#14 - Dry Deserts 干沙漠

1.Deserts occur in places where it's hard for rain to reach.
Most rain comes from clouds that form over the sea and blow onto the land. If there's a big mountain range, the clouds never reach the other side. An area called a rain shadow desert forms. Deserts also form in the middle of continents. The land there is so far from the sea, rain clouds can barely reach it.

V The Namib Desert in the southwest of Africa contains some of the biggest sand dunes in the world. V

2.The world's biggest desert used to be a swamp!
The Sahara Desert takes up most of the northern Africa. It is made up of 9 million square kilometers of dry sand, pebbles, and boulders. There are some oasis too, where freshwater springs flow out of the ground. Animal bones and objects left by ancient peoples show that around 6000 years ago, the Sahara Desert was green and swampy. Lot of hippos, crocodiles, and humans lived there.

3.The world's driest desert is the Aatacama Desert in Chile, South America.
This desert is right next to the sea! It formed because, in South America, rainclouds blow from east to west. They drop their rain on the Amazon rainforest, but cannot get past Andes mountains. On the other side of the Andes, next to the Pacific ocean, is the Atacama Desert. It is so dry that people who died there 9000 years ago have been preserved as mummies.

V Desert roses aren't plants. They occur when desert minerals, such as gypsum, combine with sand to form crystals. V

4.Deserts aren't always hot.
The hottest temperature ever recorded was 57.8°C in Libya. However, deserts can be cold, too. The average temperature in the Atacama Desert, South America is only about 10°C. In the Gobi Desert in Asia, winter temperatures can drop to -40°C. All deserts can be cold at night, as there are no clouds to stop heat escaping.

5.Even in dry deserts, there is water if you know where to look.
Desert plants, such as cactuses store water in their stems, leaves or spines. When rain does fall, it seeps into the ground and stays there. Desert people and animals chew desert plants or dig into the ground to find enough water.

VAn oasis is a freshwater spring in a desert. Oasis form when water stored deep underground meets a barrier of rock that it can't soak through, and rises to the desert surface. V

垃圾发生在难以到达的地方。
大多数雨水来自形成在海上的云彩，并吹向陆地。如果有一个很大的山脉，云层就不会到达对方。一个称为雨影沙漠的地区形成。沙漠也形成在大陆中部。距离大海很远的地方，雨云几乎无法到达。

V非洲西南部的纳米比亚沙漠包含了世界上最大的沙丘。 V

世界上最大的沙漠曾经是沼泽！
撒哈拉沙漠占据了北非的大部分地区。它由900万平方公里的干砂，鹅卵石和巨石组成。还有一些绿洲，淡水泉涌出地面。古民族留下的动物骨骼和物体显示，6000年前，撒哈拉沙漠是绿色和沼泽的。很多河马，鳄鱼和人类住在那里。

世界上最干旱的沙漠是智利，南美洲的阿塔卡马沙漠。
这个沙漠就在海旁边！形成的原因是，在南美洲，雨云从东向西吹。他们在亚马逊雨林下雨，但不能过去安第斯山脉。在安第斯山脉的另一边，毗邻太平洋，是阿塔卡马沙漠。这样干燥，9000年前死亡的人已经被保存为木乃伊。

V沙漠玫瑰不是植物。它们发生在诸如石膏等沙漠矿物与沙子结合形成晶体时。 V

警报并不总是很热。
利比亚记录的最热气温为57.8°C。然而，沙漠也可能很冷。南美洲阿塔卡马沙漠的平均气温仅为10摄氏度。在亚洲的戈壁沙漠，冬季气温可能下降至-40°C。所有的沙漠都可以在晚上感冒，因为没有云可以阻止散热。

即使在干燥的沙漠，如果你知道在哪里看，也有水。
沙漠植物，如仙人掌，将水分放在茎，叶或刺。当雨落下降时，它渗入地面并停留在那里。沙漠人和动物咀嚼沙漠植物，或者挖到地上找到足够的水。

V绿洲是沙漠中的淡水泉。绿洲形式当储存在深层地下的水会遇到不能渗透的岩石的障碍物，并升起到沙漠表面。 V