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Lesson 7-02 Heating The Atmosphere

Page history last edited by debra.krohn@gmail.com 14 years, 1 month ago

 

Lesson 7.02 – Heating the Atmosphere

Standard:  ES4.b Students know the fate of incoming solar radiation in terms of reflection, absorption, and photosynthesis.

ES6.a Students know weather (in the short run) and climate (in the long run) involve the transfer of energy into and out of the atmosphere.

 

 

INTRODUCTION

The concepts of heat and temperature often are confused. The phrase “in the heat of the day” is one common expression in which the word “heat” is misused to describe the concept of temperature. Heat is the energy transferred from one object to another because of a difference in their temperatures. Recall that all matter is composed of atoms or molecules that possess kinetic energy, or the energy of motion. Temperature is a measure of the average kinetic energy of the individual atoms or molecules in a substance. When energy is transferred to the gas atoms and molecules in air, those particles move faster and air temperature rises. When air transfers energy to a cooler object, its particles move slower, and air temperature drops.

 

 

INSTRUCTION

Energy Transfer as Heat

Three mechanisms of energy transfer as heat are conduction, convection, and radiation. All three processes, illustrated in Figure 9, happen simultaneously in the atmosphere. These mechanisms operate to transfer energy between Earth’s surface (both land and water) and the atmosphere.

Figure 9 Energy Transfer as Heat A pot of water on the campfire illustrates the three mechanisms of heat transfer.

 

 

Conduction

Anyone who has touched a metal spoon that was left in a hot pan has experienced the result of heat conducted through the spoon. Conduction is the transfer of heat through matter by molecular activity. The energy of molecules is transferred by collisions from one molecule to another. Heat flows from the higher temperature matter to the lower temperature matter.

 

 

The ability of substances to conduct heat varies greatly. Metals are good conductors, as those of us who have touched hot metal have quickly learned. Air, however, is a very poor conductor of heat. Because air is a poor conductor, conduction is important only between Earth’s surface and the air directly in contact with the surface. For the atmosphere as a whole, conduction is the least important mechanism of heat transfer.

 

 

Convection

Much of the heat transfer that occurs in the atmosphere is carried on by convection. Convection is the transfer of heat by mass movement or circulation within a substance. It takes place in fluids, like the ocean and air, where the atoms and molecules are free to move about. Convection also takes place in solids, such as Earth’s mantle, that behave like fluids over long periods of time.

 

 

The pan of water in Figure 9 shows circulation by convection. Radiation from the fire warms the bottom of the pan, which conducts heat to the water near the bottom of the container. As the water is heated, it expands and becomes less dense than the water above. The warmer water rises because of its buoyancy. At the same time, cooler, denser water near the top of the pan sinks to the bottom, where it becomes heated. As long as the water is heated unequally, it will continue to circulate. In much the same way, most of the heat acquired by radiation and conduction in the lowest layer of the atmosphere is transferred by convective flow.

 

 

Electromagnetic Waves

The sun is the ultimate source of energy that creates our weather. You know that the sun emits light and heat as well as the ultraviolet rays that cause a suntan. These forms of energy are only part of a large array of energy called the electromagnetic spectrum. This spectrum of electromagnetic energy is shown in Figure 10. All radiation, whether X-rays, radio waves, or heat waves, travel through the vacuum of space at 300,000 kilometers per second. They travel only slightly slower through our atmosphere.

 

 

Figure 10 Electromagnetic Spectrum Electromagnetic energy is classified according to wavelength in the electromagnetic spectrum.

 

 

Imagine what happens when you toss a pebble into a pond. Ripples are made and move away from the location where the pebble hit the water’s surface. Much like these ripples, electromagnetic waves move out from their source and come in various sizes. The most important difference among electromagnetic waves is their wavelength, or the distance from one crest to the next. Radio waves have the longest wavelengths, ranging to tens of kilometers. Gamma waves are the shortest, and are less than a billionth of a centimeter long.

 

 

Visible light is the only portion of the spectrum you can see. White light is really a mixture of colors. Each color corresponds to a specific wavelength, as shown in Figure 11. By using a prism, white light can be divided into the colors of the rainbow, from violet with the shortest wavelength—0.4 micrometer (1 micrometer is 0.0001 centimeter)—to red with the longest wavelength—0.7 micrometer.

 

 

Figure 11 Visible light consists of an array of colors commonly called the colors of the rainbow.

