The Atlantic Ocean has a significant effect on Britain's climate. Although the British Isles are as far north in latitude as Labrador in Canada, they have a mild climate throughout the year. This is due to the Gulf Stream, a current of warm water that flows up from the Caribbean past Britain. Prevailing southwesterly winds moving across this warmer water bring moisture and moderating temperatures to the British Isles. The surrounding waters moderate temperatures year-round, making the UK warmer in winter and cooler in summer than other areas at the same latitude. Great Britain's western coast tends to be warmer than the eastern coast, and the southern regions tend to be warmer than the northern regions. The mean annual temperature in the far north of Scotland is 6°C (43°F), and in warmer southwestern England it is 11°C (52°F). In general, temperatures are ordinarily around 15°C (60°F) in the summer and around 5°C (40°F) in the winter. Temperatures rarely ever exceed 32°C (90°F) or drop below -10°C (14°F) anywhere in the British Isles. In general, frosts, when the temperature dips below 0°C (32°F), are rare.
Winds blowing off the Atlantic Ocean bring clouds and large amounts of moisture to the British Isles. Average annual precipitation is more than 1,000 mm (40 in), varying from the extremes of 5,000 mm (196 in) in the western Highlands of Scotland to less than 500 mm (20 in) in the driest parts of East Anglia in England. The western part of Britain receives much more moisture than the eastern areas. It rains year-round, and in the winter the rain may change to snow, particularly in the north. It snows infrequently in the south, and when it does it is likely to be wet, slushy, and short-lived. Southern Britain has experienced episodes of drought in recent years, although historically these are rare occurrences. Some regard these episodes as indicators of global climatic changes.
The climate has affected settlement and development in Britain for thousands of years. The mild, wet climate ensured that thick forests rich in game, as well as rivers and streams abundant with fish, were available to prehistoric hunters and gatherers. Britain was regarded as a cold, remote, and distant part of the ancient Roman Empire in the first few centuries AD, so relatively few Romans were motivated to move there for trade, administrative, or military reasons. Preindustrial settlements clustered in southern England, where the climate was milder, the growing season longer, and the rich soil and steady rainfall produced bountiful harvests. Successive waves of invaders made the plains of southern England their primary objective. After the Industrial Revolution began in the 18th century, populations grew enormously in areas with rich resources beneath the ground, particularly coal, even though these resources were sometimes located in the colder, harsher northern regions of England or the western Lowlands of Scotland.