There are many different factors that affect climate around the world. It is the varying influence of these factors that lead to different parts of the Earth experiencing differing climates. The most important natural factors are:

• distance from the sea
• ocean currents
• direction of prevailing winds
• shape of the land (known as 'relief' or 'topography')
• distance from the equator
• the El Niño phenomenon.

It is now widely accepted that human activity is also affecting climate, and that the impact is not the same everywhere. For example, changes appear to be happening faster near the poles than in many other places. In this tutorial we will look at some of these factors in more detail.

The sea affects the climate of a place. Coastal areas are cooler and wetter than inland areas. Clouds form when warm air from inland areas meets cool air from the sea.  The centre of continents are subject to a large range of temperatures.  In the summer, temperatures can be very hot and dry as moisture from the sea evaporates before it reaches the centre of the land mass.

Ocean currents can increase or reduce temperatures. The diagram below shows the ocean currents of the world (view original source map). The main ocean current that affects the UK is the Gulf Stream.

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The Gulf Stream

The Gulf Stream is a warm ocean current in the North Atlantic flowing from the Gulf of Mexico, northeast along the U.S coast, and from there to the British Isles.

The Gulf of Mexico has higher air temperatures than Britain as it is closer to the equator. This means that the air coming from the Gulf of Mexico to Britain is also warm. However, the air is also quite moist as it travels over the Atlantic ocean. This is one reason why Britain often receives wet weather.

The Gulf Stream keeps the west coast of Europe free from ice in the winter and, in the summer, warmer than other places of a similar latitude.

Winds that blow from the sea often bring rain to the coast and dry weather to inland areas.  Winds that blow to Britain from warm inland areas such as Africa will be warm and dry.  Winds that blow to Britain from inland areas such as central Europe will be cold and dry in winter. Britain's prevailing (i.e. most frequently experienced) winds come from a south westerly direction over the Atlantic.  These winds are cool in the summer, mild in the winter and tend to bring wet weather.

Climate can be affected by mountains. Mountains receive more rainfall than low lying areas because as air is forced over the higher ground it cools, causing moist air to condense and fall out as rainfall.

The higher the place is above sea level the colder it will be.  This happens because as altitude increases, air becomes thinner and is less able to absorb and retain heat. That is why you may see snow on the top of mountains all year round.

The distance from the equator affects the climate of a place. At the poles, energy from the sun reaches the Earth's surface at lower angles and passes through a thicker layer of atmosphere than at the equator. This means the climate is cooler further from the Equator. The poles also experience the greatest difference between summer and winter day lengths: in the summer there is a period when the sun does not set at the poles; conversely the poles also experience a period of total darkness during winter. In contrast, day length varies little at the equator.

El Niño, which affects wind and rainfall patterns, has been blamed for droughts and floods in countries around the Pacific Rim. El Niño refers to the irregular warming of surface water in the Pacific. The warmer water pumps energy and moisture into the atmosphere, altering global wind and rainfall patterns. The phenomenon has caused tornadoes in Florida, smog in Indonesia, and forest fires in Brazil.

El Niño is Spanish for 'the Boy Child' because it comes about the time of the celebration of the birth of the Christ Child. The cold counterpart to El Niño is known as La Niña, Spanish for 'the Girl Child', and it also brings with it weather extremes.

The factors above affect the climate naturally. However, humans also affect the climate. Early on in human history our effect on the climate would have been quite small. However, as populations increased and trees were cut down in large numbers, so our influence on the climate increased. Trees take in carbon dioxide and produce oxygen. A reduction in trees will therefore have increased the amount of carbon dioxide in the atmosphere.

The Industrial Revolution, starting at the end of the 19th Century, has had a huge effect on climate. The invention of the motor engine and the increased burning of fossil fuels have increased the amount of carbon dioxide (a greenhouse gas - more on that later) in the atmosphere. The number of trees being cut down has also increased, reducing the amount of carbon dioxide that is taken up by forests.

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