Ireland lies west of the island of Great Britain, from which it is separated by the narrow North Channel, the Irish Sea (which attains a width of 130 mi/209 km), and St. George's Channel. More than a third the size of Britain, the island averages 140 mi (225 km) in width and 225 mi (362 km) in length. A large central plain extending to the Irish Sea between the Mourne Mts. in the north and the mountains of Wicklow in the south is roughly enclosed by a highland rim. The highlands of the north, west, and south, which rise to more than 3,000 ft (914 m), are generally barren, but the central plain is extremely fertile and the climate is temperate and moist, warmed by southwesterly winds. The rains, which are heaviest in the west (some areas have more than 80 in./203 cm annually), are responsible for the brilliant green grass of the "emerald isle," and for the large stretches of peat bog, a source of valuable fuel. The coastline is irregular, affording many natural harbors. Off the west coast are numerous small islands, including the Aran Islands, the Blasket Islands, Achill, and Clare Island. The interior is dotted with lakes (the most celebrated are the Lakes of Killarney) and wide stretches of river called loughs. The Shannon, the longest of Irish rivers, drains the western plain and widens into the beautiful loughs Allen, Ree, and Derg. The River Liffey empties into Dublin Bay, the Lee into Cork Harbour at Cobh, the Foyle into Lough Royle near Derry, and the Lagan into Belfast Lough.