At the completion of this section, you should be able to describe what is meant by "electromagnetic radiation" and how it is generated. You should also be able to explain the various types of electromagnetic radiation, specifically the portions of the electromagnetic spectrum that meteorologists use to observe the atmosphere.
If we're going to talk about remote sensing, we have to start by talking about radiation. While the mention of "radiation" may conjure up thoughts about nuclear reactors or nuclear bombs, it turns out that the scientific use of the term "radiation" is considerably more broad. Radiation is defined as the emission and transfer of energy via high-energy particles (photons) or electromagnetic waves. In fact, the vast majority of radiation that you encounter on a daily basis has nothing to do with nuclear radiation at all. From an everyday light-bulb, to the microwave that heats your frozen lunch, to the radio that you listen to on your morning commute, you're surrounded by devices that make use of radiation. Even light from the sun is a form of radiation, so radiation is occurring all around you!
At some point in a science class, you probably studied the electromagnetic ("EM") spectrum of radiation, but how is this electromagnetic spectrum created? To begin with, you probably know that the building blocks of all matter are atoms and molecules. Within these atoms and molecules are smaller particles which have positive and negative charges -- protons and electrons, respectively. These charged particles tend to oscillate or vibrate (especially electrons). Without getting into the details, physics tells us that any charged particle like an electron has an electrical field surrounding it (electrical charges and electrical fields go hand-in-hand). Furthermore, moving charges also possess magnetic fields. Thus, when an electron oscillates, its surrounding electric and magnetic fields change. Like moving your hand rapidly back and forth in a pool of water, oscillating electrons send out ripples of energy (that is, "waves") that have both electrical and magnetic properties (hence, electro - magnetic radiation).
So, how is it that different kinds of electromagnetic waves exist to create an entire spectrum? The wavelength of any wave is simply the distance between two consecutive similar points on the wave (for example from wave crest to wave crest). Now think about our pond analogy above. If you move your hand slowly in the water, you will create a few waves with long wavelengths. However, if you move your hand rapidly in the water, you create lots of waves with very short wavelengths. The same is true for an oscillating electron. If the oscillation is very quick (we say the oscillation has a high frequency), then the EM radiation produced will have a short wavelength. If the oscillation is slower (having a lower frequency) then the electromagnetic waves will have long wavelengths.
Now, the frequency at which electrons oscillate is essentially set by the temperature of the matter in which the electron resides (remember, we defined an object's temperature as the average kinetic energy of its atoms or molecules). The higher the temperature, the higher the frequency of oscillation. So, when temperature increases, the wavelength of the electromagnetic radiation emitted by the electron decreases. Conversely, as temperature decreases, the frequency of oscillation slows and the wavelength of the emitted electromagnetic radiation increases. For a visual, check out the short video below (0:57) demonstrating the relationship between oscillation frequency and wavelength.
Before leaving this discussion, let me add a quick caveat: we have discussed the generation of EM radiation by a single oscillating charged molecule. In reality, matter exists as a system of charged particles. This means that the resulting electromagnetic radiation field is much more complex than I have outlined here. Remember that earlier in the course we defined temperature as the mean motion of the molecules (and that not all molecules have the same energy state). This means than a spectrum of electromagnetic radiation is generated from any system of matter that contains many charged particles, all oscillating at different frequencies. I should also note that the vibrating molecule model for electromagnetic emission only explains the existence of low-energy waves (those having lower frequencies than visible light). High-frequency EM emissions are still generated by moving charges, but require a different mechanism to generate the high energy waves (there's more details in the Explore Further section below if you are interested.)
With that caveat out of the way, now look at the entire spectrum of electromagnetic radiation. First, note that the range in wavelengths for different types of electromagnetic radiation is staggering -- from hundreds of meters to the size of an atom's nucleus. Also note that visible light does indeed qualify as electromagnetic radiation, despite taking up only a tiny sliver of the entire spectrum. This means that human eyes are completely blind to almost all electromagnetic radiation (most wavelengths are invisible to the naked eye).
