The sun as a source of Energy

1. The sun as a source of Energy Atmospheric Processes

2. Key questions… • What is the importance of the sun for life on Earth? • What forms of radiation are emitted from the sun? • What are the characteristics of insolation and factors which causes it to change?

3. The Sun • Yellow star – radiating energy for approximately 5 billion years • 70% Hydrogen, 28% Helium, 2% heavier ‘metals’ (oxygen, carbon, neon, iron) • Nuclear fusion – heat and light the earth • Sole input to energy flows within the atmosphere is electromagnetic radiation emitted from the Sun

4. Nuclear Fusion atmosphere Sun Surface temp = 6000°C Hydrogen atoms fuse → Helium + energy = Nuclear Fusion 99% released as short-wave radiation – travelling at 297,600 km s Earth – surface temp = 14°C 148,800,000 km Note: protons overcome their mutual electric repulsion releasing energy – further reading: Nuclear force

5. Other inputs of heat energy to the atmosphere • Negligible inputs of heat energy are supplied from: • geothermal sources • volcanic eruptions • hot springs • anthropogenic sources • domestic heating • industry

6. Electromagnetic spectrum • All objects with a temperature above absolute zero (–273o C or 0 K) emit radiation in all directions as electromagnetic waves travelling at the speed of light (3 x 108 m s-1) • Electromagnetic waves are spread across a range of wavelengths – the electromagnetic spectrum • Wavelengths vary in size from <0.0001 µm (gamma rays) to several km (long-wave radio) • 1 µm (micron) = 1/1000th mm

7. Electromagnetic spectrum Hotter object • Amount and type of radiation emitted by an object is dependent on the object’s temperature • A hotter object will emit a greater amount of radiation and at a shorter wavelength than a colder object Colder object X-axis = Wavelength Y-axis = Amount of radiation

8. Solar radiation • Emitted over a wide range of wavelengths – solar spectrum • 99% - short-wave radiation with maximum output within the visible part of solar spectrum • Total amount of solar radiation reaching top of atmosphere = 1366 W m-2, but amount reaching Earth’s surface = c.338W m-2 (c.25%)

9. Absorption of solar radiation by atmosphere • UV radiation • Almost completely absorbed by ozone (O3) in stratosphere • Visible light • Atmosphere is largely transparent • Solar radiation passing through troposphere is • Scattered • Reflected • Absorbed

10. Terrestrial radiation • Earth emits radiation – long-wave radiation • Maximum output within the infrared part of spectrum

11. Absorption of terrestrial radiation by atmosphere • Atmosphere is only partially transparent to long-wave radiation from Earth’s surface • c.94% is absorbed atmosphere, including: • Water vapour (H2O) • Carbon dioxide (CO2) • Part of this is radiated back to Earth’s surface (counter-radiation) – raising surface temperature by c.38OC • c.6% escapes directly to space

13. Earth’s Energy Budget – Effect of cloud cover

14. The Greenhouse Effect

16. Earth’s Energy Budget – Variations with latitude • Equator to 35O N & S – Incoming short-wave radiation (solar radiation) exceeds outgoing long-wave radiation (terrestrial radiation) • Polewards of 35O N & S – Long-wave exceeds short-wave radiation • Continual poleward transfer of energy from area of excess to area of deficit via general circulation of atmosphere (70%-90%), ocean currents (10%-30%) and storms.

17. Earth’s Energy Budget – Variations with time of year

18. Green Flash

19. Green Flash • Rare optical phenomena that occur at sunset and sunrise • Appears as green spot visible for 1-2 seconds • Observed at low altitudes with unobstructed view of horizon (e.g. ocean) • Caused by refraction of light – blue light is dispersed and only green light remains visible Green Flash

20. Laws of electromagnetic radiationStefan-Boltzman Law • “The radiation flux (amount of radiation emitted per unit surface area) of an object is proportional to the 4th power of its absolute temperature” • E = x T4 E = Radiation flux (amount of radiation emitted)  = 5.67 x 10-8 W m-2 K-4 (Stefan-Boltzman constant) T = Absolute temperature of an object • Hotter objects emit more electromagnetic radiation per unit surface area

21. Laws of electromagnetic radiationWien’s Displacement Law • “The wavelength of peak intensity of radiation emitted by an object is inversely proportional to its absolute temperature” • peak =  ÷ T peak = Wavelength of maximum intensity of radiation emitted  = 2898 (Wien constant) T = Absolute temperature of an object • Wavelength of peak intensity of solar radiation is within visible part of electromagnetic spectrum: T = 6000 K peak = 0.483 μm (483 nm)

22. Sun v. Earth

23. Laws of electromagnetic radiationInverse square Law • “The amount of radiation reaching an object is inversely proportional to the square of the distance of that object from the radiation source” • 1 ÷ d2 d = Distance between radiation source and object receiving that radiation • Amount of solar radiation received at the top of Earth’s atmosphere (150 million km from Sun) = 1366 W m-2 • Known as the solar constant

24. Changes in the Solar Constant “The number of polar bears, the length of women's skirts, the stock market: Everything imaginable has been correlated with the solar cycle” 16 Jan 2009 14 May 2013 29 Mar 2001 RMetS Presidential Address 15 May 2013