London Geography & Climate

London sits in a shallow basin of clay and gravel on the tidal reach of the Thames, about 80 kilometres from the sea.

The river

The Thames is tidal as far upstream as Teddington, with a range of around seven metres at London Bridge — the brown colour is suspended sediment stirred by that tide, not pollution. The river is why the city exists: this was the furthest point inland that seagoing ships could reach and the furthest downstream that the river could be bridged.

North and south

The two banks are geologically different and it shows in the city. The north side rises quickly to the heights of Hampstead and Highgate on gravel and sand; the south is flatter, wetter, historically marshy, and was drained and built over later. That is a large part of why the historic centre, the wealth and most of the Underground are north of the river.

The lost rivers

A dozen tributaries — the Fleet, the Tyburn, the Walbrook, the Effra — were progressively covered over and turned into sewers. They are still down there, and they explain a surprising number of street names and dips in the road.

London clay

Stiff, impermeable and easy to tunnel through once you are below the gravel, which is why the Underground was possible at all and why deep tube lines exist here and in very few other cities.

Green space

Around 47 per cent of Greater London is green or blue space, and it has been designated a National Park City.

London has a temperate maritime climate, which in practice means it is rarely extreme and frequently grey.

Temperature

Summer highs average 22 to 24°C in July and August, winter highs 8 to 9°C with nights just above freezing. Snow is occasional and rarely lasts. The city is several degrees warmer than the surrounding countryside because of the urban heat island, which is most noticeable on winter nights.

Rain

Around 600 millimetres a year — less than Rome, Sydney or New York, a fact Londoners enjoy pointing out. The reputation comes from how it falls: frequent light drizzle spread across many days rather than heavy downpours. Carry a jacket rather than an umbrella.

Light

The real seasonal variable. In late June it is light from before five until after nine; in late December it is dark by four. That swing affects the mood of the city more than the temperature does.

Heatwaves

Increasingly frequent and increasingly serious. The city recorded over 40°C for the first time in 2022, and very little housing or public transport is built for it — the deep Tube lines in particular become genuinely unpleasant.

The fog that has gone

The pea-souper of Dickens and Holmes was not weather but coal smoke trapped under an inversion, and the Great Smog of December 1952 killed several thousand people in a few days. The Clean Air Acts that followed ended it, and London has not had one since — a rare case of an environmental problem being genuinely solved, and worth remembering when the current air quality argument comes up.

Wind and flood

Atlantic systems bring gales in autumn and winter; the Thames Barrier protects the city from tidal surge and is raised several times a year.