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