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Key Concepts

Key Concepts of Chapter 2:

1. Earth has two principal motions—rotation and revolution. Rotation is the spinning of Earth about its axis. Revolution refers to the movement of Earth in its orbit around the Sun.

2. The two most important reasons for the variation in solar energy reaching a particular location are the seasonal changes in the angle at which the Sun’s rays strike the surface and the length of daylight. The seasonal variation in the angle of the Sun affects where on Earth the solar rays are most concerned and the thickness of atmosphere the rays must penetrate.

3. The four days each year given special significance based on the annual migration of the direct rays of the Sun and its importance to the yearly cycle of weather are (1) June 21/22, the summer solstice in the Northern Hemisphere, when the vertical rays of the Sun are striking 23 1/2° north latitude (Tropic of Cancer), (2) December 21/22, the winter solstice in the Northern Hemisphere, when the vertical rays of the Sun are striking 23 1/2° south latitude (Tropic of Capricorn), (3) September 22/23, the autumnal equinox in the Northern Hemisphere, when the vertical rays of the Sun strike the equator, and (4) March 21/22, the spring, or vernal, equinox in the Northern Hemisphere, when the vertical rays of the Sun also strike the equator.

4. Energy is the ability to do work. The two major categories of energy are (1) kinetic energy, which can be thought of as energy of motion, and (2) potential energy, energy that has the capability to do work. Heat is the transfer of energy into or out of an object because of temperature differences between that object and its surroundings.

5. The three mechanisms of energy transfer are (1) conduction, the transfer of heat through matter by molecular activity, (2) convection, the transfer of heat by mass movement or circulation within a substance, and (3) radiation, the transfer mechanism by which solar energy reaches our planet.

6. Radiation or electromagnetic radiation, whether X rays, visible light, heat waves, or radio waves, travels as various size waves through the vacuum of space at 300,000 kilometers per second. Shorter wavelengths of radiation are associated with greater energy. The wavelength of visible light ranges from 0.4 micrometer (violet) to 0.7 micrometer (red). Although the Sun emits many forms of radiation, most of the energy is concentrated in the visible and near visible (infrared and ultraviolet) parts of the spectrum. The basic laws of radiation are (1) all objects emit radiant energy, (2) hotter objects radiate more total energy per unit area than colder objects, (3) the hotter the radiating body, the shorter is the wavelength of maximum radiation, and (4) objects that are good absorbers of radiation are also good emitters.

7. Approximately 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth’s surface. About 30 percent is reflected back to space. The remaining 20 percent of the energy is absorbed by clouds and the atmosphere’s gases. The wavelength of the energy being transmitted, as well as the size and nature of the absorbing or reflecting substance, determine whether solar radiation will be scattered, reflected back to space, or absorbed. The fraction of radiation reflected by a surface is called its albedo.

8. Radiant energy that is absorbed heats Earth and eventually is reradiated skyward. Because Earth has a much lower surface temperature than the Sun, its radiation is in the form of longwave infrared radiation. Because the atmospheric gases, primarily water vapor and carbon dioxide, are more efficient absorbers of terrestrial (longwave) radiation, the atmosphere is heated from the ground up. The general drop in temperature with increased altitude in the troposphere (about 6.5°C/kilometer, a figure called the normal lapse rate) supports the fact that the atmosphere is heated from below. The transmission of shortwave solar radiation by the atmosphere, coupled with the selective absorption of Earth radiation by atmospheric gases that results in the warming of the atmosphere, is referred to as the greenhouse effect.

9. Because of the annual balance that exists between incoming and outgoing radiation, called the heat budget, Earth’s average temperature remains relatively constant despite seasonal cold spells and heat waves. Although the balance of incoming and outgoing radiation holds for the entire planet, it is not maintained at each latitude. Averaged over the entire year, a zone around Earth between 38°N and 38°S receives more solar radiation than is lost to space. The opposite is true for higher latitudes, where more heat is lost through outgoing longwave radiation than is received. It is this energy imbalance between the low and high latitudes that drives the global winds and ocean currents, which in turn transfer surplus heat from the tropics poleward. Furthermore, the radiation balance of a given place fluctuates with changes in cloud cover, atmospheric composition, and most important, Sun angle and length of daylight. Thus, areas of radiation surplus and deficit migrate seasonally as the Sun angle and length of daylight change.

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