Infrared/Microwave Analogy?


Is it fair to assume that infrared radiation from the earth warms the greenhouse gases in the atmosphere like a microwave oven will heat a glass of water.

That is, radiated energy increases kinetic energy of susceptible molecules and hence temperature increases.

(Yes, I know the frequencies are different so microwaves will cause rotation rather than vibration.)

6 Answers

  • 9 months ago
    Favorite Answer

    That is exactly right. Radiation is one of the standard means of transferring heat energy. The amount of heat in anything is determined by the excitation of the atoms or molecules. When there is no movement or excitation of the atoms then that is absolute zero.

    One of the highly specialized sciences is that of Vacuum Deposition or the manufacturing of computer chips and such which have to be done under vacuum. In this operation IR is used since an object has to be heated. Since there is little or no air that would rule out either conductive or convection transfer. That object's molecules and atoms can be excited only by radiation.

    All radiation will excite molecules somewhat. Different frequencies act upon a subject in different ways. Different frequencies of lasers act in different ways. I just had a retina reattached to my eye. A laser was used. That laser's frequency was different than the one they cut steel with.

    These are just a few examples. But basically your assumption is correct. In the early days of television broadcasting we would use microwaves to transmit from the station to the tv tower or antenna. Now they use mainly fiber optics. We would heat our coffee with microwaves. But don't put your hand in that microwave. It will excite those molecules quickly and harshly. One man that I know of lost his hand from being careless.

    Whew! You really drew out the nuts on this one. One person has some units of heat as blocks, as if the sun sends us some blocks. It does not. The sun sends us radiation. Some of this radiation is in the form of light or the frequency within the band of light frequencies. It sends us other frequencies also but most of them are filtered out by our atmosphere. To make it simple we will only deal with light frequencies. The Earth's atmosphere does not filter out light, for the most part. And these light waves do strike the molecules of the atmosphere and heat up or excite these molecules. However that is insignificant to the light waves that get through and strike the Earth's surface and are CONVERTED to IR. IR now has a much more excitation power on the molecules and atoms of the atmosphere, for want of a better term. The sun doesn't send us units of heat. It sends us a form of energy that can be converted into units of heat.

    So again, your analogy is correct, if I am understanding the question right. The radiation of a microwave is sent out as just that. A certain frequency which has no heat value. Once this wave or ray strikes and interacts with an object, excitement of the molecules of the object by that ray causes a conversion of energy which can be measured by thermally sensitive instruments. This is quite different than heat transfer by conduction or convection where actual units of heat can be transferred. (or legos, if you will)

  • Cowboy
    Lv 6
    9 months ago

    too weak to be useful

  • 9 months ago

    The Earth's atmosphere is an adiabatic temperature gradient system, with heat capacities, mass, density, gravity and the Laws of Physics and Thermodynamics determining the outcome. The atmosphere is cooler than the surface and decreases in temperature further with altitude. Thus, heat flows away from the warmer surface to the cooler atmosphere. Heat can’t be “trapped” at the warmer surface by the cooler atmosphere because heat spontaneously flows from hot to cold, and this can’t be stopped.

    Energy from the Sun called short wave infrared radiation is directed towards the Earth, the albedo effect reflects about 30% back to space and most of the remainder is absorbed by Earth’s surface. Here, the Plank’s Blackbody Radiation Law is activated. This law allows us to calculate the total amount of energy in a blackbody spectrum, and what the temperature the object actually needs to be at in order to emit that amount of energy. The Earth is not a full ‘blackbody” it is rated at about 0.7, with 1 being a full black body. Next the Stefan-Boltzmann law comes into play, and it states that an object which radiates like a blackbody has a surface brightness which is proportional to the object’s temperature (“T” in degrees Kelvin) to the fourth power.

    “Let’s look at blackbody radiation. Arrange two identical blackbodies to radiate at each other. Just make sure one is warmer than the other. It will soon be apparent that the cooler body is not cooling as rapidly as the warmer body. In fact, if it was way cooler, it could warm up a bit first and then cool slowly. The cooler body is absorbing the HIGH FREQUENCY radiation from the warmer body and converting it to lower frequency radiation appropriate to its current temperature and radiating this back to the warmer body. The warmer body does NOT respond to radiation from a lower temperature range. Eventually both bodies will be at equilibrium with each other, having reached the same temperature, and continue cooling at the same rate.”

    Next, another Law is activated called Kirchhoff’s Law of Thermal Radiation. It states: “At radiative thermal equilibrium, the emissivity of a body equals its absorptivity.” In other words, the Earth will give off just as much power in radiation as it absorbs, when it’s in radiative thermal equilibrium with the Sun.

