1.1 What is climate?
1.2 Climate overview – the British Isles
1.4 Position relative to continents and oceans
1.5 Position relative to large-scale atmospheric circulation patterns
1.7 Local geographic features
This unit explains what is meant by the term ‘climate’ and outlines the factors that govern the climate of the British Isles.
According to The Meteorological Glossary, published by the Met Office, ‘climate’ is defined as ‘the synthesis of the day-to-day values of the meteorological elements that affect its locality’.
‘Synthesis’ here means more than simple averages as the climate also involves extreme values, and the ranges within which phenomena occur, and the frequencies of weather types with associated values of elements.
The main climatic elements are precipitation, temperature, humidity, sunshine, wind velocity, and phenomena such as fog, frost, thunder, gale. Cloudiness, grass minimum temperature, and soil temperature at various depths may also be included in a climatic description of any area.
Climatic data are usually expressed in terms of an individual calendar month or season and are determined over a period of about 30 years. This is long enough to ensure that representative values for the month or season are obtained and freaks or abnormal values do not exert too strong an influence. The study of the values and frequencies of the meteorological quantities is known as climatology.
The climate of a locality is mainly governed by the factors of:
(ii) location, relative to continents and oceans;
(iii) situation in relation to large-scale atmospheric circulation patterns;
(v) local geographical features, such as topography or the nature of the built-up area.
The British Isles have a unique climate – so much so, that a popular exam question of the past is ‘Britain does not have a climate, it only has weather: discuss’.
Such a statement has been made because weather conditions can vary so much from day to day, as well as from season to season. Having said this, Britain generally has cool summers and relatively mild winters. Whilst there are parts of the world with similar summers or similar winters, there is nowhere in the world with the same summers and winters as ours. The South Island of New Zealand and Tasmania, for example, have similar summers to ours, but their winters are milder chiefly because they are nearer the equator.
The British Isles are small compared with the other land masses in the northern hemisphere – hence they are more influenced by the ocean compared with other European countries, and the Gulf Stream helps to keep winters milder compared with other landlocked nations with a similar latitude.
The latitude of the British Isles means that they are influenced by predominantly westerly winds with depressions and their associated fronts (bands of cloud and rain), moving to the east or north-east across the North Atlantic, from the eastern coast of North America, bringing with them unsettled and windy weather, particularly in winter.
Between these depressions, there are often small mobile anticyclones that bring a period of fair weather. Sometimes large, stationary anticyclones become established near the British Isles where they effectively act as a ‘block’ to the regular passage of the depressions. These larger anticyclones can often last for over a month, and completely change the character of the weather. In winter, these blocking anticyclones bring dry weather, but with the cooling at night the drop in temperatures results in fog, which can be reluctant to clear the following day when the weak sunshine is unable to raise the temperature. If one of these anticyclones becomes positioned to north of the British Isles, often they become established over Scandinavia, and easterly winds on their southern side draw very cold air from the continent of Europe.
During the summer, these blocking anticyclones can lead to drought conditions, as rain-bearing fronts are ‘diverted’ around the country. Such a situation occurred in 1976 when a high-pressure cell sat over the UK for much of the summer, causing a noticeable drop in precipitation, well below the normal average.
There are many regional variations and microclimates in the British Isles, ranging from the nearly subtropical climate of Cornwall, to the dry semi-arid conditions of East Anglia to the Arctic tundra conditions which can be experienced in the highlands of Scotland. The western and northern parts of the British Isles tend to lie close to the normal path of the Atlantic depressions, giving mild and stormy winters and cool and windy summers when the depressions track a little further north. These areas also have the highest land, and the upland barriers have the effect of producing significant increases in rainfall. The lowland, mainly on the eastern side, has a similar climate, but less severe winters.
There are also other regional variations â€“ the south is warmer than the north, while the west is wetter than the east. In general, the more extreme weather tends to occur in the mountainous and hilly areas where it is often cloudy, wet and windy.
It is possible to analyse the climate of the British Isles in more detail by considering the main factors governing the climate of a locality, as listed in 1.1.
The British Isles have a latitude between 50° N and 60° N. At this latitude, the length of daylight has a significant variation between summer and winter. For example in mid-December the period between sunrise and sunset in London is 7 hours and 50 minutes, while in Lerwick, Shetland, it is 5 hours and 50 minutes; in mid June it is 16 hours and 40 minutes in London and 18 hours 50 minutes in Lerwick.
Latitude not only affects the amount of solar radiation that reaches the British Isles by hours of daylight but also the area over which a given amount of solar radiation is acting. This variation in solar radiation and length of day gives the British Isles distinct seasons.
Given this mid-latitude position, the British Isles are influenced by predominantly westerly winds, and low-pressure systems with their weather fronts (bands of cloud and rain) move generally east or north-east across the North Atlantic from North America. The conditions these low pressure systems bring are generally unsettled with periods of wet and windy weather. As the British Isles lie to the west of mainland Europe, they are often the first landfall reached by the moisture-laden depressions. Thus the majority of the rain falls over the British Isles, and the fronts are usually considerably weaker by the time they reach the mainland of continental Europe.
