Winter temperatures in northwest Europe are some 11°C or more above the latitudinal average (see Figure 3.18), a fact usually attributed to the presence of the North Atlantic current. There is, however, a complex interaction between the ocean and the atmosphere. The current, which originates from the Gulf Stream off Florida strengthened by the Antilles current, is primarily a wind-driven current initiated by the prevailing south-westerlies. It flows at a velocity of 16 to 32 km per day and thus, from Florida, the water takes about eight or nine months to reach Ireland and about a year to reach Norway (see Chapter 7D.2). The southwesterly winds transport both sensible and latent heat acquired over the western Atlantic towards Europe, and although they continue to gain heat supplies over the northeastern Atlantic, this local warming arises in the first place through the drag effect of the winds on the warm surface waters. Warming of airmasses over the northeastern Atlantic is mainly of significance when polar or arctic airflows southeastward from Iceland. The temperature in such airstreams in winter may rise by 9°C between Iceland and northern Scotland. By contrast, maritime tropical air cools on average by about 4°C between the Azores and southwest England in winter and summer. One very evident effect of the North Atlantic Current is the absence of ice around the Norwegian coastline. However, the primary factor affecting the climate of northwestern Europe is the prevailing onshore winds transferring heat into the area.
The influence of maritime airmasses can extend deep into Europe because there are few major topographic barriers to airflow and because of the presence of the Mediterranean Sea. Hence the change to a more continental climatic regime is relatively gradual except in Scandinavia, where the mountain spine produces a sharp contrast between western Norway and Sweden. There are numerous indices expressing this continentality, but most are based on the annual range of temperature. Gorczynski's continentality index (K) (Note 1) is:
sin p where A is the annual temperature range (°C) and p is the latitude angle (the index assumes that the annual range in solar radiation increases with latitude, but in fact the range is a maximum around 55°N). K is scaled from 0 at extreme oceanic stations to 100 at extreme continental stations, but values occasionally fall outside these limits. Some values in Europe are London 10, Berlin 21 and Moscow 42. Figure 10.2 shows the variation of this index over Europe.
An independent approach relates the frequency of continental airmasses (C) to that of all airmasses (N) as an index of continentality, i.e. K = C/N (per cent). Figure 10.2 shows that non-continental air occurs at least half the time over Europe west of 15°E as well as over Sweden and most of Finland.
A further illustration of maritime and continental regimes is provided by a comparison of Valentia (Eire), Bergen and Berlin (Figure 10.3). Valentia has a winter rainfall maximum and equable temperatures as a result of its oceanic situation, whereas Berlin has a considerable temperature range and a summer maximum of rainfall. A theoretically ideal 'equable' climate has been defined as one with a mean temperature of 14°C in all months of the year. Bergen receives large rainfall totals due to orographic intensification and has a maximum in autumn and winter, its temperature range being intermediate between the other two. Such averages convey only a very general impression of climatic characteristics, and therefore British weather patterns are examined in more detail below.
Was this article helpful?