wrong we need to look in more detail at heat.
Heat is one form of energy and like energy is also measured in Joules
Heat is always associated with molecular motion
Heat always moves from hotter to colder
Heat moves to establish EQUILIBRIUM
Heat exchange occurs by conduction, radiation and convection.
Conduction - flow of energy not mass. Due to (microscopic) motions of molecules.
Convection - mass motion of material.
We also have Latent heat and Sensible heat transfer
Latent heat is heat absorbed (or released) when a liquid goes to a gas (or gas to a liquid)
Sensible heat is heat moved by winds from one place to another.
TL&T 13th edition, Energy, Heat and Temperature, p. 472-474
See alternatively:
'Heat and Temperature’, 11th p 448;10th p417 (9th p 389)
‘Mechanisms for Heat Transfer’ 11th p448-449; 10th p 417-418 (9th p 389-390)
‘The Fate of Incoming Solar Radiation;11th p 450-452;10th p419-421 (9th p.391-393)
'Heating the Atmosphere: The Greenhouse Effect, 11th p452-453;10th p421 (9th p393)