The sequence of low-pressure systems is often punctuated by ridges of high pressure. These can bring periods of fine weather, and the net result is that the weather is noticeably changeable.
Another important factor influencing the weather of the British Isles is their position close to the ocean, rather than being in the centre of a large land mass. This is clear when comparing the climate of London and Moscow. The latter has a similar latitude to London, but London has cooler summers and milder winters. This is because Moscow is situated away from the warming effects of the Gulf Stream and other warm ocean currents that could keep the winter temperatures mild.
There is a reversal during the summer, when these ocean currents help to keep London’s temperatures lower than Moscow’s. This is because water has a higher specific heat than land, and in the summer, Moscow is warmer, chiefly because of its location within an extensive continental land mass.
Another factor is the influence of airstreams. Those reaching the British Isles originate in very warm or cold regions. However, by the time they reach us they have often had their temperature greatly modified. If the wind is predominantly westerly the air is modified by the ocean (warm Gulf Stream) and the fronts are brought in from the Atlantic. This modification by the ocean means that we experience cool summer weather or mild winter weather.
A third influence affecting the climate of the British Isles is the location relative to the large-scale atmospheric circulation systems that are part of the tri-cellular model.
The western and northern parts of the British Isles tend to be close to the normal path of Atlantic low-pressure systems or depressions. As a result, these areas tend to be mild and stormy in winter, and in the summer, when depressions take a more northerly route and are less deep, cool and windy.
The boundary of these large-scale pressure systems varies over time, allowing a blocking anticyclone to become established. They can draw air from the continent rather than the Atlantic, giving us summer heatwave conditions or very cold winter conditions.
Many meteorological factors vary with increasing (or decreasing) altitude. In general, wind speed increases with height, with the strongest winds being experienced over the summits of hills and mountains.
In contrast, temperatures decrease with altitude, with the average (environmental lapse rate) change being 0.65 °C per 100 metres. However, the rate of cooling varies depending on the moisture (water vapour) content of the air. Moist air rises and cools at the saturated adiabatic lapse rate of 0.5 °C per 100 metres, whilst dry air rises and cools at the dry adiabatic lapse rate of 0.98 °C per 100 metres.
Precipitation can increase with altitude as a result of relief, or orographic, rainfall. The upland barriers force up the air masses, causing them to cool, so that water vapour condenses, allowing raindrops to form and fall near the summit, or on the windward slopes. This explains why upland areas such as the Lake District, have average annual rainfall totals exceeding 2,000 mm.
Sometimes, the air temperature cools so much that snow is produced. It is comparatively rare near sea level in England, but much more frequent over hills, thus the average number of days when snow falls in England varies from 10 or less in the south-western coastal areas to over 50 in the Pennines.
Local geographic features have an effect on the local climate. These features include hills and valleys, forests and plains, lakes and rivers, as well as urban and rural locations.
Hills and valleys can experience very different climates from the regional weather conditions. The temperature at night becomes lower in the valleys than on the surrounding hills. During the night and during the winter months, cold air flows down into the bottoms of the valleys because of its greater weight. Thus, in the valley areas, the generally lower temperatures increase the risk and the frequency of fog. During the day in summer, the valley gets higher daytime temperatures than the hills. The winds are stronger on the hills than in the valleys.
In long valleys, the winds can only blow along the line of the valley. Places in the lee of hills or mountains get less precipitation than places on the windward side. This is known as a rain shadow. In winter months high temperatures can occur in the lee of high ground. These high temperatures (up to 16 Â°C on rare occasions) occur when a moist south to south-westerly airflow warms up downwind of mountains, an effect known as the föhn after its more dramatic effect in the Alps.
As far as forests and plains are concerned, summer temperatures are usually lower in the forest areas than over a plain whilst in the winter the opposite is generally true. This is because the forests can store large amounts of heat, whilst the plains react quicker to incoming and outgoing radiation. The surface of a forested area is rougher, reducing wind speed compared to the higher winds over a smoother plain. Trees extract large amounts of groundwater and store large amounts of water that is given off to the atmosphere. Hence, humidity increases over woods while there is strong evaporation over plains.
Trees are used in a number of ways to modify the local climate. They are planted as shelter or windbreaks. As shelter breaks, they reduce the effect of the wind and raise the temperature. They are also planted to reduce wind erosion which helps the growth of crops. In some desert areas, this has been well developed near oases where a shelter belt is planted as outer protection to reduce the effect of the sandstorms and to stabilise the sand. Within this protective boundary, date palms are planted which, in turn, provide shade for other cropping trees that, in turn, shade crops beneath them.
Proximity to water also has a marked affect on microclimate, in the same way that the sea affects the macroclimate of the British Isles. Locations near to lakes and major rivers generally have milder winters and cooler summers.
There are also noticeable contrasts between urban and rural areas. Urban areas have minimum temperatures which are higher than those recorded in the rural areas, whilst cities can also have higher maximum temperatures. These effects are known as the urban heat island, and are caused by firstly, the materials used in buildings storing and then releasing heat, and secondly by the release of heat due to industrial and domestic energy consumption. Convectional rainfall and smogs can often occur more regularly in urban areas.
This session aims to look at the main characteristics of the climate of the British Isles. In general terms, the climate of the British Isles can be summarised as follows.
Temperature: the south is warmer than the north.
Precipitation: the west and north-west, and more specifically the mountains in these areas are wetter than the lowlands of the east.
Wind: the north and west is in general windier than the south and east, but it is less windy inland than on the coasts, and less windy in low-lying areas than on the tops of hills and mountains.
Air temperature varies on a daily (diurnal), seasonal and geographical basis. On a daily basis, the temperature is usually lower at night, when there is no incoming solar radiation, and the energy is being released back into space. Thus the coldest nights are those on which there is little wind, and there is a covering of snow on the ground, creating a high albedo effect. The lowest temperatures occur inland, away from the moderating influence of the relatively warmer sea.
January is usually the coldest time of the year. The least-cold areas are the south-west of England and the Channel Islands whilst the coldest areas are the Grampian and Tayside regions of Scotland. The main factor in determining this distribution of winter temperature is the proximity of the coast, particularly the west coast, and the warming Gulf Stream.
Minimum temperature normally occurs shortly after dawn and the maximum temperature two to three hours after midday. In the urban areas, the minimum temperatures experienced tend not to be so low as those recorded in rural areas although maximum temperatures are often higher. Some sheltered and low-lying areas have a greater incidence of frosts, and more severe frosts, than the surrounding areas. These are known as frost hollows.
July is normally the warmest month and the highest temperatures occur inland in central areas, furthest away from the cooling influence of the sea. The higher specific heat of sea water renders it slow to heat up, but equally reluctant to cool down. These differences affect the climate in a number of ways. Firstly, the sea can warm, by the exchange of sensible and latent heat, the air flowing over it therefore making parts of the British Isles warmer. It can, of course, happen the other way, when the sea is cooler than the land, the wind from the sea is cooler. This explains why sea-surface temperatures reach their lowest values in late February or early March. Thus, around the coasts, February may be the coldest month, although there may be little to choose between January and February.
Localised heating can also take place depending on the nature of the environment. Urban areas can heat up differently to, and more quickly than, rural areas to produce their own urban heat island. It is not uncommon for the forecast minimum temperature for London to be at least 2 or 3 degrees higher than the surrounding comparatively rural locations.
On a larger scale, temperature differences can occur as a result of periodic fluctuations in sea-surface temperatures in the Atlantic Ocean. These are smaller in scale compared with the El NiÃ±o Southern Oscillation (ENSO) in the central Pacific. Even so, they are still important as far as temperatures in Britain are concerned.
The sea-surface temperature changes in the Atlantic are known as the North Atlantic Oscillation (NAO), and they are governed by the behaviour of the Azores high pressure and the sub-polar low pressure zones. The pressure gradient between the two zones helps to define the zone within which the mid-latitude westerly circulation occurs.
When air pressure is unusually low over Iceland and high over the Azores – thus a very steep pressure gradient – the NAO is said to be positive. When both pressure areas are not significantly or poorly developed, that is the pressure gradient is weak, the NAO is negative. In positive mode, the westerlies are strengthened so that warm air and water are advected towards the British Isles.
The mild winters of the 1980s and early 1990s in Britain were a time of significantly positive NAO. In the negative phase, the Icelandic low becomes displaced south-westwards, and a blocking situation develops around the British Isles with the polar high pressure penetrating southwards. The exceptionally cold winters of 1962/63 and 1941/42 are examples of this negative phase.
The British Isles have the longest instrumental record of temperature for any region of the world, known as the Central England Temperature series which extends back to 1659. Temperatures over the British Isles have risen by about 0.7 °C since 1700 and by about 0.5 °C since the start of the 20th century.
Precipitation has been recorded in the British Isles for over 200 years. Through the dedication and enthusiasm of W R Symons, the mid-19th century saw the formation of the British Rainfall Organisation whose main objective was to establish a countrywide network of rain gauges where daily measurements were made. This network has grown to over 7,000 gauges today, including many that are automatic.
As most people in the British Isles know, precipitation can be extremely variable, both in intensity and duration. The spatial distribution of precipitation during an individual month is very uneven, just as it is on an individual day. Rainfall in Britain is associated with several distinct synoptic situations; all places may get rain from most such types, but some areas get more from some types than others. Therefore, it is not surprising that patterns of temporal variation of rainfall are complex.
Precipitation over the British Isles is the result of one or more, of three basic mechanisms.
1. Cyclonic, or frontal, rain associated with the passage of low-pressure systems. Bands of rain are associated with the passage of warm and cold fronts across the UK. These rain events are caused by the uplift and cooling of moist air parcels.
2. Convectional, with local showers and thunderstorms, caused by the localised thermal heating and overheating of the ground surface. Large towering cumulonimbus clouds may be generated, producing heavy rain.
3. Orographic, or relief rain, with precipitation increasing with altitude over upland areas. The mechanism for relief rain is the uplift and cooling of moist air over upland areas. The normal rate of cooling (environmental lapse rate) is 6.5 Â°C per 1,000 metres. Therefore, near the summit on the windward side of the hill or mountain, the air will have cooled sufficiently for thick cloud, rain and possibly snow to fall.
