9/22/99

What Happens to Solar Radiation As It Passes Through The Atmosphere?

To answer this question, we have to first describe exactly what solar radiation is. It is the energy radiating out from the sun, traveling through the vacuum of space until it reaches the earth. Solar radiation is a form of electromagnetic radiation, which means it has both electrical and magnetic properties. One of these properties is that if you line up a number of positive electric charges and expose them to electromagnetic radiation, the charges will start to wiggle around. One way to describe radiation is by the wavelength which these wiggling charges exhibit (see Figure 3.15 in Danielson). For our purposes, we will measure wavelength in microns, which are millionths of a meter.

Electromagnetic radiation from the sun (or "solar radiation" or "sunlight") is made up of a variety of different wavelengths. How radiation interacts with matter depends on the wavelength, so we classify radiation according to its wavelength (See Figure 3.18). For example, blue light is a form of electromagnetic radiation with a wavelength of about 0.5 microns; red light has a wavelength of about 0.7 microns. Ultraviolet light put out by the sun (which can give you a nasty sunburn) has wavelengths of about 0.1 to 0.4 microns. Radiation at wavelengths from 0.7 to about 10 microns is called infrared radiation, and we can feel some of this as heat. Microwaves and radio waves have wavelengths of several meters (one million times longer than visible light!)

Electromagnetic radiation can also be thought of a stream of energetic, massless particles traveling through space at the speed of light - photons. The amount of energy each photon carries is indicated by its wavelength. Shorter wavelength photons carry more energy, longer wavelength photons carry less energy. Figure 3.18 shows the relationship between a photon's energy, its wavelength, and the class of radiation it represents. Note that the figure also has a temperature scale. Why? (This will become apparent when we cover blackbody radiation.)

When solar radiation passes through the atmosphere, three things can happen: it can get absorbed, it can get reflected, and it can get scattered.

• Absorption: A process in which a portion of sunlight (i.e. a photon) is lost, and it's energy is converted to another form. One such process is dissociation; another is excitation.
  
  
  • Dissociation (also called photolysis): If a photon's energy is large enough (i.e. if it's wavelength is short enough), it can break apart the molecular bonds of individual air moelecules. A good example is when ultraviolet radiation from the sun breaks apart a molecule of oxygen (O2) into two atoms of oxygen. The photon's energy is converted into the kinetic energy of the two atoms, which raises the temperature of the atmosphere (why?). A similar process involving ulatraviolet radiation and ozone (O3) is an important heat source in the stratosphere (10-50 km altitude).
    
    
  • Excitation: Atoms and molecules can absorb photon's at particular wavelengths and not break apart. Instead the photon's energy goes into changing the configuration of the atom or molecule (this includes the orbits of the electrons around an atom, or the rotational and vibrational motion of the molecule). When an atom of molecule is in this altered state it is called "excited" - this excited state represents a higher energy state. At some later time, the atom or molecule will emit a photon having the same wavelength (and thus the same energy) as the absorbed photon, and in doing so, the atom or molecule returns to the ground, or low energy, state. This process of emission will be discussed further as it applies to solid, liquids, and gases.
    
    
• Reflection: This refers to the re-direction of solar energy as it encounters a surface (i.e. solid surface or liquid water surface). At the interface between the air and the surface, a portion of the sunlight will pass through to be absorbed or transmitted, and a portion will will be reflected (i.e. redirected back into the direction it came from). Liquid water droplets in clouds reflect a great deal of incoming sunlight, which explains clouds' bright white appearance. Snow and ice are examples of surfaces that also reflect a great deal of sunlight. Albedo is a measure of how reflective a surface is. It is the fraction of incoming light that is reflected or otherwise redirected back, so it ranges in value from 0 to 1. A bright white surface like snow has a high albedo (close to 1). The albedoes of various substances are shown in Table 3.1 on page 78 of DanielsonI/it>.
  
  
• Scattering: This describes the re-direction of sunlight by very small particles in the atmosphere (e.g., air molecules or very small dust particles having sizes of millionths of a meter). These small particles scatter sunlight having shorter wavelengths more efficiently than sunlight at longer wavelenghts. Sunlight in the visible portin of the spectrum ranges from violet (short wavelengths) to red (long wavelengths). The violet and blue wavelengths, begin the shortest, are scattered the most. This gives our sky its overall blue color. At sunrise and sunset, sunlight travels through a much longer path in the atmosphere. In this case, more of the longer wavelength light becomes scattered, giving the yellow, orange and red skies we see at these times.

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9/22/99

Solar Radiation

It is common to classify radiations of different wavelengths into groups based on how it gets scattered, reflected, and absorbed by our atmosphere.

• "Shortwave" ---> refers to solar radiation at the short wavelenghts, i.e. ultraviolet (wavelengths of 0.1 to 0.4 microns) and visible (0.4 to 00.7 micron) light. Shortwave radiation gets absorbed, reflected and scattered in the earth's atmosphere.
• "Longwave" ---> refers to radation in the infrared portion of the spectrum, between roughly 0.7 and 10 microns. Longwave radiations mostly gets absorbed (and emitted) in the earth's atmosphere. Its doesn't get scattered or reflected like shortwave radiation.

