geography worksheet 2
http://www.ces.fau.edu/nasa/mod ule-2/how-greenhouse-effect- works.php
This figure shows the blackbody spectra of Earth and sun. The incoming radiation from the sun is much more intense (Y-axis) than that of outgoing radiation from the Earth because the energy emitted from a blackbody is proportionate to its temperature to the fourth (σT4) – i.e. the sun emits a far greater amount of energy than the Earth. Incoming solar radiation is shortwave (X-axis, wavelength in microns) and in the wavelength range of ultraviolet and visible radiation (shown as the rainbow spectrum of colors). Outgoing Earth’s radiation is long wave and and is in the range of infrared radiation (shown in red).
Below the blackbody spectra, molecules in the atmosphere, known as greenhouse gases, interfere with incoming and outgoing radiation. For instance, ozone (O3) in the stratosphere absorbs some of incoming radiation and is known as the ozone layer. That said, greenhouse gases (N2O, O3, CO2, and H2O) mainly interfere with outgoing radiation.
Let’s talk about the molecular motion of these greenhouse gases to understand the greenhouse effect.
Molecular Motions and the Greenhouse Gases H2O and CO2
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Here are the physical causes (molecular motion) of the greenhouse effect. But first… it may be a bit chunky, so sit back, take a deep breath!
Gas molecules can absorb or emit radiation in the infrared range in two different ways. One way is by changing the rate at which the molecules rotate. The theory of quantum mechanics describes the behavior of matter on a microscopic scale – that is, the size of molecules and smaller. According to this theory, molecules can rotate only at certain discrete frequencies as if vibrations of a piano string in that they tend to be at specific “ringing” frequencies. (The rotation frequency is the number of revolutions that a molecule completes per second.) The molecule can absorb incident wave (energy), if this incident wave has just the right frequency.
This frequency of the radiation that can be absorbed or emitted depends on the molecule’s structure. The H2O molecule is constructed in such a manner that it absorbs infrared radiation of wavelengths of about 12 micrometers and longer. This interaction gives rise to a very strong absorption feature in Earth’s atmosphere called the H2O rotation band. As shown in the previous slide, virtually 100 % of infrared radiation longer than 12 micrometers is absorbed with a combination of CO2 and H2O.
(By the way, the H2O rotation band extends all the way into the microwave region of the electromagnetic spectrum, i.e. above a wavelength of 1000 micrometer, which is why a microwave oven is able to heat up anything that contains water.)
Molecular Motions and the Greenhouse Gases H2O and CO2
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The second way in which molecules can absorb or emit infrared radiation is by changing the amplitude at which they vibrate. Molecules not only rotate, they also vibrate – their constituent atoms move toward and away from each other. As shown in the lower figures, The molecular structure of water is electrically lopsided; a molecule is bent to its lowest energy state. This is because oxygen has two pairs of electrons hanging off it, which push the hydrogen toward the other side (Mickey Mouse structure!). Hydrogen atoms hold their electrons more loosely than oxygen atoms in chemical bonds, so each hydrogen has a slightly positive charge. The oxygen end of the molecule has a slight negative charge. Thus, water has a dipole moment built into its resting structure. Rotating an H2O molecule would oscillate the electric field and generate light. Due to the complex arrangement of the nuclei in H2O, there are many modes of vibration for the water molecule, including a symmetric stretch and a bend.
The CO2 molecule can vibrate in three ways. The bending mode of vibration (upper figure). This vibration has a frequency that allows the molecule to absorb infrared radiation at a wavelength of about 15 micrometers, which gives rise to a strong absorption feature in Earth’s atmosphere called the 15-micrometer CO2 band. Also, similar to a H2O molecule, the oxygen of a CO2 molecule tends to pull on electrons more tightly than carbon does, but the oxygen atom on one side pulls the electrons just as tightly as the other oxygen on the other side. Therefore, the molecule has no permanent electrical field asymmetry (dipole moment). This imbalance makes CO2 an important one for our climate. In fact, most gases in the atmosphere do not absorb or emit infrared light at all (e.g. N2). Why? Because vibrations in their bonds do not create an imbalance in the electrical field.
Molecular Motions and the Greenhouse Gases H2O and CO2
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What does all of this information mean? Your take home note is…. in order for gas molecules to interfere with electromagnetic energy (to emit or absorb infrared light); 1) frequency of the molecular vibration must be equal to the frequency of the
light (only a specific frequency of light can cause a specific molecular vibration!), and
2) the molecule must be electronically lopsided.
I am sharing a Youtube video that is very well made and that allows us to visually perceive these molecular motions. Please see a following slide.
Youtube video - https://youtu.be/3ojaDMadZXU
Please view this Youtube video to further your understanding of molecular motion. Particularly, the part from 2:40 to 4:47 is relevant to this lecture.
NASA, Robert Rohde - http://earthobservatory.nasa.gov/Features/EnergyBalance/page7.php en:NASA Earth Observatory
Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths. Water vapor is naturally electrically lopsided and can absorb and emit lots of frequencies of infrared light. Interesting about H2O is that not only is it a greenhouse gas, when we increase the surface temperature, more water will evaporate, which significantly increase the amount of water vapor in the atmosphere. Interestingly, this then makes H2O the most concerning greenhouse gas with greater uncertainty.
Carbon dioxide is not as strong a greenhouse gas when compared to water vapor, but it absorbs energy in wavelengths (12-15 micrometers) that water vapor does not. This is an important wavelength range because it is close to the peak intensity of outgoing radiation (thus effectively absorbs outgoing energy).
(Illustration NASA, Robert Rohde)
https://cimss.ssec.wisc.edu/sage/meteorology/lesson1/AtmAbsorbtion.htm
Atmospheric Absorption of incoming shortwave and outgoing longwave radiation
Total atmospheric absorption is indicated by the bottom row. The white areas indicate regions of the electromagnetic spectrum not affected (low absorption) by the atmosphere where solar radiation can reach the Earth's surface and terrestrial radiation can escape out to space. Note the prominent role that water vapor has in absorbing Earth's long wave radiation.
Outgoing spectrum of the Earth With an atmosphere
Okay – this is the last figure of today’s lecture.
In this figure, smooth curves show blackbody spectra for temperatures ranging from 300 K, surface temperature on a hot summer day, down to 220 K, which is about the coldest it gets in the atmosphere, up near the troposphere at about 10- km altitude. There is also a jagged-looking curve (denoted as “Atmosphere”) moving among the smooth ones. This is a model-generated spectrum of the infrared light escaping to space at the top of the atmosphere. This is jagged- looking because CO2, water vapor, ozone, and methane absorb specific wavelengths of outgoing energy emitted from the ground.
So, what would the Earth’s surface temperature look like from space if the Earth had no atmosphere? – Without an atmosphere, more energy will be radiated due to a lack in the greenhouse effect. In fact, the outgoing spectrum will look like a blackbody spectrum for 270 K (= -3 C�, 26.6 F), between the 260 K and 280 K spectra shown in figure. Compare this with the mean surface temperature described in slide #12. Blackbody spectra of Earth temperature is 255 K!
August, 2016 https://eos.org/research- spotlights/which-greenhouse-gas-does- the-most-damage-to-crops