Part III – Energy Flow
The Sun (our source of energy)
All of the energy available on Earth comes to us from the Sun.
Facts about the Sun:
Size: 110 x Earth (diameter)
Mass: 333, 000 x Earth
Temp: 6000^0 (Surface)
Composition: 90% hydrogen
Energy Source: Hydrogen fusion
The Suns power output varies continuously, but with some regularity.
- 11-year sunspot cycle is correlated with solar output.
- Brightness (output) is on a long term increasing trend; has probably risen by 25% since
the protestant phase
- When the supply of hydrogen to be “burned” runs out, the sun will die out and become a
white dwarf….. in about 5 billion years.
Earth (our home)
Distance from Sun: 150 000 000 km
Diameter: 12.745 km
Mass: 5.9 x 10^24 kg
Age: 4.6 billion years Lithosphere: Solid rock crust floating on a liquid interior.
Hydrophere: Temperature allows liquid water (71% of surface)
Atmosphere: Mainly nitrogen (78%) and oxygen (21%)
Solar Energy Received:
The Earth intercepts only a tiny fraction of the Suns total power output: An average of about
1360 W/m² (the solar insolation) at the top edge of the atmosphere.
Most of this energy is delivered in the form of visible light.
The amount of solar energy that reaches the surface and is absorbed by the planet also
- Chemical composition of the atmosphere
o How much power is reflected away from the Earth (or any planet) as a fraction of
the incident power. The albedo depends on the nature of the surface and of the
Albedo = (Power Reflected)/(Power Received)
High albedo more power reflected less power absorbed
cooler (all else being equal)
Clouds and ice have a high albedo; water, rocks. And forests have a low albedo.
Earths average albedo: about 30%
Chemical Composition of the Atmosphere
A given gas allows light to pass through it at some frequencies but strongly absorbs light
at other frequencies.
Some atmospheric gases absorb strongly in the same infra-red part of the electromagnetic
spectrum in which the surface of the Earth re-emits received solar energy as heat. Greenhouse Gasses
Eg. Water vapor, carbon dioxide, methane, ….
Ozone in the upper atmosphere strongly absorbs light at ultra violet frequencies
The Greenhouse Effect:
Without greenhouse gasses the average surface temperature would be about 33
degrees colder than it actually is.
Because of greenhouse gasses in the atmosphere the Earth must raise its surface
temperature by about 33 degrees to a