 

 

Radiation

The third mechanism of heat transfer is radiation. As shown in Figure 9, radiation travels out in all directions from its source. Unlike conduction and convection, which need material to travel through, radiant energy can travel through the vacuum of space. Solar energy reaches Earth by radiation.

To understand how the atmosphere is heated, it is useful to think about four laws governing radiation.

1.    All objects, at any temperature, emit radiant energy. Not only hot objects like the sun but also Earth—including its polar ice caps—continually emit energy.

2.    Hotter objects radiate more total energy per unit area than colder objects do.

3.    The hottest radiating bodies produce the shortest wavelengths of maximum radiation. For example, the sun, with a surface temperature of nearly 6000°C radiates maximum energy at 0.5 micrometers, which is in the visible range. The maximum radiation for Earth occurs at a wavelength of 10 micrometers, well within the infrared range.

4.    Objects that are good absorbers of radiation are good emitters as well. Gases are selective absorbers and radiators. The atmosphere does not absorb certain wavelengths of radiation, but it is a good absorber of other wavelengths.

 

 

 

What Happens to Solar Radiation?

When radiation strikes an object, there usually are three different results.

1.    Some energy is absorbed by the object. When radiant energy is absorbed, it is converted to heat and causes a temperature increase.

2.    Substances such as water and air are transparent to certain wavelengths of radiation. These substances transmit the radiant energy. Radiation that is transmitted does not contribute energy to the object.

3.    Some radiation may bounce off the object without being absorbed or transmitted. Figure 12 shows what happens to incoming solar radiation, averaged for the entire globe.

4.   

 

Figure 12 Solar Radiation This diagram shows what happens, on average, to incoming solar radiation by percentage.

 

 

Reflection and Scattering

Reflection occurs when light bounces off an object. The reflected radiation has the same intensity as the incident radiation. In contrast, scattering produces a larger number of weaker rays that travel in different directions. See Figure 13. Scattering disperses light both forward and backward. However, more energy is dispersed in the forward direction. About 30 percent of the solar energy reaching the outer atmosphere is reflected back to space. This 30 percent also includes the amount of energy sent skyward by scattering. This energy is lost and does not play a role in heating Earth’s atmosphere.

 

 

Figure 13 Reflection vs. Scattering A Reflected light bounces back with the same intensity. B Scattering produces more light rays with a weaker intensity.

 

 

Small dust particles and gas molecules in the atmosphere scatter some incoming radiation in all directions. This explains how light reaches into the area beneath a shade tree, and how a room is lit in the absence of direct sunlight. Scattering also accounts for the brightness and even the blue color of the daytime sky. In contrast, bodies like the moon and Mercury—which are without atmospheres—have dark skies and “pitch-black” shadows even during daylight hours. About half of the solar radiation that is absorbed at Earth’s surface arrives as scattered light.

 

 

Absorption

About 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth’s surface and is absorbed, as shown in Figure 12. Most of this energy is then reradiated skyward. Because Earth has a much lower surface temperature than the sun, the radiation that it emits has longer wavelengths than solar radiation does.

 

 

The atmosphere efficiently absorbs the longer wavelengths emitted by Earth. Water vapor and carbon dioxide are the major absorbing gases. When a gas molecule absorbs light waves, this energy is transformed into molecular motion that can be detected as a rise in temperature. Gases in the atmosphere eventually radiate some of this energy away. Some energy travels skyward, where it may be reabsorbed by other gas molecules. The remainder travels Earthward and is again absorbed by Earth. In this way, Earth’s surface is continually being supplied with heat from the atmosphere as well as from the sun.

 

 

Without these absorbing gases in our atmosphere, Earth would not be a suitable habitat for humans and other life forms. This important phenomenon has been termed the greenhouse effect because it was once thought that greenhouses were heated in a similar manner. A more important factor in keeping a greenhouse warm is the fact that the greenhouse itself prevents the mixing of air inside with cooler air outside. Nevertheless, the term greenhouse effect is still used.

 

 

PRACTICE

  1. Take notes on the above information.
  2. Click here (http://videos.howstuffworks.com/hsw/5666-atmosphere-the-sun-and-weather-video.htm) to view a video about solar energy.
  3. Click here (http://videos.howstuffworks.com/hsw/19215-simply-science-energy-conversions-in-living-organisms-video.htm) to watch a video about solar energy and photosynthesis.
  4. Click here (http://videos.howstuffworks.com/howstuffworks/203-what-is-the-greenhouse-effect-video.htm) to watch a video on greenhouse effect.

ASSESSMENT

  1. Turn in your notes.
  2. Take the 7.02 Quiz.


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