Climate of Barcelona

Barcelona has a maritime Mediterranean climate "Csa" according to Köppen-Geiger classification, a "warm-temperate subtropical" climate ("Warmgemäßigt-subtropisches Zonenklima") according to Troll-Paffen climate classification, and a subtropical climate according to Siegmund/Frankenberg climate classification. 
The city lays on the eastern coast of the Iberian Peninsula, a leeward location relative to westerlies, so Atlantic storms usually produce little or no rain at all. However, a phenomenon called "rebuf" (slipstream effect) in local literature, when N-NW winds behind a cold front (or a trough) from the Ebro valley ("Mestral") and the Easternmost Pyrenees ("Tramuntana") converge upon the region around Barcelona, may result in rain or, in case of enough low temperatures, even snow. Especially in summer, when sea surface temperature (SST) is higher, these precipitacions can happen in the form of warm rain (i.e. from the coalescence of water droplets without ice formation) instead of a conventional thunderstorm. In either form, convective rains in Barcelona can be heavy and lead to flash floods in its lowest parts. However, the largest amounts of rainfall in the cold season are produced by easternly (backdoor) cold fronts or "Levanters", which can last several days and be enhanced when an Atlantic cutoff is located SW of the Iberian Peninsula, sometimes travelling through the Straits of Gibraltar into the Alboran Sea. 
Temperature.
General.
Its average annual temperature is during the day and at night. In the coldest month – January, typically the temperature ranges from during the day and at night. In the warmest month – August, the typically temperature ranges from during the day and about from at night. Large fluctuations in temperature are very rare.
Seasonal climate.
Winter.
Winters in Barcelona are mild. January and February are the coldest months, with average temperatures around during the day and at night. The hills around the city and in the outer metropolitan area, further from the sea (such as Montjuic), occasionally record frost at night. Frost during the day has been recorded in the city and inner metropolitan area during the strongest cold waves and snowstorms, but is very rare overall. The last time the city recorded a temperature below freezing was in 27 January 2005.
Spring.
Spring lasts usually from March to late May, although in some years it might get into June. Maxima are around and minima around . Rain is more frequent in spring than in winter and summer, but less than autumn. 
Summer.
Generally the summer season lasts from late May or early June to late September or early October. July and August are the warmest months, with average temperatures around during the day and at night. In June and September the average temperature is around during the day and at night. Daytime temperatures above are rare.
Autumn.
Autumn is the rainiest season of the year, lasting from October to December. Maxima are around and minima around .
Sunshine.
Sunshine duration is 2,524 hours per year, from 138 - average 4.5 hours of sunshine per day in December to 310 - average 10 hours of sunshine per day in July. This is the middling value for the southern half of Europe because in this part of the continent, sunshine duration varies from about 2,000 to about 3,000 hours per year. However, this is 60% larger value than in northern half of Europe, where sunshine duration is around 1500 hours per year. In winter Barcelona has about three times the sun duration of northern Europe. Barcelona has the UV index close to that of Madrid, differentiating only in the month of March but in the average with the same value being equal to 5, the value varies from 9 (June and July) to 1 (December). Although at about the same latitude New Haven, Connecticut has values a little lower than Barcelona but still close.
Daylight.
Barcelona enjoys one of the most optimal number of hours of winter daylight in Europe. Days in winter are not as short as in the northern part of the continent, the average hours of daylight in December, January and February is 10 hours (for comparison: London or Moscow or Warsaw - about 8 hours).
Precipitation.
Barcelona has on average only 55 precipitation days a year, therein average several rainy days per month (≥ 1 mm), ranging from 2 in July to 6 in October. The average annual precipitation is less than 640 mm (25 inches), ranging from 20 mm (0.79 inch) in July to 91 mm (3.58 inch) in October.
Humidity.
Average relative humidity is 72%, ranging from 69% in July to 75% in October.
Snow.
Snowfalls in the city are rare, but light snowfall events can happen ever year on the top of the mountains around the city and in the depths of the metropolitan area (far from the sea) more frequently. A heavy snowfall occurred in Christmas 1962, being the heaviest snowfall recorded in Barcelona since 1887. In the 21st century, the heaviest snowfall happened on 8 March 2010, a thundersnow during which public transport and roads were shut down. 
Temperature extremes.
The highest temperature recorded during the day was on 27 August 2010; the average August 2003 maximum temperature during the day was . The coldest temperature recorded was at night on 27 December 1962.<ref name="aemet.es/historic">"Valores Extremos for Barcelona" (pdf) (www version) – Agencia Estatal de Meteorología</ref>
Fog.
Barcelona is generally a sunny city, however, some days of fog and spells of overcast days are not rare. Sea fog is frequent in early spring, when the first warm African air masses come in over the cold sea water.
Thunderstorms.
Thunderstorms, which occasionally reach severe limits, are common from mid July until November. The most recent major summer storm was on the 31 July 2002, when over of rain were recorded at some observatories.
Winds.
Though Barcelona is normally not a windy city, it is affected by sea breezes from May/June to September and winds from the west and northwest in winter. Eastern gales sometimes cause floods on the coastline. East and northeast winds can exceed . In winter Barcelona is sometimes affected by the tramontana or mistral winds, like other places in the Northwestern Mediterranean Basin.
Sea temperature.
Average annual temperature of sea is about . In the coldest month – January, the average sea temperature is . In the warmest month – August, the average sea temperature is .
Seasonal information.
Snow is rare in Barcelona (falling only once every two years on average), although there are resorts for winter sports in the Pyrenees 100–150 km from the city. Because of the climate and geographical location, the city hopes to host the 2026 Winter Olympics. The opening and closing ceremonies, medal ceremonies and indoor sports would be held in Barcelona, while outdoor sports would be held in ski resorts in the Pyrenees, mainly La Molina. Summers in Barcelona area are warm. Summer temperatures - above - begin as early as May, although in this month the sea temperature is still mild: around . The summer season ends in October. Over the summer season (4 months), the average temperatures is during the day and the average sea temperature is . The weather in Barcelona during spring and autumn can be very changeable. During these months it can rain for quite long periods, but consecutive sunny days are also fairly common. The difference between temperatures during day and night is small because of the strong maritime influence, very rarely surpassing 10 °C.
See also.
Climate in other places in Iberian Peninsula: