24 Climate of the British Isles 



than 200 hours in the month, while western Europe 

 generally, except Denmark and Scandinavia, receives 

 more than 200. A comparison of these figures with 

 those of rainfall shows that the British Isles as a whole are 

 considerably cloudier in proportion to their rainfall than 

 is the case in most of western Europe. 



The effect of the warm south-west winds on the 

 islands as a whole, and especially on the west 

 Distribution coasts, is also well seen in the distribu- 

 empera- ^^^^ ^| ^-j^^ isotherms (Unes of equal tempera- 

 isotherms, ture), especially during the winter months 

 (Pig. 4). The course of the January isotherm 

 of 5° C. (41° F.) is particularly instructive. This isotherm, 

 which has a mainly east and west direction through 

 southern Europe, turns northwards in south-western 

 France, crosses the English Channel, bending north-west- 

 wards to the Dorsetshire coast, continues into South 

 Wales, pursues a northerly course to a point between the 

 Isle of Man and the Scottish coast, makes a great bend to 

 the south-westwards including the whole of central and 

 north-eastern Ireland, and then returns to the islands off 

 the western Scottish coast, through which it runs due 

 north into the northern seas. 



Thus the whole of south-western England, the western 

 coasts of Wales, the whole of southern and western 

 Ireland and the western islands of Scotland have a mean 

 January temperature as high as that of western and parts 

 of southern France, while Cornwall and the extreme 

 south-west of Ireland has a mean January temperature 

 as high as that of the Mediterranean coast of Provence 

 (Riviera). It is this fact of course that accounts for 

 the successful cultivation in the open air of many 

 Mediterranean and subtropical plants, particularly 

 of many evergreens, in these parts of the country, and 

 probably also for the occurrence of Arhidus Unedo in the 

 Killarney woods. Many native herbaceous plants too, 



