47 No Moisture Here!

They are the world's driest places. Sometimes rain doesn't fall there for years. These dry areas—deserts—cover about one-fifth of the earth's surface.

Perhaps you think of a desert as a place filled with sand. This is true of many deserts, but not all. Areas near the North and South poles are also considered deserts by some scientists. They define a desert as any area where the moisture that is lost, mostly by evaporation, is greater than the moisture that falls as rain or snow.

Many deserts are near the equator, where the air is dry and warm. Others are in regions blocked off from the winds by mountains. The moisture from the ocean winds evaporates as the winds pass over the mountains and cool off. But some deserts are found right next to oceans. Deserts usually form along coastlines if there is a cold current in the ocean water. The cold wind blowing across that current and onto the nearby land holds little moisture.

Most deserts get less than ten inches of rain a year. Because the land is so dry, the rain doesn't penetrate; it just washes over the surface. Some deserts almost never get any rain. For instance, it rains in the Atacama Desert on the coast of Chile about once every hundred years!

Not many varieties of plants will grow in a desert. Some cactuses survive because they store water inside themselves after a rain. When it does rain, a desert landscape may flourish. Many plants will put forth bright and beautiful flowers.

它们是世界上最干旱的地方。有时那里数年不下雨。这些干旱地区——沙漠——覆盖了地球表面约五分之一的面积。

也许你认为沙漠是满是沙子的地方。许多沙漠确实如此，但并非全部。一些科学家也将南北极附近的区域视为沙漠。他们将沙漠定义为任何因蒸发等原因失去的水分多于降雨或降雪带来的水分的区域。

许多沙漠靠近赤道，那里空气干燥且温暖。另一些则位于被山脉阻挡了风的区域。来自海洋的风在越过山脉并冷却时，其中的水分会蒸发。但有些沙漠就在海洋旁边。如果海洋中有寒流，沙漠通常会沿着海岸线形成。吹过该洋流并吹向附近陆地的冷风几乎不含水分。

大多数沙漠每年的降雨量不足十英寸。由于土地非常干燥，雨水无法渗透，只是在地表流过。有些沙漠几乎从不下雨。例如，智利海岸的阿塔卡马沙漠大约每百年才下一次雨！

没有多少种类的植物能在沙漠中生长。一些仙人掌能存活下来，因为它们在雨后会在体内储存水分。当下雨时，沙漠景观可能会变得繁茂，许多植物会开出鲜艳美丽的花朵。