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The weather is something which surrounds
us constantly.
It often affects what we do, the clothes
we wear and how we live.
When we talk about the weather, we are
really describing the conditions in the atmosphere and the
air around us at a particular moment. If you look outside
now you might see that the weather is cloudy or sunny, or
that it is wet or windy.
There are many different types of weather. Here are a few examples.
Clouds
Clouds are made up of millions of tiny water
droplets which form when moist, warm air rises into the sky
and is then cooled down. It can take between a few minutes and
an hour for a cloud to form. There are three basic families
of clouds and seven other main types made up from these families.
The kinds of clouds you see give clues about the weather.
More
facts about clouds
Rain
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One type of weather that we seem to
get a lot of here in the British Isles is rain. Rainfall
starts out high in the sky when the tiny water droplets
contained in clouds merge together and start to grow
in size. |
Eventually the droplets grow to such a size
that they become too heavy to remain floating in the air and
fall out of the cloud towards the ground. You may, however,
have noticed that not all rainfall is the same. Sometimes the
raindrops are small and not easily seen. This is called drizzle
and usually comes from low, grey clouds. At the other end of
the scale the fast heavy rainfall that we get in summer storms
tends to be from larger towering clouds. The amount of rain
that falls varies from country to country and often changes
between the months of the year. In the British Isles, it is
generally wetter in the west and drier in the east and it also
rains more in the winter than in the summer.
Snow
Snow is formed when the weather is very
cold, with temperatures near to or below 0 °C, therefore
it is usually something we only see during the winter months.
Like rain, it starts to form when tiny particles floating
in clouds bump into each other and get steadily bigger.
Eventually, they are heavy enough to fall out of the bottom
of the cloud and drop to the ground as snow. It is important
that the air is freezing though, or the snow will melt and
turn to rain before it hits the ground.
More
facts about snow
Fog
| Fog is really a cloud that is very close
to the ground. It is made up of millions of tiny water droplets,
so that if you walk or cycle through fog you will often
find yourself getting wet. Fog builds up when very moist
air is cooled down by the ground so the air condenses to
form water droplets. |
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Think of how moist, hot air from a boiling
kettle condenses to form water droplets on a cold window
pane and this will give you an idea of how fog forms. When
surrounded by fog, it is very difficult to see very far
ahead because all the water droplets that make up the fog
are in the way. The more droplets there are, the denser
the fog is. Mist is the name given to thin fog where there
are few droplets.
Thunder and lightning
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Thunderstorms are some of the most spectacular
weather events we can experience in the British Isles. You
can often tell when a thunderstorm is about to occur because
tall storm clouds develop and the sky becomes very dark. |
Lightning can be seen flashing from the clouds
as the thunderstorm approaches, followed by a boom of thunder.
Lightning itself is a giant electrical spark leaping between
the clouds or down to the ground, heating the air in its path
to 30,000 °C. This sends out large shock waves, which we
hear as thunder. Thunder and lightning happen at the same time,
but light travels faster than sound, so you see the flash of
lightning before you hear the clap of thunder.
| Next time you see a thunderstorm you
can try an experiment to find out just how far away
the storm is. Start counting the seconds from the time
when you actually see the lightning flash to when you
hear the thunder. Then divide the time you've counted
by three and this will be the distance in kilometres
between you and the storm. |
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If you do this every time there is a flash
of lightning and count to a higher number each time, the
storm is moving away. If you count to a lower number each
time, the storm is moving nearer.
Wind
Wind is the movement of the air around us from
place to place. The heat from the sun warms up parts of the
land and sea, which in turn warms the air above them. Air that
is warmed becomes lighter than the surrounding air and rises.
The wind blows because cold air moves in to replace this hotter
air. We are not able to see the wind but we can often hear it
and we can see its effect on the things around us.
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If you look outside you can see the
branches on trees moving or the washing on clothes lines
blowing about. If you go outside you can feel the wind
on your face. The strength of the wind varies a great
deal. You will probably have noticed that sometimes,
when the wind is very strong, it is difficult to walk.
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At other times it may not be as strong
as that but will still pick up any pieces of paper or dead
leaves lying on the ground and blow them about. At the other
extreme, it can be very still and the leaves on the trees
will hardly be moving at all. In 1805, Admiral Beaufort
worked out a scale for measuring the strength of the wind,
called the Beaufort Scale, which is still used today. It
ranges from Force 0, which is calm, to Force 12 which is
hurricane force.
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