Lectures 2-3 week 1-2 2009: HAS222d Solar radiation, the - - PowerPoint PPT Presentation

Lectures 2-3 week 1-2 2009: HAS222d Solar radiation, the greenhouse, global heat engine

http://en.wikipedia.org/

The 4 streams of this course (see syllabus) 1.Energy

forms of energy concetrated, diluted conservation transmission/movement transformation efficiency of transformation heat engines degradation (and entropy) storage ‘utilization’ by plants and animals carbon cycle, photosynthesis

3.Humans and energy

history of energy demand and development ….fossil fuels connections with evolution alternative energies

2.Global Environment

physical, chemical, biological atmosphere, ocean, land surface energy, air, water, ice, carbon the sun-atmosphere-ocean heat engine fluid circulations in which protective ‘niches’ of life develop

4.Arctic populations

natives: settlement Europeans: exploration assimilation, exploitation shaping of their lives by energy and food resources in a harsh environment amplified global warming in the Arctic

Let’s start with the sun

diameter: 1.38 million km distance from Earth (mean): 149.6 million km (93 million miles)* tilt of Earth’s rotation axis relative to its orbit round the sun: 23.50 the orbit is an ellipse, but only about 2% different from a circle: the oribital eccentriciy**= 0.017 rotation period: 23.9 hours length of day: 24 hours On July 4 this year the Earth is farthest from the sun (aphelion);

• n Jan 4 it was closest (perihelion); about 7% more sunlight (rate of energy falling
• n Earth) in Jan than in July. As Northern Hemisphere goes, so goes

climate! The eccentricity shifts with 100,000 year period from 0.05 to nearly zero. perihelion shifts with 21,000 year period

• bliquity (tilt of axis) shifts with 41,000 year period …..all these slight changes alter the amount of sunshine

and its distribution at the Earth’s surface, somehow leading to ice ages….cycles of cold and warm climate.

Averaged over the globe, sunlight falling on Earth in July (aphelion) is indeed about 7% less intense than it is in January (perihelion)." That's the good news. The bad news is it's still hot. "In fact," says Spencer, "the average temperature of Earth at aphelion is about 4o F (2.3o C) higher than it is at perihelion." Earth is actually warmer when we're farther from the Sun!

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www.cwru.edu, http://science.nasa.gov/headlines/y2001/ast03jul_1.htm

* (these two numbers together tell us how big the disc of the * (these two numbers together tell us how big the disc of the sun appears in the sky sun appears in the sky… ….the relationship is .the relationship is tan tan ½ ½ Θ Θ = = ½ ½ diameter/distance (see diagram above) diameter/distance (see diagram above) For small angles tan For small angles tanΘ Θ us approximately us approximately Θ Θ, measured in radians. , measured in radians. So, So, Θ Θ = 1.38/149.6 = 0.00922 radians or .00922 x 360/2 = 1.38/149.6 = 0.00922 radians or .00922 x 360/2Л Л

• degrees. This is 0.53 degrees
• degrees. This is 0.53 degrees…

….roughly .roughly ½ ½ degree,almost degree,almost the same the same angular size as the moon, which is why we have such perfect ec angular size as the moon, which is why we have such perfect eclipses) lipses)

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = * * the eccentricity of an ellipse is defined as the ratio * * the eccentricity of an ellipse is defined as the ratio √

√(1

(1-

• b

b2

2/a

/a2

2)

) where a is the where a is the largest diameter (the major axis) and b is perpendicular to it, largest diameter (the major axis) and b is perpendicular to it, the smallest the smallest diameter diameter

angle Θ disc of the sun Earth

The sun’s radiation reaches the Earth with an intensity of about 1368 watts per square meter…that would be like 13.68 one-hundred watt light bulbs illuminating a one- meter square surface, except that light bulbs put their 100 watts of power into other forms of heat as well as visible and infrared radiation. Given the distance of the sun from the Earth (149.6 million km) and the diameter of the sun (1.38 million km), how much power (energy per unit time) is radiating from one square meter of the bright outer surface of the sun?

http://commons.wikimedia.org/wiki/File:Solar-cycle-data.png

Visible light has wavelengths between 400 and 700 nanometers (0.4/1000 to 0.7/1000 of a millimeter) (often we will also use the length unit of “micron”… a millionth

• f a meter a “nanometer” is a billionth of a meter. In yet other

units, this is 4000 to 7000 Angstroms: 1 Angstrom is about the diameter of a hydrogen atom.)

• The wavelength (l) is the distance between one peak of

the wave and the next peak. It's a distance so can be measured in metres, centimetres etc. It is sometimes given the Greek letter (lambda). It's also the distance between one part of the wave and the next part which is at exactly the same stage of vibration - but 'peak-to-peak' is easier to remember.

• The frequency (f) is the number of complete waves

passing a point each second. It's a 'number per second' so it's measured in /s or s-1; usually called hertz (Hz) after a German physicist.