Air will descend and warm on the leeward side, so there is little or no rain on this leeward side of the hill or mountain. This is called a rain shadow, and sometimes there are warm winds in these sheltered areas as the air, now much drier than during its ascent, descends quickly and warms up. This is called a föhn effect, and the warm winds are called föhn winds.
Winds over the British Isles can be a resource and a hazard. Hundreds of wind turbines have been installed in exposed parts of the British Isles, adding extra megawatts of electricity to the National Grid, and exploiting this wind resource.
However, severe storms can be a hazard, causing extensive damage to commercial and domestic property, as well as laying bare areas of woodland. The severe storm on 16 October 1987 cost the insurance industry around £1.2 billion, and 15 million trees were uprooted. On Burns’ Day, 25 January 1990, another deep low pressure system swept across the country, with speeds of 107 m.p.h. recorded at Aberporth in Ceredigion and 98 m.p.h. at Herstmonceux in Sussex.
The windiest parts of the British Isles are those on the north-west and south-west coasts, and in exposed upland areas. These are the locations across which mature depressions usually pass, and on 7 February 1969, Kirkwall in the Orkney Islands recorded a gust of 136 m.p.h. as an extremely deep and vigorous depression developed.
Coastal areas bear the brunt of gales from the Atlantic, especially during the winter when the low pressure cells are particularly active and well developed. These gales can pose extreme hazards for fishermen and sailors, and during the ‘Great Storm of 1703’ on 26 and 27 November, hundreds of ships in the English Channel and North Sea were destroyed, and an estimated 8,000 people were killed.
There are local variations with warm and cool sea breezes, plus valley winds and föhn winds in the lee of upland areas. The sea breezes can create diurnal variations, with higher wind speeds in the afternoon, due to heating from the ground. Urban areas, especially those with tall skyscrapers and an irregular skyline, can also be windier than the surrounding suburbs and rural areas as the buildings disturb the local airflows and create turbulence.
Mean daily sunshine figures reach a maximum in May or June and are at their lowest in December. The key factor, of course, is the length of day throughout the year, but wind and cloud play their part as well.
Visibility means the distance at which the outlines of a building can be seen in daytime, or at night, at which ordinary house lights can be discerned. Before the First World War, the custom was to use terms such as haze, mist, or fog in referring indirectly to atmospheric visibility. Qualitative descriptions such as exceptional visibility were also used. These terms were not very precisely defined.
The First World War and the years that followed saw the development of aviation that required more-precise information and this led to the development of numerical observations for visibility. In places where visibility observations are made it has been the practice to maintain a list of permanent objects. Measuring equipment such as the Gold visibility meter and the transmissometer have also been developed for measuring visibility and their use has become more common, from airfields to motorways and ports, as well as stations recording meteorological data. Visibility variation also depends to some extent on the colour and illumination of an object being observed against its background, as well as the optical vision of the observer!
Visibility is, broadly speaking, related to the concentrations of water droplets or solid particles. Conditions favouring a decrease or an increase of these constituents in the atmosphere will lead to an improvement or deterioration of visibility. When water droplets are present in suitable concentration and sizes, mist or droplet fog may be formed. The occurrence of fog is usually associated with radiation cooling or advection cooling, so that air temperatures fall below dew point.
This section looks at the different climates in each region of the UK. The notes below highlight the importance of local features in producing subtle variations.
Temperature – over England the mean annual temperature at low altitudes varies from 8.5 °C to 11 °C, with the highest values occurring around or near the coast of Cornwall. The mean annual temperature decreases by approximately 0.5 °C for each 100 metres increase in height so that for example, Great Dun Fell in Cumbria (at 857 m) has an annual mean temperature of 4 °C.
Winter temperatures in the British Isles are influenced by the thermal characteristics of the surrounding sea. They reach their lowest values in February or early March, so in coastal areas, February is normally the coldest month. Inland, away from the influence of the sea, the coldest period occurs a little earlier and January and February are the coldest months. Indeed, it was in an inland valley that the lowest temperature recorded in England was made – at Newport in Shropshire on 10 January 1982, -26.1°C. As a comparison, the coldest temperature ever recorded in Plymouth in Devon was -8.8 °C on 2 January 1979.
July is normally the warmest month in England. Again, the extreme temperatures occur inland, away from the influence of the sea. The UK record temperature of 37.1 °C at Cheltenham on 3 August 1990, was beaten by a number of stations on 10 August 2003, with Brogdale near Faversham in Kent, reporting the highest at 38.5 °C.
The Lake District is the wettest part of England, with annual totals exceeding 2,000 mm, comparable to the Highlands of Scotland. The Pennines and the moors of south-west England are almost as wet. Conversely, all of East Anglia and much of the Midlands, eastern and north-eastern England, as well as parts of the south-east experience less than 700 mm in a year.
The typical occurrence for rain is about one day in three in England, especially in winter, rather than in summer, although in most years, there are likely to be long dry spells.
