The River Thames before sunrise

The Thames was once a much broader, shallower river with extensive marshlands; at high tide, its shores reached five times their current width. 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 climate with regular but generally light precipitation throughout the year - unlike the rest of the UK and even the nearby coast. London is in fact amongst 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 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.

Often called "The Green City," London has a number of open spaces. The largest of these in the central area are the Royal Parks of Hyde Park and its neighbors Kensington Gardens and Holland Park Gardens at the western edge of central London, and Regent's Park on the northern edge. This park is located near the tourist attractions of Madame Tussauds Wax Museum and Baker Street, where the fictional Sherlock Holmes lived. Closer to central London are the smaller Royal Parks of Green Park and St. James's Park. Hyde Park in particular is popular for sports and sometimes hosts open-air concerts.

A number of large parks lie outside the city centre, including the remaining Royal Parks of Greenwich Park to the south east, and Bushy Park and Richmond Park to the south west. Primrose Hill to the north of Regent's Park is a popular spot to view the city skyline. Some more informal, semi-natural open spaces also exist, including the 791-acre Hampstead Heath of north London. This incorporates Kenwood House, the former stately home and a popular location in the summer months where classical music concerts are held by the lake, attracting thousands of people every weekend to enjoy the music, scenery and fireworks. Outer South East London is noted for its open spaces and extensive wooded areas.