Greater London covers an area of 609 square miles (1,579 km² ), making it one of the world's largest cities by area. Its primary geographical feature is the Thames , a navigable river which crosses the city from the southwest to the east. The Thames Valley is a floodplain surrounded by gently rolling hills such as Parliament Hill 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. 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, but a more substantial barrier further downstream may be necessary in the near future.

London has a temperate climate with regular but generally light precipitation throughout the year - London is in fact amongst the driest of Europe's capitals. 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 ). 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 uncommon, particularly because heat from the urban area can make London 5 °C (9 °F) hotter than the surrounding areas in winter. London is in USDA Hardiness zone 9, and AHS Heat Zone 2.