Map from Google - London

In London, the climate is influenced by the ocean, and is therefore cool, humid and rainy, with Atlantic fronts regularly affecting the city throughout the year, bringing a variable weather, with cloudiness, rains and showers, alternating, at least in spring and summer, with a few hours of sunshine.
However, the weather is a bit milder than in the rest of Britain: the city is located in the south, and in a relatively sheltered area, called London Basin, between the Chiltern Hills to the north and the North Downs to the south: in this area the climate is slightly warmer than in the surrounding region, except when in winter a high pressure sets in, because it can cause thermal inversions and night fogs.
Here are the average temperatures of the Heathrow Airport.
Average temperatures - London
Min (°C)2246912141411853
Max (°C)881114182123232016118
Min (°F)363639434854575752464137
Max (°F)464652576470737368615246

Rainfall amounts to about 600 millimetres (23.5 in) per year, and is less abundant than in many other European cities: the perception of London being rainy is mostly due to the frequency of the rains, which may occur quite often even in summer. The rains are therefore frequent but not abundant, often light or intermittent, and usually take the form of drizzle or brief shower.
Here is the average precipitation.
Average precipitation - London
Prec. (mm)554040455045455050706055605

Winter, from December to February, is cold, with quite frequent rains; it is on average above freezing (0 °C or 32 °F), and much milder than the latitude would suggest: the average temperature in January is about 5 °C (41 °F).
However, there are different types of weather. In periods of high pressure, the temperature is low, close to freezing (0 °C or 32 °F), and the weather is cold, gray and sometimes foggy. In periods of Atlantic currents, the temperatures can be mild: even 12/13 °C (54/55 °F) or more, with lows around 10 °C (50 °F), but the wind can blow, sometimes intensely. When cold currents prevail, the northern ones of polar origin, or better the southeastern ones that come directly from the European continent, the temperatures drop quite a bit, and there may be snow and frosts, though usually for short periods. In the winter months, there are approximately 7/8 days per month with minimum temperatures below freezing; usually frosts are not intense, and the temperature rarely drops below -5 °C (23 °F). However, at Gatwick Airport (which is actually located outside the city, in the countryside of Sussex), the temperature dropped to -12 °C (10 °F) in December 1981, -11 °C (12 °F) in January 1985, -14 °C (7 °F) in February 1986, and -11 °C (12 °F) in December 2010.

In spring, from March to May, weather disturbances still frequently pass, the average temperatures slowly rise, and the first warm days alternate with returns of cold; in March you can still experience some snow showers; in April the temperatures can still be cold, winter-like, even though the days are much longer (the sun sets much later than in winter). Typically the temperatures become mild in a more stable manner from the second half of May.

Summer, from June to August, is not very warm, but in this season that of London is the warmest area of the entire UK. In July and August, lows are around 14 °C (57 °F), and highs around 23/23.5 °C (73.5/74.5 °F). June is a bit cooler, though this is the month with the longest days, and there is also a bit more sunshine: there are on average 7 hours of sunshine a day in June, compared with 6 hours in July and August.
Even during summer, as happens in the rest of the year, in London there's no shortage of rainfall: it rains on average for one day out of three or four.
Sometimes, the London area can be reached by hot air masses from Spain: in these cases the temperature can reach 28/32 °C (82/90 °F). However, these periods are short, because usually the Atlantic currents return after a few days. Even in summer, cloudy and rainy days are possible, with highs about 16/17 °C (61/63 °F).

In autumn, from September to November, the temperature gradually decreases. The weather is often still pleasant in September, especially in the first half, then gradually becomes cold, cloudy and rainy. In November, the days are short, it's cold enough, there is little sun and the wind often blows.

As happens in big cities, in the central and most densely populated districts, the so called urban heat island effect occurs, so that the temperature is a few degrees higher than the surrounding rural areas; this effect is more noticeable at night and in the winter months.


When to go

The best time to visit London is the summer, or more generally from mid-May to mid-September: the temperatures are generally good for outdoors activities, although it's better to bring an umbrella, and a sweatshirt or sweater for the evening or cool days. In summer you can even hope to find a period of good weather, with warm days, in which the maximum temperature is above 25 °C (77 °F) or can even approach 30 °C (86 °F), and in which you can remove the outer layers of clothing, stroll in the numerous city parks and even sunbathe.