The Atmosphere Study Guide for McGraw-Hill's ASVAB
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.
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