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Planetary Atmospheres, the Environment and Life (ExCos2Y)
Topic 4: Solar Radiation
Chris Parkes
Rm 455 Kelvin Building
3. Structure of Planetary Atmospheres
• 4 distinctive layers with boundaries – Troposphere, Stratosphere,
Mesosphere, Thermosphere
• Temperature profile– Greenhouse gases
– Ozone in stratsophere
• Comparison of atmosphere’s of Earth, Venus & Mars
Revision
Electromagnetic Radiation
• All bodies emit EM radiation
• Distribution of intensity as a function of wavelength depends on temperature of the body
• The peak of the distribution from the sun is in the visible region
• The Earth’s radiation distribution peaks in the infra-red region
Peak wavelength 1/temperature
Stefan-Boltzmann Law: I =σT4
Wien displacement law: λmT = 2.90×103 m·K
Perfect black body
The most perfect black body radiation ever measured:The cosmic microwave background radiation at 2.725 Kelvin
= 1 / wavelength
Absorption of radiation by atmospheric gases
Solar radiation Earth radiation
Visible light gets through
UV absorbed by Ozone
Earth’s incoming and outgoing radiationCharacteristic Blackbody
radiation shapes
Actual solar spectrum at sea level shows gaps where absorption occurs
Likewise earth radiation reaching upper atmosphere show gaps
There are short and long wavelength “windows”
Atmospheric Ozone responsible for absorbing UV radiation
Wm
-2μ
m-1
μm
Sun - Incoming
Earth - Outgoing
UV absorbed by Ozone
Ozone Depletion in Stratosphere revisited - Chemical Reactions
Production mechanism:
UV + O2 2O
O + O2 O3
Loss mechanism:
UV + CFC Cl
Cl + O3 ClO + O2
ClO + O Cl + O2
Ozone absorbs UV:
O3 + O 2O2
UV + O3 O + O2
Will recover ~2070Chlorofluorocarbons (CFCs): Cl, F, C
Exposure to UV radiation
TOMS (Total Ozone Mapping Spectrometer)
Ozone and clouds both
absorb UV radiation
Measure UV on ground
level need to combine: incoming UV
reflected UV
cloud cover
Variation of “Insolation” with latitude
At an angle less energy density on surface
Solar energy
Solar energy
Direct or diffused shortwave solar radiation received in atmosphere or at surface
Central Australia = 5.89 kWh/m2/day - HighHelsinki, Finland = 2.41 kWh/m2/day - Low
Insolation: Incident Solar Radiation – Energy received per unit are per unit time
θ
d
d / cos θ
e.g. cos 30o = 0.5, half as much insolationHence, poles colder than equator
But heat also transmitted in atmosphere (convection…)
Daily variation due to rotation of Earth on axis
Absorbed radiationdependent on time ofday
Daytime has net surplusenergy input
Night time has net loss of Infrared radiation
Temperature ofatmosphere lags behindabsorbed radiation dueto heat capacity ofatmosphere
sun
Variation in insolation due to time of year
Greater tilt more extremeSeasons
Smaller Tilt Polar regionscolder
Seasons on Mars
• Elliptical Orbit – closer to sun in
southern hemisphere summer,
– further southern hemisphere winter
• Extreme seasons in south– -130oC in winter in
south – CO2 dry-ice caps
• Year nearly twice as long as Earth– Orbital period 1.88 x
Earth
AlbedoDefined as the fraction of incident radiation which is reflected
Object AlbedoGlobal cloud 0.23 (different types have different
albedo)
Forests 0.15 (depends on type of tree)Water 0.10 (highly dependent on incident
angle)Snow 0.8Sand 0.3Grass 0.2
Planet Earth 0.31
Extremes:•Water efficiently absorbs•Snow/Ice effficiently reflects
Radiation reflected from Earth
Top: shortwave
Bottom: longwave
Globally annual average of in & out is balanced
but there are seasonal & regional variations
CERES (Clouds & the Earth’s Radiant Energy System, NASA)
The greenhouse effect• The earth takes energy from solar radiation • Earth is in a “steady state” (constant surface temperature) • re-emit in “blackbody” radiation (longwave)
in order to keep energy balance• Greenhouse gases absorb longwave radiation from
earth’s surface and re-emit part of this back to surface• To maintain energy balance surface temperature must be
increased to increase the output radiation energy– The higher the surface temperature the more energy is being emitted
Greenhouse Gas – self-regulation
• Negative feedback mechanism– Interacts with CO2 cycle
• Solar radiation 30% higher in past
Earth – rescue from ice age
• Deep ice ages – ‘snowball Earth’– Ice reflects, water absorbs more cooling
– CO2 buildup greenhouse effect heating
Venus – high temperatures
• Increased temperature – water evaporates, CO2 released– Water vapour also greenhouse gas
• Venus lost its water early, prob. never forming oceans– CO2 dominated atmosphere
Temperature Profiles Revisited: Venus, Earth, Mars
• Temperature difference due to greenhouse effect– Mars: +6oC – lost CO2 in atmoshere– Earth: +31oC – moderate CO2, stored in rocks due to
water cycle– Venus: +500oC – CO2 dominated atmosphere
Example exam questions
Q1. Explain why on average the surface temperature along the equator is higher than that of the poles?
Q2. Sketch the daily variation of earth’s input and output radiation. Explain how this relate to the temperature variation.
Q3. Draw a diagram explaining the radiation budget of the earth?
Next lecture – convection in the atmosphere