The water cycle

Four-fifths of the world’s surface is covered by water. The heat of the sun makes water evaporate from oceans, rivers and lakes. As it rises, water vapour cools and condenses to form clouds of tiny droplets. Further cooling of the clouds produces larger drops of water which fall to the ground as rain or, if it is cold enough, as snow. Rain water trickles through soil and porous rocks until eventually it finds its way back to the sea. Then the cycle begins all over again.

As rain water falls through the air, it dissolves oxygen, nitrogen and carbon dioxide. When carbon dioxide dissolves, it forms a solution of the weak acid, carbonic acid. As rain water trickles through porous rocks, it dissolves chemicals from the rocks. The salts which rain water dissolves find their way to the sea. When water evaporates from the sea, the dissolved salts remain.

In industrial regions, the air contains pollutant gases. These dissolve in rain water to make acid rain.

So, the water cycle has the following stages:

1. Heat from the sun makes water evaporate from the oceans.

2. Transpiration: the loss of water through the leaves of plants.

3. As water vapour rises, it cools. Condensation produces tiny droplets of water.

4. Winds blow clouds towards the land.

5. As clouds rise when they meet land masses, further cooling occurs. Larger drops of water fall as rain or snow.

6. Rivers flow to the sea. They receive sewage and industrial waste.

7. Rain water flows back towards the sea, completing the cycle. It dissolves chemicals from the rocks through which it passes.

SUMMARY NOTE

There is a natural water cycle. Water vapour enters the atmosphere by evaporation. Water vapour leaves the atmosphere by condensation, followed by precipitation as rain or snow. Air dissolves in rain water. The oxygen that dissolves in water is vital to fish. If too much organic matter, e.g. sewage, is discharged into a river, the dissolved oxygen is used up in oxidizing the organic matter, and the water becomes unable to support life.

VOCABULARY AND COMPREHENSION

2. Translate the following words using a dictionary:

tiny, droplet, further, transpiration, eventually, to trickle, to dissolve, to evaporate.

3. Choose the proper ending of the sentences:

1. The heat of the sun makes water …

2. As it rises, water vapour cools and condenses to form …

3. Further cooling of the clouds produces larger drops of water which …

4. Rain water trickles through soil and porous rocks until eventually it …

5. When carbon dioxide dissolves, it forms …

6. The salts which rain water dissolves find …

4. Find factual information in the text to prove the following:

a) Acid rain is formed in industrial regions.

b) If too much organic matter is discharged into a river, the water becomes unable to support life.

5. Look through the text again and ask your partner questions covering the content of the text. Use the following phrases as starters:

“I’m sure you know…”, “Would you tell me…?”, “By the way, what…?”, “Do you remember…?”, “I hope you know…”.

Describe a natural water cycle.

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Read the following text and carry out assignments given below it.

Дата добавления: 2015-10-30