Near the south coast there is an appreciable summer minimum and winter maximum of rainfall, with totals in July about half those in January. The western, northern and eastern coasts are likely to see the driest month in spring and the wettest month in late autumn. It can be a different situation in inland areas, with parts of the Midlands experiencing a summer rainfall maximum. This reflects the higher frequency of thunderstorms (convectional rain) in the more central and south-eastern parts of England. For example thunder occurs on average 15 days per year in London and Birmingham, but in the west and north-west the frequency declines to around eight days per year.
The maximum rainfall recorded in one day was 279 mm at Martinstown (Dorset) on 18 July 1955.
|Fig 2: 30-year (1961-90) average number of days in month with snow lying at 0900 hours at selected stations.|
Snow is comparatively rare near sea level in England, but is much more frequent over hills. The average number of days each year when sleet or snow falls in England varies from about 10 or less in some south-western coastal areas to over 50 in the Pennines. Snow rarely lies on the ground at sea level before December or after March and the average number of days with snow lying in England varies from five or less around the coasts to over 90 in parts of the Pennines.
The number of days of snowfall and snow cover varies enormously from year to year. At many places in England in the last 50 years, it has ranged from none at all in a number of winters, to in excess of 30 days during the winters of 1946/47 and 1962/63. Even places near the coast experienced prolonged snow cover during these two winters, especially over higher ground, resulting in severe disruption to transport. Fortunately, such prolonged spells of winter snowfalls are comparatively rare.
|Fig 3: Monthly average number of days (1961â€“90) with gales at selected stations.|
The strongest winds in the United Kingdom are associated with the passage of deep depressions across or close to the British Isles. These are most frequent during winter, when the depressions are most active over the open ocean. The western coasts are most exposed to the stronger winds at low altitudes, with Ireland affording protection to much of England. The most exposed areas are the coasts of Devon and Cornwall where there are about 15 days of gale a year. Inland the number of days of gale decreases to fewer than five days a year.
The highest gust recorded at a low-level site, was 103 knots (118 m.p.h.) at Gwennap Head, Cornwall, on 15 December 1979.
|Fig 4: 30-year (1961â€“90) average monthly duration of bright sunshine in hours for selected stations.|
On sunny days in summer, the formation of convective (cumulus) cloud takes place over land, whilst skies over the sea remain cloud-free. Therefore, the sunniest parts of England (and the whole of the British Isles) are along the south coast of England. Many places along this coast achieve annual average figures of around 1,750 hours of sunshine. The maximum duration in one month of bright sunshine is 383.9 hours at Eastbourne, East Sussex, in July 1911.
The dullest parts of England are the mountainous areas with annual average totals of less than 1,000 hours. The minimum duration in a month, was zero hours, recorded at Westminster, Greater London, December 1890.
Visibility – many parts of England, especially those remote from industrial and urban areas, enjoy good visibility. This is particularly true of most coastal areas, the mountains and the moorlands.
Over high ground in England, hill fog can be both extensive and frequent , and it is a potential hazard to be borne in mind by walkers. Indeed, Great Dun Fell (857 metres) in Cumbria had an average of 233 days of fog per year between 1963 and 1976.
Temperature: over Scotland the mean annual temperature at low altitude ranges from about 7 °C in the Shetland Islands, in the far north, to 9 °C on the coasts of Ayrshire and Dumfries and Galloway in the south-west. Due to the environmental lapse rate, there is a temperature decrease of about 0.6 °C for each 100 m rise in height, and as a result the temperatures over higher ground are generally colder. For example, Braemar (at 339 m above mean sea level) has an annual mean temperature of 6.4 °C while the corresponding value on Ben Nevis (altitude 1,344 m ) is 0.3 °C.
In common with other parts of the British Isles, winter temperatures are influenced by the surrounding areas of sea. The North Sea is cooler than the waters off the west coast, so the east coast of Scotland is generally cooler than the west coast. In general, January and February are the coldest months when the daytime maximum temperatures over low ground average around 5-7 °C. On rare occasions in the lee of high ground, a föhn effect can cause temperatures to reach around 15 °C. The lowest temperatures occur inland away from the moderating influence of the sea, and in valleys into which cold air sinks. In these conditions, the temperature dropped to -27.2 °C at Braemar on 10 January 1982 and more recently at Altnaharra on 30 December 1995. (This is the lowest temperature ever recorded in the British Isles.) In coastal areas, such cold nights are not experienced and as an example, the lowest temperature recorded at Lerwick in the Shetland Islands in the 30 years between 1961 and 1990 is only -9 Â°C.
In summer, the more northerly latitude of Scotland explains the drop in the amount of solar radiation. Thus, the temperatures in Scotland are on average a few degrees cooler than in England. For example, in July the average daily maximum temperature at Glasgow is 19 °C compared with London 22 °C. July and August are normally the warmest months in Scotland. As the sea has a cooling influence on coastal areas, the highest temperatures occur inland. The highest temperature recorded in Scotland was 32.9 °C at Greycrook (Scottish Borders) on 9 August 2003, beating the previous Scottish record of 32.8 °C at Dumfries on 2 July 1908 and on several occasions at other places in the 19th century.
|Fig 5: Monthly averages (1961-90) of rainfall (mm) for a selection of stations in Scotland.|
There is a misconception that the whole of Scotland experiences high rainfall. In fact, rainfall in Scotland varies widely, with a distribution closely related to the topography, ranging from over 3,000 mm per year in the western Highlands (similar to the total rainfall experienced in the mountains of the Lake District in England and Snowdonia in Wales) to under 800 mm per year near the east coast (comparable with the Midlands of England).
