2. Concept: Key factors that influence an area’s climate are incoming solar energy, the earth’s rotation, global patterns of air and water movement, gases in the atmosphere, and the earth’s surface features. It is important to understand the difference between weather and climate. Weather is a set of physical conditions of the lower atmosphere, including temperature, precipitation, humidity, wind speed, cloud cover, and other factors, in a given area over a period of hours or days. Weather differs from climate, which is the general pattern of atmospheric conditions in a given area over periods ranging from at least three decades to thousands of years. In other words, climate is the sum of weather conditions in a given area, averaged over a long period of time. Ocean currents are mass movements of surface water driven by winds blowing over the oceans. These currents help to determine regional climates and are a key component of the earth’s natural capital. Three major factors affect the circulation of air in the lower atmosphere. Uneven heating of the earth’s surface by the sun. Air is heated much more at the equator, where the sun’s rays strike directly, than at the poles, where sunlight strikes at an angle and spreads out over a much greater area. These differences in the input of solar energy to the atmosphere help explain why tropical regions near the equator are hot, why Polar Regions are cold, and why temperate regions in between generally have both warm and cool temperatures. The intense input of solar radiation in tropical regions leads to greatly increased evaporation of moisture from forests, grasslands, and bodies of water. As a result, tropical regions normally receive more precipitation than do other areas of the earth. Rotation of the earth on its axis. As the earth rotates around its axis, the equator spins faster than the regions to its north and south. As a result, heated air masses, rising above the equator and moving north and south to cooler areas, are deflected in different ways over different parts of the planet’s surface. The atmosphere over these different areas is divided into huge regions called cells, distinguished by the direction of air movement. The differing directions of air movement are called prevailing winds—major surface winds that blow almost continuously and help to distribute heat and moisture over the earth’s surface and to drive ocean currents. Properties of air, water, and land. Heat from the sun evaporates ocean water and transfers heat from the oceans to the atmosphere, especially near the hot equator. This evaporation of water creates giant cyclical convection cells that circulate air, heat, and moisture both vertically and from place to place in the atmosphere. 3. As energy flows from the sun to the earth, some of it is reflected by the earth’s surface back into the atmosphere. Molecules of certain gases in the atmosphere, including water vapor , carbon dioxide , methane , and nitrous oxide , absorb some of this solar energy and release a portion of it as infrared radiation (heat) that warms the lower atmosphere. Thus, these gases, called greenhouse gases, play a role in determining the lower atmosphere’s average temperatures and thus the earth’s climates. The earth’s surface also absorbs much of the solar energy that strikes it and transforms it into longer-wavelength infrared radiation, which then rises into the lower atmosphere. Some of this heat escapes into space, but some is absorbed by molecules of greenhouse gases and emitted into the lower atmosphere as even longer-wavelength infrared radiation. Some of this released energy radiates into space, and some adds to the warming of the lower atmosphere and the earth’s surface. Together, these processes result in a natural warming of the troposphere, called the greenhouse effect. Without this natural warming effect, the earth would be a very cold and mostly lifeless planet. 4. As the drier air mass passes over the...
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