• 1 kilohertz = 1 kHz = 1000 Hz

1 megahertz = 1 MHz = 1,000,000 Hz For example: 100 complete sound waves enter your ear in a second (you'd hear a deep hum). f = 100 per second = 100 /s = 100 s-1 = 100Hz

• The speed of a wave (v) is just what it says. It's the

speed at which the vibrations in the wave move from one point to the next. Wave speed is measured in metres per second (m/s, ms-1).

• For example:

speed of sound in air = 330 m/s (approximate) speed of light in space = 300,000,000 m/sec = 3 x 108 meters per second http://www.bbc.co.uk/schools/gcsebitesize/physics/waves/waveequationsrev2.shtml

www.andor.com/image_lib

The incoming solar radiation (in watts per square meter, per micron of wavelength) outside the atmosphere (upper solid curve) and at the ground (lower solid curve) under typical atmospheric conditions. The horizontal axis is wavelength of the light in microns (millionths of a meter).

(this comes from Planck’s law for radiation of a ‘black body’ as a function

• f wavelength

and temperature) Wien’s law: peak radiation occurs at a wavelenth

• f 2897/T (in microns...10-6 m, and T in degrees Kelvin)

The total of this radiation, summed

• ver all frequencies is equal to

σT4 where σ is called the Stephan Boltzmann constant, and the peak of the curve B

• ccurs at a wavelength which varies

inversely with the temperture as you see in the figure at right. Here σ = 5.67 x 10-8 watts per meter squared, per degree Kelvin written as w m-2K-1

To make some plots from Planck’s law (previous page) you can go to http://csep10.phys.utk.edu/guidry/java/planck/planc k.html and select a temperature by sliding the slide-bar.

The temperature of a hot object determines the color (wavelength) of its

• radiation. Here the colors of various stars correspond to the peak in

these curves of radiation intensity (note that the twinkling of stars in the night sky is due to irregularities in the air temperature…refraction).

solar radiation ….

arriving at the top of the atmosphere and at the Earth’s surface

• W. Connelly HADCM3 data Wikipedia

Notice that sunlight observed at the ground has some dark lines in its spectrum… some specific colors (wavelengths) are blocked by water vapor, ozone…

short waves long waves Isaac Newton’s experiment with a glass prism showed sunlight splitting into a rainbow of colors. This is because the speed of light is slower inside the

• glass. Wavefronts of light slow down in glass, bending the rays. The amount
• f slowing varies with wavelength. Hence light rays with different wavelengths

(i.e.,different colors) are split apart, making a rainbow as above. 400 nm 500 nm 700 nm

http://bass2000.obspm.fr/solar_spect.php

sunlight enters the atmosphere with much energy in the visible wavelenghts Earth radiates at a lower temperature and hence at longer wavelength…in visible infrared

• r ‘heat’

radiation

Hartmann, Global Physical Climatology, Academic Press 1994

Infrared radiation upward from the Earth, assuming a 280K temperature red+yellow+blue = total radiation of the earth at +7° C in the range between 400 and 1800 cm-1. 10 microns (10 millionth of a meter) appears here as 1000 cm-1 blue = radiation that is absorbed by greenhouse gases. yellow = radiation that is allowed to pass by greenhouse gases. (red = absence of an absorption spectrum due to technical reasons concerning the measurements.)

http://www.espere.net/Unitedkingdom/water/uk_watervapour.html

contributions of various gases to the greenhouse effect (other than water vapor which has the greatest effect)

solar radiation (kilowatt-hours per square meter, per day) varies with latitude and season (here neglecting the great effect of cloudiness)

www.fao.org/DOCREP/003/X6541E/X6541E03.htm

Australia, with few mountains, cannot catch the moisture from the sea and has been experiencing severe drought. This is expected to worsen with global warming

The air temperature on 2 Jan 1993 at the surface of the Earth. The cold (blue) air forms a dome in the Arctic, which is dense (heavy) and tends to slide southwater beneath less dense air. This leads to both the overturning circulation and the westerly winds and jet stream.

The air temperature on 2 Jan 1993 at the surface of the Earth. The cold (blue) air forms a dome in the Arctic, which is dense (heavy) and tends to slide southwater beneath less dense air. This leads to both the overturning circulation and the westerly winds and jet stream.

today in Iqaluit (the capital of Nunivut, the new Canadian territory), T = -150C (= -50F) to +50F

reflected sunshine (upper) and infrared (long-wave) ‘heat’ radiation (lower) CERES satellite radiometer Jan 2002

clouds, snow, ice, deserts are bright absorbing areas are dark..roughly 30% of incoming radiation is reflected heat emitted from Earth is proportional to the 4th power

• f temperature

source: Wikipedia Incoming 342 watts/m2 reduced by simple reflection (albedo ~ 0.3) to 235. Human modification due to gas emissions/deforestration ~ 2.5 watts/m2

incoming solar radiation, I some is simply reflected back to space

the rest is absorbed by ocean and land and atmosphere yet re-radiated as infrared heat, both upward and downward (red arrows)

Earth’s surface I I I 2I The net effect of the ‘blanket’ of atmosphere is to have more downward radiation toward the Earth’s surface than just the incident sunlight.