The frequency of thunderstorms in Scotland is around three to nine days per year. This is relatively low compared with an average of 9-15 days per year over England. The number of thunderstorms can vary widely from year to year, but in general the northern and eastern coasts of Scotland average only three or four days with thunder per year, whilst inland values range from nine in the south to six in the north.
|Fig 6: 30-year (1961-90) average number of days in month with snow lying at 0900 hours at selected stations.|
Temperatures generally decrease with height, so precipitation falling to the ground as rain at low-level sites may fall as snow over higher ground. Consequently, there is a marked increase with altitude in the number of days with falling snow and also in the number of days with snow lying on the ground.
The average number of days with sleet or snow falling in Scotland ranges from 20 or less near the west coast to over 100 days in the Cairngorm Mountains and some other high peaks. Snow rarely lies on the ground at sea level before November or after April. On low ground in the Western Isles and in most coastal areas of Scotland, snow lies on an average for less than 10 days per year, although this increases to around 15-25 days for coasts in the north and north-east. However, over the mountains, snow typically lies for more than 50 days per year.
In heavy snow, there can be quite extensive drifting, especially over higher ground. Snow deposited in natural hollows, such as high-level corries, can persist for some considerable time, an effect utilised to good effect by the development of the skiing industry in Scotland, and a few of these high level, north-facing, snow beds are semi-permanent, only disappearing in very occasional summers. On the highest summits, such as Ben Nevis, snow cover typically persists for around six or seven months of the year.
|Fig 7: Monthly average number of days (1961-90) with gales in selected stations.|
The most common direction from which the wind blows in Scotland is from the south-west, but the wind direction often changes markedly from day to day with the passage of weather systems. There is a close relationship between surface isobars and the wind speed and direction over open, level terrain. However, in mountainous areas local topography also has a significant effect, with winds tending to blow along well defined valleys.
Since many major Atlantic depressions pass close to or over Scotland, the frequency of strong winds or gales is higher than in other parts of the British Isles. Over low ground the windiest areas are the Western Isles, the north-west coast and the Orkney and Shetland Islands with over 30 days with gales per year in some places.
|Fig 8: 30-year (1961-90) average monthly duration of bright sunshine in hours for selected stations.|
Generally, Scotland is more cloudy than England, due mainly to the hilly nature of the terrain and the proximity of low-pressure systems from the Atlantic. Despite this, parts of Angus, Fife, the Lothians, Ayrshire and Dumfries and Galloway average over 1,400 hours sunshine per year. This compares favourably with coastal areas of Northern Ireland and the north of England, but is less than the annual totals of over 1,700 hours experienced along the south coast of England. The dullest parts of Scotland are the more mountainous areas, with an annual average of less than 1,100 hours of sunshine over the mountains of Highland Region. Mean daily sunshine figures reach a maximum in May or June and are at their lowest in December. Wind and cloud play their part but the key factor is day length throughout the year due to the relatively high latitude of Scotland.
Visibility â€“ Scotland often enjoys excellent visibility, largely because of its remoteness from the industrial and urban areas of Britain and mainland Europe. In the industrial areas of central Scotland the switch away from coal fires and the decline in traditional heavy industry has reduced the incidence of smoke haze caused by local air pollution. However, cloud with a very low base can often shroud high ground with hill fog. Extensive hill fog often develops, especially in the west, when a moisture-laden south-westerly airstream covers the country. The resulting low visibility and drizzle, can pose a hazard for hill walkers and motorists.
Radiation fog may form overnight in low-lying inland areas on clear, calm nights, particularly in winter. Sea fog formed by advection cooling can develop over the North Sea. Known locally as haar, it sometimes ruins what would otherwise be a fine day on or near the east coast, or in the Northern Isles, between April and September. Both these types of fog tend to break up and disperse during daytime, although inland, during the winter, mist and fog does sometimes persist. At Edinburgh, the midday visibility is less than 1,000 m on 3% of December days.
Temperature – Over Wales, the mean annual temperature at low altitude varies from about 9.5 °C to 10.5 °C, with higher values occurring around or near the coasts. The mean annual temperature decreases with the environmental lapse rate, by approximately 0.5 °C for each 100 m increase in height, so that for example Bwlchgwyn, Wrexham, at 386 m, has an annual mean temperature of 7.3 °C. On this basis, Snowdon at 1,085 m would have an annual mean temperature of about 5 °C.
In winter the coldest areas are away from the coasts, where the sea has little influence on temperature. Cold air drains into the floors of inland valleys, and it is here that the lowest temperatures are found. Indeed, it was under such conditions that the lowest recorded temperature in Wales has been recorded, -23.3 °C at Rhyader on 21 January 1940.
