week 1 Flashcards
(15 cards)
what is electromagnetic radiation
- light and other electromagnetic waves transmit energy through space
- they consist of jointly oscillating electric and magnetic fields
- EM radiation always travels at the speed of light (c)
- c = 3*10^8 ms-1
equation describing EM wave propagation
- c = lambda * f
- wave speed (c) - speed of propagation (ms-1)
- wavelength (lambda) - distance between wave peaks (m)
- frequency (f) - number of waves per second (s-1)
- wave number (v) - waves per unit distance (m-1)
describe the electromagnetic spectrum
- visible light is only a tiny fraction of the EM spectrum
- the greater the frequency (the smaller the wavelength) the greater the energy
- EM radiation interacts most strongly with objects around its wavelength
describe black body radiation
- all objects emit EM radiation known as black-body radiation
- the warmer an object is:
- the shorter the wavelength (higher the frequency) of peak emission
- the more energy it emits
what is Wien’s displacement law
lambdamax = b/T
b = 2.0 x 10^-3mK (Wien’s displacement constant)
the wavelength of peak thermal emission (lambdamax) is inversely proportional to the temperature of the object
kelvin vs celcius
melting point of ice:
0 degrees celcius = 273.15K
absolute zero:
0K = -273.15 degrees celcius
what is the Stefan-Boltzmann law
j* = sigmaT^4
sigma = 5.67 x 10^-8 Wm^-2K^-4 (Stefan-Boltzmann constant)
the power per unit area (j*) of thermal emission from a surface is proportional to the fourth power of temperature
the sun and earth as idealised black-bodies
the sun’s photosphere has a temperature of ~6000K
lambdamax = b/T = ~480nm
the earth’s atmosphere has a temperature of roughly -20 degrees celcius
lambdamax = b/T = ~11um
describe shortwave and longwave radiation
- as the spectrum of incoming and outgoing radiation from Earth are distinct, we label them:
- shortwave radiation = visible light, plus UV and near-IR
- longwave radiation radiation = thermal or infrared radiation
what is insolation
- the power of incoming solar radiation per unit area (Wm^-2) (integrated over a specified time)
- the sun emits energy equally in all directions. equal energy passes through the surfacce of any sphere centred on the sun.
- area of a sphere = 4piR^2
- insolation drops as the area of the sphere does i.e. it follows the “inverse square law”
- insolation(Is) is proportional to 1/R^2
incoming light at the earth
- the average insolation at the top of Earth’s atmosphere (Is) is 1368 Wm^-2. this is the solar constant
- the earth is a sphere so it casts a circular shadow
- area of circle = pir^2
- the total energy flux (W) on earth : Insolation (Wm^-2) x area (m^2)
- F solar,in = Is x pir^2
what is albedo
- not all light reaching the Earth is absorbed, some is reflected
- albedo is the fraction of light that is reflected by a body
- earth’s albedo is ~0.3
- albedo varies as a function of angle and wavelength
assumptions about the Earth
- a sphere
- a perfect black-body
- has no atmosphere
- has a uniform surface temperature
energy budget of the pale blue dot
- if a system is in equilibrium, it is unchanging: fluxes in = fluxes out
- for the energy budget of a planet we have:
- absorbed solar insolation (W) = outgoing thermal radiation (W)
- insolation (Wm^-2) x fraction absorbed x area of shadow (circle)(m^2) = outgoing black-body radiation (Wm^-2) x area of planet (sphere)(m^2)
Is x (1-a) x pirp^2 = sigmaTp^4 x 4pirp^2
assumptions about the atmosphere of the earth
- a single layer
- transparent to shortwave
- fully absorbing of longwave
- has a uniform temperature
