The sun as a source of Energy

Atmospheric Processes

Key questions…

1. What is the importance of the sun for life on Earth?

2. What forms of radiation are emitted from the sun?

3. What are the characteristics of insolation and factors which causes it to change?

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

Nuclear Fusion

148,800,000 km

Earth – surface temp

= 14°C

atmosphere

SunSurface temp = 6000°C

Hydrogen atoms fuse → Helium +

energy = Nuclear Fusion

Note: protons overcome their mutual electric repulsion releasing energy – further reading: Nuclear force

99% released as short-wave radiation

– travelling at 297,600 km s

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

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

Electromagnetic spectrum

• 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

Hotter object

Colder object

X-axis = Wavelength

Y-axis = Amount of 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.338 W m-2 (c.25%)

Solar radiation

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

• Earth emits radiation – long-wave radiation

• Maximum output within the infrared part of spectrum

Terrestrial radiation

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

Earth’s Energy Budget – Effect of cloud cover

The Greenhouse Effect

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.

Earth’s Energy Budget – Variations

with time of year

Green Flash

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

Laws of electromagnetic radiation

Stefan-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

Laws of electromagnetic radiation

Wien’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)

Sun v. Earth

Laws of electromagnetic radiation

Inverse 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

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”

RMetS Presidential Address 15 May 2013

16 Jan 200929 Mar 2001 14 May 2013