the driest. Atlantic currents, warmed by the Gulf Stream, bring mild winters; especially in the west where winters are wet and even more so over high ground. Summers are warmest in the southeast of England and coolest in the north. Heavy snowfall can occur in winter and early spring on high ground, and occasionally settles to great depth away from the hills. The average total annual sunshine in the United Kingdom is 1339.7 hours, which is just under 30% of the maximum possible (The maximum hours of sunshine possible in one year is approximately 4476 hours). The hours of sunshine vary from 1200 to about 1580 hours per year, and since 1996 the UK has been and still is receiving above the 1981 to 2010 average hours of sunshine United Kingdom is ranked 4 out of 180 countries in the Environmental Performance Index. A law has been passed that UK greenhouse gas emissions will be net zero by 2050. Topography England accounts for just over half (53 per cent) of the total area of the UK, covering . Most of the country consists of lowland terrain, with more upland and some mountainous terrain northwest of the Tees–Exe line; including the Lake District, the Pennines, Exmoor and Dartmoor. The main rivers and estuaries are the Thames, Severn and the Humber. England's highest mountain is Scafell Pike () in the Lake District. Scotland accounts for just under one-third (32 per cent) of the total area of the UK, covering . This includes nearly 800 islands, predominantly west and north of the mainland; notably the Hebrides, Orkney Islands and Shetland Islands. Scotland is the most mountainous country in the UK and its topography is distinguished by the Highland Boundary Fault – a geological rock fracture – which traverses Scotland from Arran in the west to Stonehaven in the east. The fault separates two distinctively different regions; namely the Highlands to the north and west and the Lowlands to the south and east. The more rugged Highland region contains the majority of Scotland's mountainous land, including Ben Nevis which at is the highest point in the British Isles. Lowland areas – especially the narrow waist of land between the Firth of Clyde and the Firth of Forth known as the Central Belt – are flatter and home to most of the population including Glasgow, Scotland's largest city, and Edinburgh, its capital and political centre,