Climate of the British Isles
Climate of the British Isles
?To what extent is the climate of the British Isles a product of the air masses that affect it? (40 marks) The British Isles are a group of islands located off the north-western coast of continental Europe. The climate is usually considered to be temperate maritime; however, the weather is very changeable from place to place and from time to time. Air masses greatly affect the weather experienced in the British Isles and therefore, its climate. An air mass is a body of air with uniform levels of temperature, humidity and pressure acquired from prolonged contact in its source region. Despite having an effect on British climate, there are other factors which play major roles and need to be considered. Much of the weather that the British Isles are subjected to is brought along by one of the five major air masses which affect the islands from five main source areas. The air masses can be split into two categories: maritime and continental. The three maritime air masses – tropical, polar and arctic – and their influences tend to dominate British weather. The most frequent of all the air masses affecting the Isles is the Polar Maritime (PM) air mass, which is thought to account for 25% of weather experienced in the UK.
The air starts off very cold and dry in its source area of Northern Canada and Greenland, but as it tracks west across the relatively warm waters of the North Atlantic Ocean, it warms from below and picks up moisture, becoming unstable. These wet, unstable characteristics bring cumulus clouds and showers throughout the year, particularly along the west coast of Britain, where the air mass first hits. The cooling effect of the sea brings below average temperatures of just 16?C in the summer, and conversely, the warming effect brings highs of around 8?C in the winter. The arctic maritime air mass is very similar to the PM; however, due to its shorter track over the sea, it brings much colder conditions, often responsible for winter snowfall, especially in Scotland where its effects are particularly prevalent. The second most frequent air mass to arrive at the UK is the Tropical Maritime (TM) air mass, and it is thought to affect around 15% of the UK’s weather. It tracks northeast across the ocean, from its source area in subtropical Atlantic Ocean between the Azores and Bermuda, picking up moisture as it moves over the water.
Therefore, the air mass is broadly described as ‘warm and wet’, bringing warm and muggy weather in the summer with temperatures of 25?C, and mild and damp weather in the winter with temperatures of 15?C. It also often brings stratus clouds, and overcast conditions with persistent drizzle. These conditions are particularly prevalent in the SW peninsula, as this is where the air mass first hits the British Isles. Dartmoor, Dyfed and Ireland can be shrouded by mild, damp conditions whether it is summer or winter. However, in contrast to this, TM air has far less influence on the climate of east of England, where there is a considerably drier climate. This is because moisture from the air evaporates and clouds disperse as the air moves over the UK towards the east. Some parts of Southeast England only receive 50cm of precipitation per year compared with an average of 100cm in the southwest. It can be said, therefore, that this particular air mass, along with the polar maritime air mass, has a far greater effect on the climate of just the south-western area of the British Isles, than the overall climate of all the islands. The air masses which generally tend to affect the more eastern areas of the islands and bring the drier conditions sometimes experienced are the two continental air masses – the Polar Continental (PC) and Tropical Continental (TC).
As they have a source area of continental land, and track mainly over land to reach the British Isles, they characteristically bring dry weather conditions. The PC air mass moves from Russia over freezing land in the winter to bring sub-zero temperatures to the UK, which can be as cold as -10?C at night. The TC air mass conversely tracks over Spain and France, to bring very hot, dry weather with temperatures of at least 25?C in the summer, particularly in the southern areas hit first. The effects of these continental air masses are less prevalent than maritime influences; however they still have important influences on weather in the British Isles, and are also fundamental in explaining temporal and spatial differences in climate across the country. It is not only the weather that the air masses bring that affects the climate of the UK, but also, the complex interactions between the air masses. Most importantly, the interactions between the Polar Maritime and Tropical maritime air masses can lead to the formation of areas of significantly low pressure called depressions. These depressions usually form in the Atlantic Ocean at the boundary between PM and TM air, and bring characteristic cloudy, wet and windy conditions. These depressions are what are responsible for many of the winter storms. They are regular occurrences and so contribute to the overall pattern of weather over a long period i.e. the climate of the British Isles.
The interaction between continental air masses often leads to anticyclones which are the opposite of depressions – areas of significantly high pressure. They bring settled weather, with clear skies and light winds. In the summer they can be responsible for temperatures of over 30?C. Many say it is the pattern of depressions and anticyclones passing over Britain that is responsible for its notoriously changeable weather and irregular climate. The difference in temperature between two air masses, polar and continental, leads to the formation of the Jet Stream – a very fast flowing ribbon of air – at the boundary. This ribbon of air can flow at speeds of up to 200mph and, despite being around 5-7 miles high in the atmosphere, has very significant effects on the UK’s weather. The winds leaving the stream are rapidly diverging, creating low pressure in the tropopause. Air from the surface rises up to replace this air, leaving low pressure at the surface. It is this process that also helps in the explosive cyclogenesis that forms large depressions. The depressions are carried via the Jet Stream towards the UK as it flows from east over the Atlantic. These depressions are a major feature of the UK’s climate, as they bring wet, windy and stormy conditions typical of many a winter’s day.
