Part of a lecture about the water cycle — the continuous journey

Section 4. You will hear part of a lecture about the water cycle — the continuous journey water makes between the land, the sea and the sky. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good morning. Today's subject is the water cycle, the endless circulation of water around our planet. The striking fact to hold on to is that the total amount of water on Earth never changes, the very same water has been cycling for billions of years, so the water in your glass may once have fallen as rain on a dinosaur. Let me take you round the cycle stage by stage. MAN: The engine of the whole system is the Sun. Its heat warms the surface of the oceans, and turns liquid water into an invisible gas, water vapour, which rises into the air. This first stage is called evaporation. Now, plants add to this too. They draw water up through their roots and release it as vapour from their leaves, and this particular contribution has its own name, transpiration. Together these lift enormous quantities of water into the atmosphere. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: As the vapour rises, the air around it grows colder, and cold air cannot hold as much moisture. So the vapour turns back into tiny liquid droplets, the reverse of evaporation, and this stage is called condensation. It is these countless droplets, clustered together, that we see as a cloud. A cloud, in other words, is not vapour, it is already liquid water, suspended in the air. MAN: The droplets in a cloud collide and merge, growing heavier, until the air can no longer hold them up, and they fall. This falling water, in whatever form, rain, snow, hail or sleet, is given the general term precipitation. Where the water goes next depends on where it lands. Some of it soaks down into the ground and is stored in the soil and rock, forming what we call groundwater. The rest flows downhill across the surface, gathering into streams and rivers, this movement over the land is called run-off, and it eventually carries the water back to the sea, where the whole cycle begins again. MAN: I want to end with why this matters beyond the textbook. First, the cycle is what makes fresh water available on land at all, evaporation leaves the salt behind in the sea, so the rain that falls is fresh, in effect the Sun distils the ocean for us. And second, human activity is now altering the cycle, a warmer atmosphere holds more moisture and can release it more violently, which is why a heating climate is expected to bring both heavier floods and deeper droughts, sometimes in the very same region. Understanding the cycle, then, is the first step to understanding those changes. That's all for today. That is the end of Section 4. You now have some time to check your answers.