Page 47 - 新思维科学学生用书7 样章

2 Materials and their structure

What happens during the water cycle?

Water moves into the atmosphere

Energy from the Sun heats the Earth and the temperature
of the water in the rivers, lakes and oceans increases.
When this happens, some of the liquid water forms water

vapour, which moves into the atmosphere. This is called
evaporation. This happens because some of the particles
in the liquid water gain enough energy to break free from
the forces holding them together and they change to a gas.

Water can also evaporate into the atmosphere from plants;
this is called transpiration.

Water in the atmosphere cools down

As the water vapour goes up into the atmosphere, it cools
and changes back into little droplets of water in the air,
forming clouds. This process is called condensation. It
happens because the particles in the water vapour lose

energy and cannot move so quickly. Air currents high in the
atmosphere move the clouds around the world.

Water falls from clouds

When a lot of water has condensed, the water droplets in
the clouds become too heavy for the air to hold them. The
droplets fall back to Earth as rain. If the drops become
colder they may form snow, hail or sleet. This process is

called precipitation.

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