For atmospheric remote sensing, we use electromagnetic radiation in the microwave, infrared, and visible bands. Perhaps most familiar to you is the visible portion of the electromagnetic spectrum. Indeed, wavelengths of EM radiation that span from approximately four tenths of a micron (a micron is one-millionth of a meter) to a little more than seven tenths of a micron compose the part of the spectrum that meteorologists use to generate "visible" satellite images (which we'll cover later in the lesson).
Beyond the longest wavelengths associated with visible light lies the infrared ("beyond red") band of the electromagnetic spectrum. A majority of the infrared spectrum, spanning from approximately 3 to 100 microns, essentially constitutes "terrestrial radiation" because the oscillating charges that emit at these wavelengths are consistent with temperatures commonly observed on this planet as well as the Earth's atmosphere. Thus, terrestrial radiation lends itself to be used in infrared satellite imagery (of which there are several applications we'll study soon).
Microwaves are next in line in the electromagnetic spectrum's hierarchy, with wavelengths spanning from 100 microns to about 30 centimeters. Most radar imagery used in weather forecasting employs artificially produced microwaves ranging in wavelength from 3 to 10 centimeters (more on radar later in the lesson).
Now that you know the terminology behind the different regions of the electromagnetic spectrum, we need to discuss the properties by which objects emit radiation. These properties have been grouped into what I call the "four laws of radiation." Read on.
As I mentioned previously, the discussion in this section focused on the generation of low-energy electromagnetic waves (those with lower frequencies than visible light). If you want explore further than what I present here, there are many online sources that discuss the various regions of the electromagnetic spectrum. For starters, check out: the Wikipedia page on the electromagnetic spectrum.
Above the visible portion of the electromagnetic spectrum is the very short wavelength region that includes gamma rays, x-rays and ultraviolet light. The shortest wavelengths belong to gamma rays, which have wavelengths that are as short as one trillionth of a meter (unimaginably small). It turns out that the energy required for matter to emit electromagnetic radiation with wavelengths on the order of a few microns (or less), surpasses that which can be generated by an oscillating molecule. In fact, at such energies, the molecular and atomic bonds may break down completely, leaving only single atoms (or even single electrons!). Therefore, a few new mechanisms are needed to explain very short-wave EM emissions.
Perhaps you remember the Bohr model of an atom from high school chemistry that shows the electrons orbiting a nucleus of protons and neutrons (like a mini solar system). Suffice to say, things are a bit more complicated than that, but I'll stick with this model for simplicity. In an unenergized state (called the base state), an atom has a number of electrons in various orbits (or shells) around the nucleus. However, if sufficient energy is added to the atom, one or more of its electrons will be ejected into higher orbits around the nucleus (because they have more energy, they can better overcome the pull of the nucleus). Then, when these electrons fall back down to their original orbit, they must jettison the extra energy. They emit this energy in the form of a photon (a small packet of EM radiation), that has a frequency which corresponds to the energy released. Such photons are typically found in the near-IR, visible, and ultraviolet portions of the EM spectrum.
At even higher temperatures, the electrons may even break their bonds with the atomic nucleus itself, forming what is known as a plasma. Plasmas are a fourth state of matter (not a solid, liquid, or gas) that consist of positive ions (left over atomic nuclei) and free electrons. In a plasma, electromagnetic radiation is generated when the speed or direction of an electron is altered by a positive ion or another electron. Because of the unrestrained nature of electrons within a plasma, they can travel at tremendous speeds and thus can generate very high-energy photons. The generation of ultraviolet waves, X-rays, and gamma rays are typically from plasmas.
Although such high-energy radiation can be generated artificially (the medical use of X-rays, for example), most of the sources for natural high-energy EM emission originate in space. The plasma of our sun emits copious amounts of X-rays and ultraviolet radiation, as well as gamma rays during eruptions of solar flares. Furthermore, the most prodigious gamma-ray bursts come from interstellar events such as supernovae, black holes, and quasars. Check out the image below, which shows gamma ray emission from the entire sky. Note that the strongest gamma ray emissions are concentrated along the disk of the Milky Way Galaxy.