Coastal areas do not experience such cold nights, for example the lowest temperature recorded at Brawdy in Pembrokeshire was -10.7 °C on 13 January 1987. On the opposite extreme some of the highest winter temperatures recorded in the British Isles have occurred in Wales up to 18 °C. These occur when a powerful föhn effect develops downwind of Snowdonia in a moist south to south-easterly airflow.
July is normally the warmest month in Wales, with the highest temperatures occurring away from the cooling influence of the sea. The highest temperature recorded in Wales was 35.2 °C at Hawarden Bridge in Flintshire on 2 August 1990.
|Fig 9: Monthly averages (1961-90) of rainfall (mm) for a selection of stations in Wales.|
Rainfall in Wales varies widely, with the highest average annual totals being recorded in the mountainous areas of Snowdonia, with the wettest parts having over 3,000 mm per year, and the Brecon Beacons, where the yearly fall is comparable to the English Lake District or the Western Highlands of Scotland. In the east, close to the border with England, annual totals are similar to those over much of the English Midlands (around 1,000 mm per year), whilst similar totals are also found in the coastal areas.
Throughout Wales, the months of October to January are significantly wetter than those from February to September, unlike places in south-east Scotland and the English Midlands where July and August are often the wettest months of the year. This is a reflection of the relatively low frequency of thunderstorms in Wales compared with England. For example at Cardiff, thunder occurs on an average of 11 days a year, compared with 15-20 in many places in England. In the west and north-west, the frequency declines to around eight days per year.
|Fig 10: 30-year (1961-90) average number of days in month with snow lying at 0900 hours at selected stations.|
Snow is comparatively rare near sea level in Wales but much more frequent over the hills. The average number of days each year when sleet or snow falls in Wales varies from about 10 or less in some south-western coastal areas to over 40 in Snowdonia. Snow rarely lies on the ground near sea level before December or after March, and the average number of days with snow lying in Wales varies from six or less around the coasts to over 30 in Snowdonia.
The number of days of snowfall and snow cover varies enormously from year to year. At many places in the last 50 years, it has ranged from none at all in several winters, to in excess of 30 days during the Winters of 1946/47 and 1962/63. Even places near the coast experienced prolonged snow cover during these two winters.
In heavy snowfalls there can be quite extensive drifting of snow in strong winds, especially over higher ground, resulting in severe dislocation of transport. Fortunately, such occasions are rare, but one of the worst snowstorms in the 20th century in South Wales occurred on 7 and 8 January 1982, when depths of one metre or more were commonplace, with severe drifting and power lines brought down.
|Fig 11: Monthly average number of days (1961-90) with gales in selected stations.|
The strongest winds in the British Isles are associated with the passage of deep depressions across or close to the country. These are most frequent in winter and it is then that gales are most frequent. As these depressions are most intense over the open Atlantic Ocean, low-lying parts of Wales have more-frequent gales. For example, in the extreme south-west of Dyfed about 30 days of gales occur on average per year. Further north, Wales is more protected by Ireland. Other coastal areas have 15 days or more of gale with the number of days decreasing to five days or fewer inland.
In general, wind speed increases with height, with strongest winds being observed over the summits of hills and mountains. There are no wind recording stations at high altitudes in Wales, so no data can be given but, as an indication, Snaefell on the nearby Isle of Man (at 615 metres) has, on average, over 200 days of gales a year.
|Fig 12: 30-year (1961-90) average monthly duration of bright sunshine in hours for selected stations.|
Wales is generally cloudier than England as a result of the hilly nature of the terrain that forces up moist, maritime air moving in from the Atlantic. Even so, the south-western coastal strip of Dyfed has an annual average of over 1,700 hours of sunshine – a total also achieved by many places on the south coast of England. The dullest parts of Wales are the mountainous areas, with average annual totals of less than 1,100 hours.
Mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December. The key factor is the length of day throughout the year, but prevailing winds and cloud play their part as well.
Visibility – Much of Wales enjoys excellent visibility. The traditional areas of heavy industry are close to the southern coast – a location that is, in itself, relatively breezy and free from serious reductions of visibility by reason of smoke.
Fog statistics are scarce but, given the mountainous nature of the country and the proximity to the sea, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers in Snowdonia and the Brecon Beacons.
Temperature – over Northern Ireland the mean annual temperature at low altitudes varies from about 8.5 °C to 9.5 °C with higher values occurring around or near the coasts. The mean annual temperature decreases by about 0.5 °C for each 100 metre increase in height so that, for example, Parkmore Forest in County Antrim (at 235 m) has an annual mean temperature of 7.4 °C. On this basis, Slieve Donard, the Province’s highest mountain (at 852 m) would have an annual mean temperature of about 4.5 °C.
In winter, the temperatures of coastal areas are influenced by the surface of the surrounding sea. Inland, the lowest temperatures are to be found, where cold air drains into the bottoms of valleys. The lowest temperature recorded in Northern Ireland was -17.5 °C at Magherally, County Down, on 1 January 1979, whilst in coastal areas the lowest recorded temperature at Helens Bay during the 30-year period between 1961 and 1990 was -5.4 °C.
