Around the Earth is something called an atmosphere, which is a mixture of gases that is held against the Earth’s surface by gravity. All the different kinds of weather we experience are caused by the daily heating and cooling that takes place within the atmosphere.
In some parts of the world, the weather stays very much the same for most of the year. Elsewhere, like the UK, the weather may change daily. The pattern of weather over a long period of time is known as the climate of an area.
From outer space, the Earth is seen to be covered in swirling clouds. These are only in the lower part of the atmosphere, up to around 11 km.
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You can feel the weight of the air by putting a large piece of paper on a flat surface and putting a ruler underneath it. Bang the ruler. You will find that it is difficult to make the paper lift; this is due to air pressure. |
| Fig 1: Paper and ruler pressure test. |
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The winds
Over some areas air piles up to give a high pressure; between these areas there is low pressure. If the Earth did not spin then the winds would simply blow from high to low pressure. The spin of the Earth makes the winds blow around these instead. Near the equator these winds are very regular and were once used by sailing ships. These are known as the Trade Winds.
Water cycle
| Water evaporates from the sea and ground and turns from liquid into a gas called water vapour. The air takes up the moisture and as the air rises above the hills the water turns from gas into tiny water droplets. This produces clouds and the water falls back to the ground as rain. The rain is soaked up by the ground and also runs back down the rivers and into the sea. The whole process then starts again. |
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Fig 2: The water cycle |
Rain
Water droplets in a low cloud join together to make larger droplets which fall as rain. In a higher cloud, the droplets freeze and join together to form ice crystals which then melt as they fall.
Snow
In winter the ice crystals do not melt as they fall and instead fall as snow. If a low pressure area crosses the country it is sometimes possible that snow will fall.
Fog
Fog can be thought of as a cloud at ground level. In valleys, night fog is common. This is produced because cold air sinks to the bottom of the valley to produce fog. Often the hilltops may be clear as they poke out through the top of the fog, whilst the valley is still foggy.
Let's find out
Write a story about getting lost in the fog.
Produce a dance based on snowflakes falling.
Snow can be great fun. Produce a large picture showing a range of snow sports and activities.
Heavy rain can sometimes cause severe flooding. Work out some ways you could protect you home in case it floods.
Write a poem about the rain and decorate it with raindrops.
Things to make
Here is another way of showing how strong the air pressure is.
You will need:
- a plastic beaker;
- a piece of card.
Fill the beaker with water.
Hold it over the sink and press the piece of card over the top. Now carefully turn the beaker up the other way.
The pressure of the air on the card produces enough force to keep the water in the beaker. |
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Fig 3: Air pressure testing. |
Amazing facts
If 10 centimetres of snow are melted in a glass it will be would produce only about 1 centimetre of water.
A rainbow lasting over 3 hours was reported in Wales in 1979.
To find out how far away a thunderstorm is, count the number of seconds between the flash and the bang. Divide this by three to get the distance of the storm in kilometres.
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Fig 4: Thunder and lightning. |
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