    Now we must look at photons. What defines the energy in a photon? For this we need to look up the basic principles of molecular emission/absorption. That slant is important, a photon CANNOT emit AND absorb at the same time. Just one or the other. Low emissivity warms, high emissivity cools. Oxygen and Nitrogen have low emissivity, where as the so-called greenhouse gases have a higher emissivity, thus cool the atmosphere.

    The total energy of a photon is the sum of rotational energy plus the vibrational energy plus the electronic energy plus the translation energy. The translation energy is acquired by kinetic collisions with other photons.A photon does not have a temperature rating. It has an energy rating appropriate to its frequency. This energy is defined as Planck’s Constant times the frequency. Thus high frequency photons are more energetic than low frequency photons.

    From “Thermodynamics”, G. J. V. Wylen, John Wiley & Sons, 1960:

    “Heat is defined as the form of energy that is transferred across a boundary by virtue of a temperature difference or temperature gradient. Implied in this definition is the very important fact that a body never contains heat, but that heat is identified as heat only as it crosses the boundary. Thus, heat is a transient phenomenon.

    Therefore, there is no heat transfer from the atmosphere to the surface, or from a cooler object to a warmer object in general. And since positive heat flow is what is required for temperature increase, then no cooler object raises the temperature of a warmer object by utilization of its thermal energy. That is why there is no “back radiation” or “heat trapping” as the IPCC Greenhouse Gas theory suggests and why it is a myth.

    These are the Laws of Physics that are the foundation into understanding a very complex process that has been has been subject to much controversy and abuse through ignorance.

    • Al P
      Lv 7
      9 months agoReport

      This answer is almost completely wrong.

  • 9 months ago

    I think of it in terms of lego.

    Sun throws lego bricks at you. You throw lego bricks away. But you're a bit slow. So you have a pool of lego bricks sitting by your feet before you are able to throw them away at the same rate Sun is throwing them at you. This pool of bricks represents your temperature in the absence of greenhouse gases.

    I come in, and as you throw bricks away, I start throwing some back. The pool of bricks at your feet starts growing. Your temperature rises. It takes you a while to get up to speed so you are throwing more bricks to compensate for the ones I'm returning. When you manage this, the rate of bricks arriving from Sun, and the rate of bricks you are throwing away match. The pool of bricks at your feet stays the same but because you took time to react, it is now larger than before I started chucking bricks back.

    I don't really retain any bricks. I catch some and return them quickly.

    • graphicconception
      Lv 7
      9 months agoReport

      Elizabeth: You have done a Dirac on me. You have provided an overview of the whole greenhouse effect. I was just asking about the GHG warming part by infrared radiation from the earth,

  • How do you think about the answers? You can sign in to vote the answer.
  • Dirac
    Lv 4
    9 months ago

    Yes and no. The key to the greenhouse effect is warming the SURFACE of the planet, not the atmosphere. That depends on the greenhouse gases RADIATING infrared back toward the surface of the planet and changing the surface equilibrium. At its most abstract level you could just think about a single infrared absorber/radiator in the atmosphere at a particular altitude. That absorber would be completely transparent to sunlight and completely opaque to the longwave infrared and is simply at the ambient temperature of the atmosphere at that level.

    In reality it is much more complicated, since greenhouse gases are distributed throughout the atmospheric column, and near the surface collisional (thermal) processes do become important.

    It is crucial to the greenhouse effect that the temperature of the radiating layer is lower than the temperature of the surface. That's why parts of the Antarctic Plateau actually have a NEGATIVE greenhouse effect, because the surface can be colder than the stratosphere. That's why all those experiments that are supposed to demonstrate the greenhouse effect in a Coke bottle or some other transparent container are misleading--the greenhouse effect is NOT about a greenhouse gas absorbing IR and then warming its surrounding gases.

    An alternative way of thinking about it is the following: over a the long term the planet has to be in radiative equilibrium with the sun. With no greenhouse gases then that takes place at the planet's surface, with greenhouse gases that equilibrium level is moved upward, and the surface is at the radiative equilibrium temperature plus the lapse rate x altitude of the equilibrium layer.

    • Dirac
      Lv 4
      9 months agoReport

      Conduction is not particularly important for transporting heat upward, but it is important for warming the lowermost portion of the atmosphere. Collisions between the air and the ground cause the air to warm up (at least during the daytime) and then convection can overturn that warmed air.

  • 9 months ago

    The entire electromagnetic spectrum is capable of heating anything that absorbs it.

Still have questions? Get your answers by asking now.