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Lecture 3.1
Solar energy
This week we’ll contemplate little things like…
• Why there’s life on Earth
• Why you don’t want to live at the South Pole
• Why you don’t want to live in San Antonio
• Why the weather changes every day these days
Today
• We’ll deal with solar radiation
• What’s the “greenhouse effect”?
• Return homework
Radiation• What heats the Earth??? The Sun!!!• How does it do it???
– Radiation -- Energy transfer from one place to another by electromagnetic waves.
• Light• Radio Waves• Microwave• Infrared• Ultraviolet
• Note EM radiation does not require a ‘medium’ to pass through, it can get from the sun to the earth through the vacuum
Radiation
• Incoming Solar Radiation (Insolation)
– The sun radiates a huge amount of energy but in all directions.
– The amount reaching a point in space depends on the distance from the sun.
• Solar Constant: The amount of solar energy arriving at the top of the atmosphere perpendicular to the sun’s rays. (Not really “constant” but close enough for government work!)
• = 1375 W m-2
– (Sometimes written as 1365 W m-2, depending on source.)
Radiation
Incident Solar Radiation and Albedo
Radiation
NASA -- Apollo 8
Albedo• But we must consider reflections:
Albedo = Amount reflected (x 100%) Amount incoming
Earth’s albedo = 30%
• This 30% is due to:
– clouds– dust, haze, smoke– scattering by air molecules– reflections from land, oceans, ice
Radiation
• Only one half of the earth intercepts sunlight. From the sun, it looks like a disc.
SolarRadiation
Which half of the Earth is light?
• The Earth rotates on its own axis– Only the daytime side receives energy directly
from the sun– The nighttime side often receives a smaller
amount of energy reflected off the moon
Radiation
• All things, whose temperature is above absolute zero, emit radiation They radiate!!!
• Radiation is emitted at all wavelengths -- some more so than others
• Examples– Dogs The atmosphere– Snow Your Books– Trees and …..– The oceans You!!!
Radiation
E = T4
• E =The amount of energy (W m-2) emitted by an object per unit area
= Stefan-Boltzmann constant = 5.67 x 10-8 W m-2 K-4
• T = Temperature (K)
Stefan-Boltzmann Law: Anything that has
a temperature radiates energy. Hotter
objects radiate a lot more energy.
Wien’s Law
• This tells us the peak wavelength that an object will emit
λmax = 2900 / T
Where λmax is the wavelength in micrometers
T is the temperature in Kelvin
Wien’s Law
• The sun has a surface temperature of about 6000K:– λmax = 2900 / 6000 ≈ 0.48μm– This is green light
• The Earth has a surface temperature of about 290K:– λmax = 2900 / 290 ≈ 10μm– This is infra red radiation
Radiation• OUTPUT
– The earth’s surface has a temperature so it radiates according to the Stefan-Boltzmann Law.
– Wien’s Law tells us this is primarily infrared (IR) radiation. But, only 6% of this passes directly to space.
Solar and Terrestrial Radiation
© 1999 Prentice-Hall -- From Aguado and Burt, Understanding Weather and Climate Wavelength
Wavelength
SolarRadiation
TerrestrialRadiation
Notice that the earth’s radiationis much, much less than that ofthe sun!
Radiation
• What have we discovered about the radiation of the sun compared to the earth?
– The sun has a radiation maximum in the visible part of the spectrum.
– The Earth has a radiation maximum in the infrared part of the spectrum.
Radiation
GOES-8Full-diskVisible
Radiation
GOES-8Full-disk
IR
Radiation
• For the Earth’s temperature to remain constant over a long period of time (decades), the amount of solar radiation absorbed must equal the amount of long wave radiation emitted to space.
Solar absorbed = Long Wave emitted
Input = Output
RadiationEarth-Atmosphere Energy Balance
© 1998 Wadsorth Publishing -- From Ahrens Essentials of Meteorology
Scattering of Radiation
• Radiation can be scattered or absorbed by the gases and particles (dust) in the atmosphere
• Different wavelengths of light are scattered in different ways
• A certain proportion will be scattered straight back into space
Absorption of Radiation
• Radiation can be absorbed by molecules of gas in the atmosphere
• Different gases absorb different wavelengths of light
• The major atmospheric gases absorb infra-red, but not visible, radiation
• When the gas absorbs radiation it gains energy (is warmed)
Atmospheric AbsorptionAtmospheric Absorption
Solar radiation passes rather freely through Solar radiation passes rather freely through earth's atmosphere, but earth's re-emitted earth's atmosphere, but earth's re-emitted longwave energy either fits through a narrow longwave energy either fits through a narrow window or is absorbed by greenhouse gases window or is absorbed by greenhouse gases and re-radiated toward earth.and re-radiated toward earth.
Figure 2.11Figure 2.11
The Atmosphere is transparent to solar radiation.
Radiation
• As a first approximation --
Radiation
• Thus the earth’s atmosphere is essentially opaque (not transparent) to IR radiation from the earth’s surface.
Absorption by:
a. H2Ov c. CO2
b. Clouds d. O3
Radiation
• The atmosphere radiates IR both upwards and downwards.......
• The downward portion re-warms the earth’s surface and is known as the
Greenhouse Effect.
Summary
• We’ve seen what the Greenhouse Effect is and what it isn’t and why we should avoid the term altogether
• Next time we’ll talk about ‘climate variation’ and why it happens
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