July is normally the warmest month in Northern Ireland, and the highest temperatures of all have occurred inland, furthest away from the cooling influence of the Atlantic. The highest recorded temperature was 30.8 °C at Knockarevan, County Fermanagh, on 30 June 1976, and at Shaw’s Bridge, Belfast, on 12 July 1983.
|Fig 13: Monthly averages (1961-90) of rainfall (mm) for a selection of stations in Northern Ireland.|
Rainfall in Northern Ireland varies widely, with the highest annual average totals being recorded in the Sperrin, Antrim and Mourne Mountains where the yearly fall of around 1,600 mm is about half that of the English Lake District, or the western Highlands of Scotland. In the east, close to the coast, and near to the southern and eastern shores of Lough Neagh, the annual totals of just under 800 mm are similar to those near the Firth of Forth in Scotland. Generally, rainfall distribution is closely related to topography.
The seasonal variation of rainfall in Northern Ireland is not large, but throughout the province the wettest months are between August and January, unlike places in south-east Scotland or in the English Midlands, where July and August are often the wettest months of the year. This is partly a reflection of the relatively low frequency of thunderstorms in Northern Ireland, compared with that of England. For example, at Armagh thunder occurs on an average of less than four days per year, compared with 15-20 at many places in England. Only in a few places, mainly away from the coasts, does the frequency of thunder exceed five days per year.
|Fig 14: 30-year (1961-90) average number of days in month with snow lying at 0900 hours at selected stations.|
Snow is comparatively rare near sea level in Northern Ireland, but much more frequent over the hills. The average number of days each year when sleet or snow falls in Northern Ireland varies from around 10 near the east coast to over 30 in the mountains of Sperrin, Antrim and Mourne. Snow rarely lies on the ground at sea level before December or after March and the average annual number of days with snow lying in Northern Ireland varies from less than five around the coasts to over 30 in the mountains.
The number of days of snowfall and snow cover varies enormously from year to year. At many places in the last 50 years it has ranged from none at all in several winters, to in excess of 30 days during the winters of 1962/63 and 1981/82. Even places near the coast experienced prolonged snow cover during these two winters. With heavy snowfalls there can be quite extensive drifting of the snow in strong winds, especially over the higher ground, resulting in severe disruption of transport. Fortunately, such occasions are comparatively rare.
|Fig 15: Monthly average number of days (1961-90) with gales in selected stations.|
The strongest winds in the United Kingdom are associated with the passage of deep depressions across or close to the British Isles. These are most frequent during winter, which is when gales can occur on exposed western and northern coasts of both Britain and Ireland. For example, the Hebrides experience on average about 35 days of gale a year, and the extreme south-west of England about 30. In Northern Ireland, the coastal areas are not so exposed as these areas are afforded some protection both by the rest of Ireland and adjacent parts of Scotland. Thus, the coastal areas of the counties of Antrim and Down have about 15 days of gale per year, while the number of days decreases inland to five or fewer.
In general, wind speed increases with height, with strongest winds being observed over the summits of hills and mountains. There are no wind recording stations at high altitudes in Northern Ireland, so no data can be given but, as an indication however, Snaefell on the nearby Isle of Man (at 615 metres) averages over 200 days of gale a year.
|Fig 16: 30-year (1961-90) average monthly duration of bright sunshine in hours for selected stations.|
On the whole, Northern Ireland is cloudier than England because of the hilly nature of the terrain and the proximity to the Atlantic. Even so, the coastal strip of County Down manages an annual average total of over 1,400 hours of sunshine. This compares favourably with many coastal areas of England and Wales, though not with the 1,750 hours achieved in places along the south coast of England. The dullest places in Northern Ireland are the more-mountainous areas, where the annual average falls below 1,100 hours.
Mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December.
given Northern Ireland’s peripheral location, away from the industrial and urban areas of Britain and Europe, much of the Province enjoys excellent visibility. Any early morning mist or fog that might develop will usually clear rapidly, though it can be much more persistent during winter months.
Given the hilly nature of the country and its relative proximity to the sea, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers.
|4. Additional UK climate information|
1. Make a concise definition of the term climate.
2. What are the five main factors which influence the climate of a locality?
3. What is the name given to the type of climate which affects the British Isles?
4. Explain why Moscow, on a similar latitude to London, has warmer summers and colder winters than the English capital.
5. Explain the meaning of the term ‘environmental lapse rate’.
6. Outline the meaning of the following phrases:
a. orographic, or relief rain,
b. föhn effect,
c. rain shadow.
7. Explain the different mechanisms that produce frontal and convective rain.
8. What is the correct value for the number of days with snow falling over the Cairngorms 40, 60, 80 or 100?
9. Which English weather records are held by Martinstown in Dorset and Gwennap Head in Cornwall?
10. What is an urban heat island? What are the meteorological conditions that may favour its formation?
11. Why do temperatures in the inland parts of the UK differ from those localities that are nearer the sea?
12. Describe and explain the type of weather hazards which walkers might face in upland parts of the UK.
13. Outline the meteorological conditions by which fog may develop in parts of the UK.
14. The highest recorded temperature in England was made at Brogdale near Faversham in Kent:
a. Was it 36.5, 37.5, 38.5 or 39.5 °C?
b. In which year did this occur 1990, 1995, 2000 or 2003?
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