London is the capital city of both the United Kingdom and the constituent country of England

Topography and climate
Climate chart for London

Greater London covers an area of 609 square miles (1,579 km²), making it the 37th largest urban area in the world. Its primary geographical feature is the Thames, a navigable river which crosses the city from the south-west to the east. The Thames Valley is a floodplain surrounded by gently rolling hills such as Parliament Hill, Addington Hills, and Primrose Hill. These hills presented no significant obstacle to the growth of London from its origins as a port on the north side of the river, and therefore London is roughly circular.

The Thames was once a much broader, shallower river with extensive marshlands; at high tide, its shores reached five times their current width. Since the Victorian era It has been extensively embanked, and many of its London tributaries now flow underground. The Thames is a tidal river, and London is vulnerable to flooding. The threat has increased over time due to a slow but continuous rise in high water level by the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound. In 1974, a decade of work began on the construction of the Thames Barrier across the Thames at Woolwich to deal with this threat. While the barrier is expected to function as designed until roughly 2030, concepts for its future enlargement or redesign are already being discussed.

London has a temperate marine climate, like much of the British Isles, with regular but generally light precipitation throughout the year—unlike the rest of the UK and even the nearby coast. London is in fact among the driest of Europe's capitals, with water resources per head of population equivalent to Israel. The warmest month is July, with an average temperature range at Greenwich of 13.6 °C to 22.8 °C (56.5 to 73.0 °F). Record high temperatures of up to 38.1 °C (101 °F) were recorded in different parts of London on 10 August 2003. The coolest month is January, averaging 2.4 °C to 7.9 °C (35.6 to 46.2 °F). Average annual precipitation is 583.6 mm (22.98 in), with February on average the driest month. Snow is relatively uncommon, particularly because heat from the urban area can make London up to 5 °C (9 °F) hotter than the surrounding areas in winter. However light snowfall is seen on some days most winters. London is in USDA Hardiness zone 9, and AHS Heat Zone 2.