The Jet Stream is key in determining which air mass affects us, as it meanders north and south over Britain, allowing warmer air masses to dominate as it moves north and colder air masses to dominate as it moves south. The air masses and their influences on the climate of the British Isles are tied in to the Jet Stream very closely, as both the Jet Stream and the air masses affect each other. With all this said, it is important to remember that the air masses and their impacts would not affect the British Isles at all, if it were not for the UK’s geographical location on the planet. The location also plays a key role in some of the climatic features of the British Isles. The UK is situated in the temperate mid-latitudes, roughly between 50? and 60?N of the equator. This position greatly affects the average insolation levels received by the Isles, and so has a large influence on temperature. Due to the curvature of the earth, the angle of incidence of the sun in the sky is relatively low, even in summer.
This means that the sun’s rays have to pass through a greater amount of atmosphere than at the equator, and there is more chance of it being reflected back out to space. The rays are also far more dispersed than at the equator. Both these factors mean that insolation levels are lower in the UK than in areas of lower latitude. This keeps temperatures down. The latitude of the UK, along with the tilt of the earth at 23.4?, is also responsible for the seasons which are a main feature of the climate of the British Isles. The seasons allow the Isles to experience snow and minus temperatures in the winter, and heatwaves in the summer. It is important to remember that the air masses bring different weather depending on the season, so the seasons are actually affecting the air masses themselves and the extent to which they affect UK weather and climate. Ocean currents, in particular the North Atlantic Drift, also play a large role in the climate of the British Isles, and again this would not be so if it were not for the position of the Isles on the exposed western edge of the European continent.
The UK is surrounded by this comparatively warm ocean water, the temperature of which varies only slightly from month to month. This keeps annual fluctuations in temperature minimal because of the differential heating capacities of land and sea. When compared to a landlocked country on the same latitude, such as areas of Russia, this means summer temperatures are lower than might be expected because air moving over the sea toward the British Isles is cooled by the relatively cold summer sea, yet winter temperatures are warmer for the opposite reason. Russia experiences very hot dry summers and extremely cold winters with temperatures of -30°C. Temperatures of this sort could never be expected in the UK due to the warming influence of the sea in the winter. As well as the overall climate of the British Isles, it is also important to consider local, smaller scale climates. Some may call these microclimates. An example of this is that it is far wetter in the highlands than the lowlands.
The wettest place in the British Isles is Snowdonia in Wales where average annual totals exceed 3,000 mm of rain a year, followed by the Highlands of Scotland. This is mainly due to the effect of altitude on precipitation. Where moist air, which could be from a maritime air mass, is forced over a physical barrier such as a mountain range, water vapour cools and condenses to fall as rain. This is known as relief rainfall. Despite the moist air almost certainly being supplied by a maritime air mass, the rain would not have fallen if the mountain range had not been present. This indicates that it is not only the air masses themselves affecting both regional and national climates, but outside factors which amplify the effects of the air mass. Urbanisation is another factor influencing microclimates within the British Isles. In built up areas like London, man-made materials such as concrete absorb much heat during the day, keeping temperatures in the cities warmer than the surrounding rural areas.
In the summer this can occur with temperatures up to 5? warmer in central London than outside the city. This is known as the urban heat-island effect. Human activity also causes there to be more cloud cover over urban areas. They can receive thicker and up to ten per cent more frequent cloud cover than rural areas. The reason for this is that convection currents are generated by the higher temperatures of the urban microclimates so more moisture in the air will condense and form clouds. Human activity is also thought to be having some effect on climate as a whole, and not just in urban areas. Climate change is affecting global and regional climate patterns, and it is thought to be a result of intense industrialisation and burning of fossil fuels. However small, climate change will be influencing the climate experienced in the British Isles.
Overall, it seems that the air masses do have a very large effect on the weather brought to the UK, and therefore its climate too. As the British Isles are exposed to five different main air masses, it is no wonder that the climate here is notoriously changeable and rather different to countries located on the same latitude. This also explains the smaller scale variations in climate between the west and east, for example. However, factors affecting the air masses themselves must also be considered. The weather that the air masses bring is dependent upon the climatic conditions in the source area, the season, and the vertical characteristics of the atmosphere at the time. As discussed above, there are also many other factors which influence the air masses themselves, but also which directly influence the weather and climate of the British Isles.
Therefore, in my view, it would be wrong to say that the air masses alone are responsible for producing the climate experienced on these Islands. It is very hard to isolate one factor as the key-factor as they are all interrelated, and all play a large role in climatic conditions here. The air masses are certainly crucial to the weather patterns seen across the country, however I would arguably say that the location of the British Isles, with its exposure to the sea, and latitude of 50?-60?N, is the main cause for our current climate. The air masses may be important, but they only affect the UK because it is situated where it is.