The climate of the British Isles is influenced by the movement of all the major air masses, including maritime tropical (mT), maritime polar (mP), maritime Arctic (mA), continental polar (cP) and continental tropical (cT) air. The British Isles, residing in the mid-latitudes, lies in the path of air mass convergence between warm tropical airflow and cold polar airflow. At this convergence zone, the lighter warm air rises over the heavier cold air, producing the typical weather phenomena associated with this regime, fronts, depressions, and rain. As a consequence, the British climate can be very changeable, and all types of weather may be experienced within a single day. In general however, the British climate is relatively mild for its latitude, since it is influenced by the Gulf Stream. Warm maritime tropical (mT) air retains much of its heat because it is in contact with the Gulf Stream, a warm ocean current that originates in the Gulf of Mexico and travels northeast across the Atlantic. In contrast, Newfoundland, on the west side of the North Atlantic at a similar latitude to the British Isles, can be up to 10°C colder in winter.
Although the British Isles makes up only a relatively small geographical area of the Earth's surface, differences in climate across the region do exist. The climate of the western half of the British Isles is dominated by maritime tropical and maritime polar air, whilst the climate further east is often influenced by more continental regimes. In addition to the major air masses, the strength of sunlight received at different locations in the British Isles, which varies according to latitude, also has an influence on climate. Consequently, the climate of the British Isles can be divided into four quarters. The northwest quarter, including Northern Ireland, the Lake District and the Western Isles of Scotland, is characterised by mild winters (average 6°C) and cool summers (average 15°C), whilst the northeast of Scotland has cold winters (average 3°C) and cool summers. The southwest quarter, containing the Irish Republic, Wales and Southwest England, experiences mild winters and warm summers (average 17°C), whilst the Midlands and Southeast England have cold winters and warm summers. In general, the western half of the British Isles experiences a more maritime climate during winter, whilst the east receives influence from the cold air streams from the continent. In summer, climatic differences are dominated more by latitude.
The greater influence of maritime air in the western half of the British Isles means that it receives considerably more rainfall than in the east. The generally higher ground in the west forces incoming Atlantic air to rise, further enhancing precipitation. Parts of highland Scotland, for example, can receive over 250 cm or 100 inches of precipitation per year. The east, by contrast, is lower and flatter, lying in the rain shadow of the Welsh Mountains, the Lake District and the Scottish Highlands, and is consequently much drier. Some parts of the Southeast of England may receive only 50 cm or 20 inches of precipitation per year.