Heating the Atmosphere 2: The Atmosphere

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 the figure. Most of this energy is then reradiated skyward.

Laws of 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 do colder objects. 3. The hottest radiating bodies produce the shortest wavelengths of maximum radiation. For example, the sun, with a surface temperature of nearly 6,000°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.

Laws of Solar Radiation

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 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. The figure shows what happens to incoming solar radiation, averaged for the entire globe.

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 atmosphere and clouds directly absorb about 20 percent of the solar radiation that strikes Earth. How does the sun indirectly warm the atmosphere?

Earth's land and sea absorb solar radiation, then reradiate it to the air.

Fill 2

In

Introduction

It's summer in the desert southwest. Air temperatures hover in the triple digits. At ground level, the air appears steamy and is noticeably hotter than air higher up. As the sun's light passes through the atmosphere and reflects off the ground, the light is scattered as it moves through the layers of hot air. The result? The air looks rippled as if it were made of water.

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.

How is conduction different from convection?

Objects must be in contact with one another to transfer heat through conduction. Convection is a movement of heat through liquids created by mass movement of molecules.

How is radiation different from conduction and convection?

Radiation can transfer energy as heat through empty space.

Reflection and Scattering

Reflection occurs when an electromagnetic wave 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. Scattering disperses waves 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 heat the Earth's atmosphere.

Small Dust Particles and Scattering

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 daytime through windows in the absence of direct sunlight. Scattering also accounts for the brightness and even the blue color of the daytime sky. About half of the solar radiation that is absorbed at Earth's surface arrives as scattered radiation.

How is the atmosphere affected by radiation?

Solar radiation can be absorbed, transmitted, or reflected. The radiant energy that is absorbed is converted to heat and causes an increase in temperature.

Photosynthesis

Some incoming solar radiation is not absorbed and reradiated. Instead, it is absorbed by the chlorophyll in green plants. Plants use the energy from this radiation in photosynthesis. Thus solar energy is the main energy source for virtually all life on Earth.

Conductors

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.

Insolation and Angles

The amount of incoming solar radiation, called insolation, that reaches the Earth's surface differs from place to place. Consequently, different locations have different climates. The two main factors that determine how much solar energy, and therefore how much heat, a location receives are the angle of the sun and the duration of daylight. The angle at which sunlight strikes the Earth's surface at a particular location depends on the location's latitude, the time of day, and the season. Because of the curvature of the Earth, the sun is at a lower angle in the sky and solar rays must travel through a larger area of atmosphere to reach the surface at the Earth's poles. The equator, on the other hand, receives more direct sunlight.

Atmosphere and Gas

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 these 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 rest travels earthward and is again absorbed. In this way, Earth's surface is continually supplied with heat from the atmosphere as well as from the sun.

Energy Transfer as Heat

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. 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 the air, those particles move faster, and air temperature rises. When air transfers energy to a cooler object, its particles move more slowly, and air temperature drop

Radiation

The pan of water in the figure 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 the convective flow.

Electromagnetic Waves

The sun is the ultimate source of energy that creates our weather. 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 the figure. All radiation, whether X-rays, radio waves, or infrared waves, travels through the vacuum of space at 300,000 kilometers per second. Radiation travels only slightly slower through our atmosphere.

Radiation 2

The third mechanism of heat transfer is radiation. As shown in the figure, 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.

Tilt and Radiation

The tilt of the Earth on its axis is the key to the seasons and duration of daylight. During the summer, when the Northern Hemisphere is tilted toward the sun, sunlight strikes the surface at a more direct angle, and this hemisphere experiences longer days. As a result, the Northern Hemisphere receives more solar heating. At the same time, the Southern Hemisphere is tilted away from the sun, so sunlight strikes it at a more oblique angle and the days are shorter. Thus, when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. Differences in the amount of incoming solar radiation play a major role in weather and climate across the globe. Although incoming solar radiation is absorbed, reflected, or re-radiated differently from place to place, there is a balance of energy transfer into and out of Earth's atmosphere. As a result, the atmosphere's average temperature tends to remain constant from year to year. But the atmosphere's average temperature can and does change. It changes in response to factors that disturb its energy balance.

Why are carbon dioxide, water vapor, and other greenhouse gases important to life on Earth's surface?

They absorb radiant energy emitted by Earth's surface.

Visible Light

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. By using a prism, white light can be divided into the colors of the rainbow, from violet with the shortest wavelength—400 nanometers (1 nanometer is 1.0 x 10-7 centimeters) to red with the longest wavelength—700 nanometers.

Greenhouse

Without these absorbing gases in our atmosphere, Earth would not be a suitable habitat for most types of living things found on Earth today. This 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 that the greenhouse itself prevents the mixing of air inside with cooler air outside.)

You know what happens when you touch something hot, such as food that has been heated in an oven. The heat is transferred to your hand in a natural process known as _____.

conduction

Electromagnetic Waves Length, Movement, and Radio Waves

electromagnetic waves move out from their source and come in various sizes. Electromagnetic waves are classified by their wavelength, or the distance from one crest to the next. Radio waves, like the AM waves shown in the figure, have the longest wavelengths, ranging to tens of kilometers. Gamma waves are the shortest—less than a billionth of a centimeter long.

What is heat?

energy transferred because of a temperature difference