Radiative Processes

The next lectures are chiefly on solar radiation.

Electromagnetic radiation

All objects that are warmer than zero Kelvin, radiate electromagnetic (EM) radiation is emitted by all objects that are warmer than zero Kelvin.

Gamma rays, X-rays, ultra-violet, visible light (all colours that you can see),infra-red light, microwaves and radio waves....ALL these are electromagnetic waves.

EM waves are so categorised according to their wavelength.

The wavelength is the length of one complete wave, e.g. The length from crest to crest (or trough to trough).

The frequency of a wave is the number of crests (troughs) that passes by in a given interval.

All wavelengths of Electromagnetic waves travel at the same speed. This is the speed of light which is 300.000 km/s in vacuum.

The frequency of the waves is inversely related to wavelength.

Long wavelength implies low frequency.
Short wavelength implies high frequency

The higher the frequency of an EM wave, the more energetic it is.

Special interest: UV, Visible & IR

Three types of EM waves are especially important in this course:

Ultra violet light (UV rays for short)
Visible light (violet, blue, green, yellow, orange, red)
Infra-red light (IR for short)

UV rays are too energetic for our skin. We can only tolerate UV in small doses. Fortunately the ozone layer screens outmost of the UV radiation.

Visible light is the light that we normally see...all the different colours

Infra-red light is often called heat-radiation. We cannot see it,but we can "feel" it. A hot stove emits a lot of IR. The earth emitts IR to space (This is also called Thermal radiation)

The electromagnetic spectrum emitted bya radiating object

If you measure the EM radiation that a "hot" object emits, you will find that it emits over a range of wavelengths (or frequencies).

The intensity will usually be small for some short wavelength, and then increase with wavelength until a wavelength of maximum intensity of radiation is reached. For wavelengths higher than that the intensity decreases with wavelength.

The curve you would get from your measurements would be "a single hump curve"

Its shape can be explained with a radiation law called Plancks law.

Radiation Laws

The EM spectrum emitted by objects in thermal equilibrium(not undergoing any reactions) is described by Plancks law.

For an object at a given temperature Plancks law gives theintensity as a function of wavelength.

Plancks law also explains how the intensity of the radiationdepends on the temperature of the object.

Strictly speaking the law is only valid for Black bodies.

The earth and the sun are close to being black bodies,although neither is black...the name applies to an object that absorbs all radiation incident on it at every wavelength.

Radiation Laws II

Plancks law is usually explained with two special cases, Wiens displacement Law and the Stefan-Boltzmann Law (those were initially found experimentally).

Wiens law:
If the objects temperature is T, then the wavelength of maximum intensity of the emitted radiation will be:

L= Constant / T

(The constant is about 2.9 *10-3 m K, remember that T is in Kelvin degrees!!!)

Stefan-Boltzmann Law:
The total amount of energy radiated from an object oftemperature T is given by

Total energy radiated = Constant * T⁴

(The constant is by convention refferred to as the greek letter "sigma" and equals 5.67 * 10-8 W/(m2 K4) And again the T is measured in Kelvin degrees!)

Examples: Solar and Terrestrial radiation

The temperature of the sun's surface (the photosphere) isabout 5800K.

Hence the intensity peaks at a wavelength of about0.5 micro meters which lies is visible light.

The surface of the earth has a surface temperature ofabout 288 K (15 C)
Hence the intensity peaks at about 10 micro meters(which is in the so called "Long Wave" part of the spectrum)

Try using Wiens displacement law to verify these results.

Obviously the sun is much hotter than the earth...how much more radiation does it emit?

Using Boltzmanns law we can see that the sun emits 160.000 times more energy than the earth

How is the earth heated up?

The sun heats up the earth by bathing it in EM radiation.

The earth absorbs much of the radiation that the sun shineson it, and this heats up the earth.

But the warmer the earth is, the more heat radiation it selfwill emit.

An equilibrium is reached when the earth is radiating intospace as much as it receives from the sun.

We will now discuss in detail these two factors:

The input of solar radiation

The terrestrial radiation

The insolation

The incoming solar radiation is referred to as the insolation.

The intensity of direct sunlight at the top of theatmosphere is 1380 W/m2 which is often referred to as the solar constant

How much insolation a certain area on the surface will get depends on the latitude. This happens in two ways:

The intensity of the sunlight goes down as the sun is lower on the horizon.
When the sun is lower on the horizon it has to traverse a "thicker atmosphere" (the optical pathlength increases). This increases the distance over which sunlight can be scattered back into space. Thus the albedo of the atmosphere is larger when for higher latitudes, because the sun is lower on the horizon in those latitudes.