So What Happens to Solar Radiation As It Passes Through The Atmosphere?

If we assume that the insolation at the top of the atmosphere represents 100 "units" of energy, then that 100 units gets distrubuted as follows:

	Scattering back to space		=  6 units
	Absorption by air molecules		= 16 units
	Reflection by clouds			= 20 units
	Absorption by clouds			=  4 units
	Reflection by the earth's surface	=  4 units

Subtracting these losses means that, on average, only about 50 units of the 100 incoming units will be absorbed at the earth's surface. Note that the albedo of the earth/atmosphere combined is about 30 units (20 due to clouds, 6 by molecular scattering, and 4 by surface reflection). Now the earth's surface albedo is not the same everywhere - the snow covered poles have a higher albedo than the continents or the oceans. These numbers represent global averages.

The 50 units of energy absorbed at the earth's surface goes into heating the surface. How much heating that actually occurs depends on the type of surface. This is determined by the specific heat of a surface. The definition of specific heat is the amount of energy (in calories) required to raise the temperature of a substance one degree Celsius. Table 3.2 on p. 79 in Danielson give the specific heat for various substances. Note that water has a high specific heat - i.e. it takes more energy to raise the temperature of water than any other substance. This means water has a moderating effect on climate - it won't heat up a lot in sunlight, and it won't cool down a lot in its absence (mixing is also important here).

9/24/99

Absorption and Emission of Electromagnetic Radiation

As mentioned above, an atom or molecule may absorb a photon with a given wavelength, and then emit a photon at some later time having the same wavelength. [Remember that shorter wavelength photons carry more energy than longer wavelength photons.]

From above, you can see that about half of the sun's energy passing through the top of the atmosphere gets absorbed at the earth's surface (50 of the original 100 units). The earth's surface then emits radiation upward as a blackbody. Remember that the sun emits electromagnetic radiation over a wide range of wavelengths, from ultraviolet to visible to infrared. The earth therefore absorbs and emits radiation over a wide range of wavelengths as well. The radiation emitted by a solid (like the earth's surface), a liquid, or a dense gas is described as blackbody radiation. The term "blackbody" means you assume that the object is a perfect absorber and emitter of radiation. In other words, it will absorb all the radiation falling on it and then emit all the radiation it absorbs, with no losses.

Characteristics of blackbody radiation:

• Radiation is emitted over a wide range of wavelengths (this is called a continuous spectrum); Figure 3.19 in Danielson shows the continuous spectrum of emission for a blackbody at room temperature, at 1000 degrees Kelvin, and at 5800 degrees Kelvin (equivalent to the surface temperature of the sun).
  
  
• The amount of radiation emitted varies with wavelength; the wavelength at which the most energy is emitted depends on the object's temperature (Wien's Law). As an object grows hotter and hotter, the wavelength of maximum energy emission grows shorter and shorter. In the case of a room temperature object in Figure 3.19, most of the energy is emitted at a wavelength of 10 micrometers. An object with a temperature of 5800 degrees Kelvin emits most of its energy at 0.5 micrometers (the wavelength of green light in the visible portion of the spectrum). The wavelength of peak emission is inversely proportional to the object's temperature.
  
  
• The total amount of energy emitted by a blackbody is found by adding up the energy emitted at each wavelength across this continuous spectrum. In mathematical terms, this is equivalent to finding the area under the blackbody curve in Figure 3.19. The Stefan-Boltzmann formula tells us that the total amount of energy emitted by a blackbody is directly proportional to the temperature of the object raised to the fourth power. If you double an object's temperature, it will emit 16 times as much energy (2 to the fourth power equals 16).

These characteristics listed above only apply to solids, liquids, or dense gases. Our atmosphere does not qualify as a dense gas, so it does not emit radiation as a blackbody. Instead of emitting radiation over a broad range of wavelengths, the gases in our atmopsphere only emit (and absorb) radiation at specific wavelengths. Instead of following a continuous spectrum, we say these gases exhibit a line spectrum. If we plot the amount of energy emitted as a function of wavelength for such a gas, it would look like a line (see Figure 3.20). Another way to describe this is to say that the gases in our atmosphere are selective absorbers and emitters of radiation - they only absorb and emit radiaion at selected wavelengths. The particular wavelength depends on the internal structure of the gas molecule.

Gases that are particularly strong absorbers and emitters of infrared radiation are known as greenhouse gases. Water vapor, carbon dioxide, methane, and chlorofluorocabons (CFC's) are the most important greenhouse gases in the earth's atmosphere. They allow shorter wavelength solar radiation radiation to pass through the atmosphere and heat the earth's surface, but they absorb the longer wavelength radiation emitted upward by the earth. Some of the energy absorbed by these gases is emitted back down towards the earth. This additional energy source keeps the surface about 35 degrees Celsius (63 degrees Fahrenheit) warmer than it would be if there were no atmosphere.