单选题 Are you planning a vacation? If you like hot and extremely dry summers, go to Phoenix, Arizona. For hot temperatures but lots of rain and thunderstorms, try Miami, Florida. If you want average temperatures and rainfall, St. Louis, Missouri, is the spot. Or if you're a cold weather fan, head to Fairbanks, Alaska. Its winters are very cold with little precipitation. Each of these cities has a certain type of weather. The average weather for a place over many years is called climate, and in no two places in the world is it exactly the same. How can this be?
Many things in nature, such as sunshine, temperature, and precipitation, affect climate. Nearness to mountain, oceans, and large lakes affects it too. Another factor is altitude, or height above sea level. Air cools as altitude increases. So a city at a higher altitude may be colder than one at a lower altitude. Finally, winds affect climate. They move heat and moisture between the oceans and continents. Winds keep the tropics from overheating. They keep the polar regions from getting overly cold.
Climate changes over long periods of time. Some scientists think, for example, that the earth's climate changed at the time of the dinosaurs. They think the dinosaurs died because of the change. What causes a climate to change? One possible cause may be changes in the sun. Sunspots, for example, are cool, dark spots that form on the sun. Sunspots may decrease precipitation on the earth and cause unusually dry periods. Changes in the atmosphere may change climate too. Volcanic eruptions, for instance, release solid particles into the air. These particles may form a cloud that blocks out the sun's heat. Human activity is another cause of climate change. Air pollution and the reduction of forest cover may have long-term effects on climate.