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 with some information about them.

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.

Rain

One type of weather which is so very important to every part of the world 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.

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.

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.

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 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.

Think of how moist, hot air from a boiling water 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

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.

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 warm 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.

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. 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.

Hurricanes

Hurricanes are extremely strong storms which can be very destructive and dangerous to both people and their environment. They can be up to five or six miles (eight or nine kilometres) high and from 300 to 600 miles (500 to 1,000 km) wide. A hurricane moves forwards like a giant spinning top, with winds blowing at speeds of 74 miles (120 km) per hour or more. There is, however, a small area at the centre of a hurricane where the sky is clear and the winds are light, known as the 'eye' of the storm.

Hurricanes begin over the warm parts of the oceans and are given different names depending on where in the world they occur. The word 'hurricane' refers to those that arise in the Caribbean, in the Pacific they are called 'typhoons', in the Indian Ocean 'cyclones' and off the coast of Australia they are known as 'willy-willies'. The highest wind speed ever recorded was 217 miles (349 km) per hour in 1969 during Hurricane Camille.

Weather forecasts

In the past, forecasts were mainly based on the observations made by people on the ground. Reports from observers in many different areas would be studied to gain an overall picture of what the weather was like. From this, the forecasters could predict what the weather would be. There were many areas of the world, particularly the oceans, where there were no people to observe the weather and so the forecasters would have to estimate what was going on in those places. Nowadays, improvements in technology have changed the way that the weather is predicted. Computers, radar and satellites are used as well as ground observations. The estimation of what is happening in the remote areas such as the oceans is now done by computers instead of people. This means that the forecasts are now more accurate than before and can be used to cover longer periods of time.

Weather satellites

The first weather satellite was launched on 1 April 1960. Satellites look down to earth from space and give information to weather forecasters about what is actually happening in different parts of the globe and which cannot be seen from the ground. There are two types of satellite. One type appears to 'hang' over the same spot above the equator and orbits the earth at a height of 35,780 km taking 24 hours to complete each orbit.

If a satellite stops orbiting and falls back down to Earth it will be broken into pieces and burned up when it re-enters the Earth's atmosphere. Sometimes pieces of a satellite will fall down to the ground or into the sea. This could be dangerous if large pieces fell onto a town, however, it is usually possible to tell where they are likely to land.

Rainbows

Rainbows occur when it is raining and the sun is shining at the same time. The light from the sun looks white but is in fact made up of seven different colours. We can only see these individual colours when the rays of sunlight are split up, for instance by raindrops, giving us a rainbow.

The colours in the rainbow are red, orange, yellow, green, blue, indigo and violet. They always occur in that order, with red on the outside and violet on the inside. There are sayings you can learn to help you remember the order, for example 'Richard of York gave battle in vain'. To see a rainbow, the sun has to be low in the sky and behind you and the shower of rain has to be in front of you. Sometimes the sunlight is split up twice by the raindrops and then a second, fainter, rainbow can be seen around the outside of the first one, with the colours in reverse. From the ground we see a rainbow as a semicircle, but if you were in an aeroplane you could see it as a whole circle.

Climate

'Weather' refers to the conditions existing at one time or on a particular day. The word 'climate' is used to sum up what the weather is usually like at a particular place over a longer period of time. There are many different climates around the world. Britain's climate is temperate, meaning we rarely experience either very hot or very cold temperatures, and maritime because we are near an ocean. We have seasons with generally warm summers and coldish winters and some rainfall all the year round. Other areas of the world, including most of Europe, some of the United States and New Zealand, also have a temperate climate. These are popular areas for people to live in.

Other types of climate include 'Mediterranean', with hot, dry summers and mild, wet winters, occurring mainly around the Mediterranean; 'Tropical rainforest', where the temperature is hot and the rainfall heavy all year round, occurring near to the equator; 'Polar', which is cold with ice-covered lands and occurs in the Arctic and Antarctic; and 'Desert', where there is little or no rainfall and little variation between the seasons, occurring for example in North Africa.

Who does the weather effect?

Most of us are affected by the weather in some way. Many people like to listen to or watch the weather forecasts so that they can see what the weather is going to be like. It's no fun having a picnic in the pouring rain and there's no point going skiing if there's no snow. The weather has a great effect on our free time, but there are also people doing many different jobs who need weather forecasts to plan ahead and keep things running smoothly.

Here are a few examples of them:

Pilots

Pilots need to have up-to-date weather information to help them prepare their routes by avoiding some types of weather and taking advantage of others. If the wind is behind an aircraft, the flight will take less time and need less fuel than if it is flying into the wind. They will also need to know the weather conditions at the airport where they are planning to land, in case these are bad enough to prevent them from landing and they have to use another airport.

Farmers

Different crops may need different conditions to grow in, therefore the type of crops farmers grow may depend on the type of weather occurring in that particular area. Farmers will also want to know what sort of weather is expected from day to day, to help them plan their sowing or planting, to decide when to control weeds and insect pests and to plan harvesting.

Sailors

It is important for sailors to know what the conditions at sea are likely to be, as high winds and rain can make a journey at sea very difficult and sometimes dangerous. Many sailors listen to the Shipping Forecasts on BBC radio before they set off so they can work out which areas are safe, or decide whether it is wise to sail at all.

Builders

Forecasts are very important to builders because rain and wind can often slow down or stop building work. By looking at the weather predictions, they can see when there will be bad weather and then they can plan ahead.

Supermarkets

Supermarkets want to know what the weather will be like to help them decide what to order. If it is going to be hot and sunny people will buy more cold drinks, ice cream and salads. If good weather is forecast at the end of the summer, shops may want to carry on selling charcoal for barbecues for longer. When rain is expected, shops may move their display of umbrellas near to the doors to sell more.

Energy suppliers

The gas and electricity companies need to know when the weather is likely to be cold so they can make sure that there is enough power available. If it is very cold when you all go home from school, you will be more likely to put on the television and turn on the heating, than go outside to play in the garden. This will cause a power surge because everything has been turned on at once, and if the companies are not expecting it there may not be enough power to go round. The people who build oil rigs and tow them out to sea also need to know what the weather will be like. If it is going to be very windy they may not be able to move the rigs in case they are blown over.