The Atmosphere Study Guide for McGraw-Hill's ASVAB (page 3)
Composition and Structure
Most of Earth's atmosphere is made up of nitrogen and oxygen.
The atmosphere is divided into various layers, determined by temperature. In the troposphere, the layer in which we live and in which our weather occurs, the temperature decreases dramatically as the altitude increases. The troposphere contains most of the atmosphere's mass and thus air pressure.
Above the troposphere is the stratosphere. Very little weather occurs in the stratosphere. Occasionally, the top portions of very large thunderstorms poke into this layer. The lower portion of the stratosphere is also influenced by the polar jet stream and the subtropical jet stream. Ozone gas molecules, which absorb ultraviolet sunlight, concentrate in this layer, causing this layer to experience a sharp rise in temperature.
The mesosphere contains the coldest temperatures, and the thermosphere has the highest temperatures. The highest temperatures come from the absorption of solar radiation by the oxygen molecules.
Air Masses Air masses are large bodies of air that have similar temperature and humidity. They can be cold air masses (polar) or warm air masses (tropical). If they are formed over water, they are called maritime (with much moisture), and if they are formed over land, they are called continental (dryer).
The line along which two air masses meet is called a front. Much of our severe weather takes place at or near a front. A cold front is the interface in the atmosphere where a cold, dry, stable air mass displaces a warm, moist, unstable subtropical air mass. Cirrus clouds are found before the front reaches an area and are followed by precipitation from cumulus and cumulonimbus clouds. A warm front is located at the place where an advancing warm, subtropical, moist air mass replaces a retreating cold, dry, polar air mass. An occluded front is produced when a fast-moving cold front catches and overtakes a slower-moving warm front.
Types of Cloud Clouds are classified according to their appearance and height. Clouds form when water vapor condenses onto dust particles floating in the air. Cloud formation also occurs when warm and cold air (or land) meet.
Winds and Storms
Wind is the movement of air in our atmosphere. Wind occurs because of pressure differences between one part of the atmosphere and another. The greater the pressure difference (called the gradient), the stronger the wind. Atmospheric pressure is measured by barometers and is shown on maps as isobars, or areas of equal pressure.
Wind Movements Wind does not necessarily travel in a straight line. In a high-pressure area (anticyclone), the wind flows out and around like water poured over an inverted bowl. In a low-pressure area (cyclone), the wind flows inward. Adding to the complexity, Earth's rotation creates another force called the Coriolis force. The Coriolis force deflects the air so that it moves to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Effects of Mountains and Prevailing Winds When a prevailing wind meets up with an obstacle like a mountain range, the air rises and cools, forming clouds, and loses some moisture in the form of precipitation. This occurs on the windward side of the mountains. After the air passes over the mountain range, it descends and heats up. Thus the leeward side of a mountain range is often dry and sometimes will become a desert.
Storms Two important kinds of storms are thunderstorms and hurricanes.
Thunderstorms These occur when moist, unstable air is lifted vertically. When lifting begins, the air starts to cool until the dew point is reached and condensation takes place, forming a cumulus cloud. For a thunderstorm to form, the uplifting needs to continue until a cumulonimbus cloud forms. Within these clouds are found strong winds, downdrafts, hail, thunder, lightning, and even tornadoes.
Hurricanes These are large cyclonic storms (areas of very low pressure) that develop over the oceans in the tropics. Other names for hurricanes are cyclones, tropical cyclones, and typhoons. The center of the hurricane has the lowest air pressure. The magnitude of the pressure gradient at Earth's surface from the outside to the inside of the hurricane determines the strength of the winds. Around the clear and calm eye are bands of thunderstorms, severe rain, and winds. Damage from hurricanes comes from strong winds, flooding, and storm surge. Storm surge is created when strong winds push ocean water upward and inland, much like a huge water wall. The water crushes and often washes away many of the obstacles in its way.
Climate is the general temperature and humidity of a region over time. For example, a climate can be tropical, desert, or polar. Climate is dependent upon several factors, including
- The latitude of the area
- What air masses generally influence the area
- Proximity of oceans and ocean currents
- Location of mountain barriers
- Direction of the prevailing winds
Over Earth's history, the climate has changed. Sometimes the planet was warmer and more tropical; other times it was colder and covered with ice. Today scientists suspect that Earth is warming, primarily because of atmospheric pollution caused by human activities.
Climate changes have been tracked through the chemical composition of rocks, the types of animals that lived in the past, and indicators such as tree rings.
The Greenhouse Effect The greenhouse effect is a phenomenon of Earth's atmosphere. It is similar to the heat buildup in your car on a sunny day. A huge percentage of the rays of visible light from the sun pass through Earth's atmosphere, warming the planet's surface. Part of the energy is re-radiated or bounced back toward space by long-wave infrared radiation and is absorbed by carbon dioxide molecules and water vapor in the atmosphere. This energy is reflected back to Earth's surface in the form of heat. Basically, all this is a good thing; without it, Earth's surface would be too cold to allow us to survive. Even the oceans would freeze if it were not for the greenhouse effect.
However, scientists worry that as we increase the amount of carbon dioxide in the atmosphere through industrial pollutants and other particles, the greenhouse effect can intensify. A significant and persistent warming of the planet can cause long-term climate changes. An increase in chlorofluorocarbons, nitrous oxide, and methane from human activity can aggravate this situation.
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