January 14, 1998

Heat Transfer
Absorption, Emission, Equilibrium
Global Energy Balance
Greenhouse Effect

Heat Transfer

We have discussed heat and its relationship to energy. Now we will discuss how heat travels from one location to another.

The four primary means of heat transfer are:

Conduction

An example of conduction would be when the handle of a metal pot that is on a stove heats up even though it is the bottom of the pot that is being heated. Atoms and molecules in the pot that are heated move faster and crash into their neighboring mole cules causing them to vibrate and move faster, thus heat is conducted. The effect is analogous to a chain reaction.

Convection

If you open up an oven, you will feel a hot burst of air. The air in the oven which is very hot is convected out. Convection occurs due to the tendency of air to move from higher temperature regions toward lower temperature regions. Convection curre nts are often called thermals. We will cover this in more detail later.

Advection

If the temperature in Orlando is 90 degrees F and the temperature in Cocoa Beach is 70 degrees F and the wind is blowing from the west from Orlando toward Cocoa Beach, you can visualize how the wind will be blowing warmer air from Orlando toward Cocoa Beach. The temperature in Cocoa Beach would likely increase. This is an example of warm air advection. Advection refers to any quantity, not just heat, that is transported by the wind, including moisture advection.

Radiation

Between the Earth and the Sun is primarily empty space, however, heat obviously reaches Earth from the Sun despite the absence of material to conduct the heat or gases for convection to take place. The heat reaches Earth through electromagnetic wav es. Energy carried in electromagnetic waves is called radiation.

A wave is a disturbance in a medium. When you throw a rock in a pond, a disturbance in the water medium is created. The wave pushes the water first upward and then downward as the wave passes a point in the water. Light and energy travel in w aves as well. Electromagnetic radiation essentially occurs due to the vibration of atoms and molecules.

The two fundamental dimensions of a wave are its frequency and wavelength. The amount of energy a wave carries is proportional to its frequency - the higher the frequency, the more energy the wave contains.

All matter radiates energy. The energy that is emitted by matter varies tremendously in wavelength and frequency. The wavelength and frequency of radiation is primarily determined by an objectÕs temperature. The hotter the object is, the faster its atoms and molecules will be vibrating. Intuitively, you can imagine that the faster the atoms and molecules vibrate, the higher the frequency of the radiation that is emitted will be.

All the different wavelength and frequency possibilities represent the electromagnetic spectrum. Electromagnetic waves with a wavelength of between .4 and .7 micrometers is classified as visible light and triggers a response in our eyes and all ows us to see. This is an extremely tiny portion of the electromagnetic spectrum.

The visible light spectrum goes from violet at .4 micrometers to red at .7 micrometers. Below .4 micrometers is the ultraviolet portion of the EMS (electromagnetic spectrum) while above .7 micrometers is the infrared.

Once again to reiterate what was said earlier, electromagnetic waves are the result of vibrating atoms and molecules. The faster the vibrations, the higher the frequency of the emitted radiation and the more energy the wave will carry.

As a result, the energy released from an object is directly proportional to its temperature. Energy in an object is actually proportional to the fourth power (T^4) of temperature. This relationship is the Stefan-Boltzman Law.

Global Energy Budget

Energy is constantly arriving at Earth from the sun. Why then does the temperature of the Earth not increase continuously as the sun heats the Earth at a basically constant rate? The answer lies in the Stefan-Boltzman Law. We have learned that all o bjects radiate energy at a rate proportional to the fourth power of their temperature. The Earth is nearly in Energy Balance. In other words, the amount of radiation received from the sun is equal to that that is radiated back into space.

Absorption/Emission

Different substances, depending upon their physical composition and characteristics, emit and absorb different amounts of radiation. For instance, black concrete is a much better absorber of radiation than white snow is.

Also, different substances, depending upon their physical composition and characteristics, emit and absorb radiation of different frequencies in differing quantities. For instance, carbon dioxide absorbs infrared radiation quite well but does not abso rb ultraviolet radiation. On the other hand, oxygen absorbs ultraviolet radiation quite well but does not absorb infrared radiation well.

It is the ability of certain gases that are anthropogenically being introduced into the atmosphere to absorb radiation at broad regions of the electromagnetic spectrum that is responsible for the Greenhouse Effect. In the Greenhouse Effe ct, energy is absorbed and retained within the atmosphere and as a result, more radiation comes into the atmosphere from the sun than leaves Earth for space and as a result, the Earth grows hotter.

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