Map from Google - London

In London, the climate is influenced by the ocean, and is therefore cool, humid and rainy, with Atlantic fronts passing one after the other throughout the year, and bringing a variable weather, with cloudiness, rains and showers, which alternate, at least in spring and summer, with a few hours of sunshine.
However, the climate 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 high pressure prevails during winter, because it can cause thermal inversions and night fogs.
Here are the average temperatures of the Heathrow Airport.
Average temperatures - London
LondonJanFebMarAprMayJunJulAugSepOctNovDec
LondonJFMAMJJASOND
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 as a rainy city is mostly due to the frequency of the rains, which can occur quite often also 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
LondonJanFebMarAprMayJunJulAugSepOctNovDecYear
LondonJFMAMJJASONDY
Prec. (mm)554040455045455050706055605
Prec. (in)2.21.61.61.821.81.8222.82.42.223.8
Days1199998888111010109

Winter, from December to February, is cold, with quite frequent rains, but it's not freezing, and is 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 temperature 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 temperature drop quite a bit, and there may be snow and frost, 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 the Gatwick Airport (which is actually located outside the city, in the countryside of Sussex), the temperature reached -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 the return of cold weather; in March you can still experience some snow showers; in April the temperature can still be cold, winter-like, even though the days are much longer (the sun sets much later than in winter). Typically the temperature becomes mild in a more stable manner in the second half of May.

Summer, from June to August, is not very warm, but in this season the area of London is the warmest 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 it is the month with the longest days, and there is also a bit more sunshine: there are on average 7 hours of sunshine per day in June, compared with 6 hours in July and August.
Even in summer, as happens in the rest of the year, in London there's no shortage of rainfall: it rains on average one day out of three or four. On cloudy and rainy days, highs can remain about 16/17 °C (61/63 °F).
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-lived, because Atlantic currents usually return after a few days.

In autumn, from September to November, the temperature gradually decreases. The weather is still often pleasant in September, especially in the first half, then gradually becomes more cold, cloudy and rainy. In November, the days are short, it's cold enough, the sun is rarely seen 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 slightly higher (about 2 degrees Celsius) than in surrounding rural areas; this effect is more noticeable at night and in the winter months.

London

When to go

The best time to visit London is the summer, or more generally from mid-May to mid-September: temperatures are generally good for outdoors activities, although it's better to bring an umbrella, and a sweatshirt or sweater for the evening or for cool days. In summer you can 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 parks of the city and even sunbathe.