London has a temperate oceanic climate (Köppen: Cfb). Rainfall records have been kept in the city since at least 1697, when records began at Kew. At Kew, the most rainfall in one month is 7.4 inches (189 mm) in November 1755 and the least is 0 inches (0 mm) in both December 1788 and July 1800. Mile End also had 0 inches (0 mm) in April 1893. The wettest year on record is 1903, with a total fall of 38.1 inches (969 mm) and the driest is 1921, with a total fall of 12.1 inches (308 mm).

The average annual precipitation amounts to about 600 mm, which is half the annual rainfall of New York City, but also lower than Rome, Lisbon, and Sydney, Australia. Despite its relatively low annual precipitation, London still receives 109.6 rainy days on the 1.0 mm threshold annually. However, London is vulnerable to climate change in the United Kingdom, and there is increasing concern among hydrological experts that London households may run out of water before 2050.

Temperature extremes in London range from 40.2 °C (104.4 °F) at Heathrow on 19 July 2022 down to −16.1 °C (3.0 °F) at Northolt on 1 January 1962. Records for atmospheric pressure have been kept at London since 1692. The highest pressure ever reported is 1,049.8 millibars (31.00 inHg) on 20 January 2020.