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. Solar Radiation

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 –DogsThe atmosphere –SnowYour Books –Treesand ….. –The oceansYou!!!

Radiation E =The amount of energy (W m -2 ) emitted by an object per unit area  = Stefan-Boltzmann constant = 5.67 x 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 λ max = 2900 / T Where λ max is the wavelength in micrometers T is the temperature in Kelvin

Wien’s Law The sun has a surface temperature of about 6000K: –λ max = 2900 / 6000 ≈ 0.48μm –This is green light The Earth has a surface temperature of about 290K: – λ max = 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 Solar Radiation Terrestrial Radiation Notice that the earth’s radiation is much, much less than that of the 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-8 Full-disk Visible

Radiation GOES-8 Full-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

Radiation Earth-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 Absorption Solar radiation passes rather freely through earth's atmosphere, but earth's re-emitted longwave energy either fits through a narrow window or is absorbed by greenhouse gases and re-radiated toward earth. Figure 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. H 2 O v c. CO 2 b. Cloudsd. O 3

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