英语听力：从南极到北极 13

The sun beating down on tropical waters powers the weather systems of the globe. Moisture evaporates from the warming ocean and rises to create great storms. The winds generated out at sea sweep inland across the continents. As they travel across the Sahara, they create the biggest of all sand storms, blowing sand halfway around the world to fertilize the Amazon jungle. Winds blowing across the Indian ocean collect moisture and sweep northwards towards the Himalayas. As the air rises, so it cools. The water it carries condenses into clouds and then falls as the life-giving rains of the monsoon. So air currents powered by the sun carry wet air to the middle of continents. Without water, there could be no life. But its distribution over the land is far from even. Deserts cover one-third of the land surface. And they are growing bigger every year. This is the Kalahari desert in South Africa. It’s the dry season. And thousands of elephants have started to travel in a desperate search for water.