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Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: Air temperature changes as altitude increases. In some layers of the atmosphere, the temperature decreases. In other layers, it increases. You can see this in Figure 15.12. Refer to this figure as you read about the layers below. The troposphere is the lowest layer of the atmosphere. In it, temperature decreases with altitude. The troposphere gets some of its heat directly from the Sun. Most, however, comes from Earths surface. The surface is heated by the Sun and some of that heat radiates back into the air. This makes the temperature higher near the surface than at higher altitudes. Look at the troposphere in Figure 15.12. This is the shortest layer of the atmosphere. It rises to only about 12 kilometers (7 miles) above the surface. Even so, this layer holds 75 percent of all the gas molecules in the atmosphere. Thats because the air is densest in this layer. Air in the troposphere is warmer closer to Earths surface. Warm air is less dense than cool air, so it rises higher in the troposphere. This starts a convection cell. Convection mixes the air in the troposphere. Rising air is also a main cause of weather. All of Earths weather takes place in the troposphere. Sometimes air doesnt mix in the troposphere. This happens when air is cooler close to the ground than it is above. The cool air is dense, so it stays near the ground. This is called a temperature inversion. An inversion can trap air pollution near the surface. Temperature inversions are more common in the winter. Can you explain why? At the top of the troposphere is a thin layer of air called the tropopause. You can see it in Figure 15.12. This layer acts as a barrier. It prevents cool air in the troposphere from mixing with warm air in the stratosphere. The stratosphere is the layer above the troposphere. The layer rises to about 50 kilometers (31 miles) above the surface. Air temperature in the stratosphere layer increases with altitude. Why? The stratosphere gets most of its heat from the Sun. Therefore, its warmer closer to the Sun. The air at the bottom of the stratosphere is cold. The cold air is dense, so it doesnt rise. As a result, there is little mixing of air in this layer. The stratosphere contains a layer of ozone gas. Ozone consists of three oxygen atoms (O3 ). The ozone layer absorbs high-energy UV radiation. As you can see in Figure 15.14, UV radiation splits the ozone molecule. The split creates an oxygen molecule (O2 ) and an oxygen atom (O). This split releases heat that warms the stratosphere. By absorbing UV radiation, ozone also protects Earths surface. UV radiation would harm living things without the ozone layer. At the top of the stratosphere is a thin layer called the stratopause. It acts as a boundary between the stratosphere and the mesosphere. The mesosphere is the layer above the stratosphere. It rises to about 85 kilometers (53 miles) above the surface. Temperature decreases with altitude in this layer. There are very few gas molecules in the mesosphere. This means that there is little matter to absorb the Suns rays and heat the air. Most of the heat that enters the mesosphere comes from the stratosphere below. Thats why the mesosphere is warmest at the bottom. Did you ever see a meteor shower, like the one in Figure 15.15? Meteors burn as they fall through the mesosphere. The space rocks experience friction with the gas molecules. The friction makes the meteors get very hot. Many meteors burn up completely in the mesosphere. At the top of the mesosphere is the mesopause. Temperatures here are colder than anywhere else in the atmosphere. They are as low as -100 C (-212 F)! Nowhere on Earths surface is that cold. The thermosphere is the layer above the mesosphere. It rises to 600 kilometers (372 miles) above the surface. The International Space Station orbits Earth in this layer as in Figure 15.16. Temperature increases with altitude in the thermosphere. Surprisingly, 
 Question: The shortest layer of the atmosphere is the
Output:
troposphere.