GED Science Practice Test: Atmosphere

Read the following passage and answer the question below:

The atmosphere of Earth is a layer of gases surrounding the earth.  Those gases stay around earth due to the earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

The common name given to the atmospheric gases used in breathing and photosynthesis is air. By volume, dry air contains 78% nitrogen, 21% oxygen, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 2%.

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Layers of the Atmosphere:

As you move farther and farther away from earth’s surface into the upper layers of the atmosphere, the atmosphere becomes thinner and thinner.  Thus, the highest amount of air pressure is experienced near the surface of the earth in the troposphere layer.  The troposphere has over 75% of the atmosphere’s mass, due to this air pressure, and it contains 99% of the water vapor found in the atmosphere.  These characteristics make the troposphere the only atmospheric layer suitable for the survival of terrestrial plants and terrestrial animals.  Conversely, the least amount of air pressure is experienced far from the surface of the earth in the exosphere. In the exosphere, molecules present in air are still gravitationally held to the planet, but don’t interact with each other, because they are so spread out. Outside of the exosphere is outer space, however, there is no definite boundary between the atmosphere and outer space.  When spacecraft re-enter the earth’s atmosphere, the friction between the molecules in the air and the spacecraft create a great deal of friction and heat that must be carefully managed.  These atmospheric effects become noticeable upon re-entry at an altitude of about 120 kilometers, well below the exosphere layer, in the thermosphere layer.  The following diagram shows the troposphere, exosphere, and the layers in between:

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As you can see, several layers can be distinguished in the atmosphere.  Such layers are characterized by temperature, composition, and pressure.  The characteristics of the atmosphere mean that some types of energy from the sun make it through some layers, and some types of energy from the sun get trapped in some layers.  The following diagram shows how far different types of electromagnetic radiation, such as UV rays and infrared rays, can penetrate into the earth’s atmosphere:

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