|
Geografia d'Europa: textos de suport |
The complex and fragmented structure of Europe contributes to the variable meteorological and climatological conditions: from Mediterranean in the south, through temperate oceanic in the west and temperate continental in the east, to boreal in the north. These main types contain many subdivisions linked to altitude (eg, Alpine climate), position and aspect in plains or valleys, and the distance from the sea. Temporal variations of climate have also influenced historical and economic development. The following succinct description is based on the monograph of Martyn (1992).
Europe lies across three climatic zones:
The influence of the Atlantic is strongly felt owing to the warm North Atlantic Current or Drift originating from the Gulf Stream which reaches the shores of western Europe beyond 45ºN (mainly fetching up on the coasts of Ireland, Britain and Norway, as well as all the way to the Russian island of Novaya Zemlya). Thanks to this current, the whole area experiences a positive temperature anomaly in winter which gradually decreases south and eastwards across the continent.
Europe's latitudinal position on the globe, between 36 and 71ºN, determines the duration of incoming light and heat from the sun. Annual sunshine totals vary from 1000 hours in the cloudiest areas of eastern Iceland, the Faeroes and western Scotland, 1400 to 1600 hours in northwest central Europe to 2800 to 3000 hours in the southern Mediterranean, and even over 3400 hours in Portugal and southeast Spain.
The atmospheric circulation over Europe is established in the presence of permanent cyclonic centres stationed over the North Atlantic (the Icelandic Low), and of the Azores High. In winter, Europe may come under the influence of the seasonal Asian high, and lows moving in from the Atlantic into the Mediterranean basin or forming further north. Summer is characterised by the presence of the Arctic High in the Svalbard region and the South Asian Low. This pressure system gives rise to prevailing southwesterlies over Northern, Western and Central Europe, and northwesterlies and westerlies in Southern Europe. The characteristic feature of the climates of Northern, Western and Central Europe is the changeability of the weather. Summer in Southern Europe is much more reliable under the influence of tropical air masses.
These insolation and circulation conditions are well reflected by the air temperature distribution. In January, when thermal conditions are determined by the atmospheric circulation, the isotherms run latitudinally. The 0ºC isotherm can be regarded as the boundary between the warm south and west of Europe and its cool centre and north. In July, insolation affects the continent far more than the atmospheric circulation does, although all along the Atlantic fringe the reverse is still the case. The shape of Europe and the dominant westerlies cause the continentality of the climate to increase eastwards, as shown by the annual temperature amplitude. It increases from 8 to 10ºC in Iceland, along the Irish and British coasts and Portugal, and from 14 to 16ºC along the Atlantic coast of Scandinavia to 20ºC in the Iberian Meseta, the Lombardy plain, the lee of the Scandinavian mountains, the Baltic States, central Poland, the Middle Danube plain, the Dinaric Alps and southern Greece. A more continental climate is found in the Middle Danube and Wallachian plains (25 to 26ºC), Lapland and eastern Finland (26 to 28ºC), as well as central Russia.
Moist air masses from the Atlantic cause the largest amounts of precipitation to fall on western coasts and mountain slopes. In such places the annual precipitation may exceed 1000 to 2000 mm and, in particularly exposed spots, may even reach 3000 to 4500 mm (western parts of Britain and Ireland, Norway, the Iberian and Balkan peninsulas, southern and northern slopes of the Alps and the western Pyrenees). Precipitation is scantiest (300 to 500 mm per annum) in the southeast of the Mediterranean peninsulas, chiefly southeast and central Spain, southeast Italy and eastern Greece. Under 500 mm per annum falls in Sweden, in the Baltic States, central Poland, the Middle Danube plain, the Dinaric Alps and southern Greece.
Although small, Europe has a wide mosaic of seasons of maximum precipitation, reflecting the influence of orography. Southern Europe has a dry season (spring and summer) which lengthens eastwards and southwards. The rest of Europe has precipitation all the year round, western coasts receiving more in autumn and winter. As a sign of increasing continentality, the proportion of more abundant summer rain grows eastwards from southeast Sweden and west-central Europe. In the Alps and the interior lowlands, however, rainfall is more plentiful in spring. Both the particular climate and the orography contribute to the accumulation of waters in the Alps. Thus Switzerland is the water tower of Europe. The main watercourses leaving the country flow either towards the North Sea (Rhine), or towards the Mediterranean Sea (Rhone). A smaller area of territory belongs to the catchment areas of the Po (Adriatic) and the Danube (Black Sea).
The interplay between natural conditions and cycles and anthropogenic
influences can sometimes lead to undesirable conditions being reinforced.
In winter, for example, maximum emissions of pollutants often combine with
periods of maximum precipitation leading to washout and greater than expected
environmental impacts. Under drier, colder, conditions (due to latitude
or altitude), many pollutants have longer residence times, leading to a
build-up in the atmosphere during winter; at the start of spring
a critical phase in biospheric cycles the pollutants can still be
abundant.
REFERENCES
|
Fuente:
European Environment Agency
The Dobris Assessment
Chapter 03: The Continent