Barcelona has a Mediterranean climate (Csa) according to Köppen-Geiger classification, a warm-temperate subtropical climate (Warmgemäßigt-subtropisches Zonenklima) according to Troll-Paffen climate classification,^[3] and a subtropical climate according to Siegmund/Frankenberg climate classification.^[4]

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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.^[5] Barcelona has an average seasonal UV index close to that of Madrid, different only in March but with the same annual average of 5, with the values varying from 9 in June and July to 1 in December.^[7][8] Although at about the same latitude New Haven, Connecticut has values a little lower than Barcelona but still close.^[9]

In winter, days are not as short as in the northern part of the continent, as the average hours of daylight in December, January and February is 10^[11] (for comparison: London^[12] or Moscow^[13] or Warsaw^[14] - about 8 hours).

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.^[5]

As the city lies on a leeward location relative to the westerlies, the largest amounts of rainfall in the cold season are produced by easternly (backdoor) cold fronts or "Levanters",^[15] which can last several days^[16] and be enhanced when an Atlantic cutoff travels through the Straits of Gibraltar into the Alboran Sea.^[17] So although Atlantic cyclones usually produce little or no precipitation at all, the slipstream effect ("rebuf" in Catalan) behind a cold front or a trough, when N-NW winds from the Ebro valley ("mestral") and the easternmost Pyrenees ("tramuntana") converge around Barcelona,^[18][19] brings rain or, in case of enough low temperatures, snow, as in November 1999.^[20] Especially in summer, when sea surface temperature (SST) is higher, convective precipitation can happen in the form of warm rain^[21] (i.e. from the coalescence of water droplets without ice formation) instead of a conventional thunderstorm. Either way, these storms can be heavy and lead to flash floods in the lowest parts of the city.^[22][23]

Average relative humidity is 72%, ranging from 69% in July to 75% in October.^[5]

Snowfalls in the city are rare, but light snowfall events can happen any year on the top of the mountains around the city and in the depths of the metropolitan area (far from the sea) more frequently.^[24][25] A heavy snowfall occurred in Christmas 1962, being the heaviest snowfall recorded in Barcelona since 1887.^[26] In the 21st century, the heaviest snowfall happened on 8 March 2010, a thundersnow during which public transport and roads were shut down.^[27][28][29]

The highest temperature recorded during the day was 37.4 °C (99.3 °F) on 27 August 2010; the average August 2003 maximum temperature during the day was 32.8 °C (91.0 °F). The coldest temperature recorded was −8.0 °C (17.6 °F) at night on 27 December 1962.^[31]

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, which occasionally reach severe limits, are common from mid July until November. The most recent major summer storm was on the 31 July 2002,^[34] when over 200 mm (7.87 in) of rain were recorded at some observatories.

Though Barcelona is normally not a windy city,^{[citation needed]} 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 100 km/h (62.14 mph). In winter Barcelona is sometimes affected by the tramontana or mistral winds, like other places in the Northwestern Mediterranean Basin.

Average annual temperature of sea is about 18 °C (64 °F). In the coldest month – January, the average sea temperature is 13 °C (55 °F). In the warmest month – August, the average sea temperature is 24 to 25 °C (75 to 77 °F).^[11][35]

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 such as La Molina.^[36] Summers in Barcelona area are warm. Summer temperatures - above 20 °C (68 °F) - begin as early as May, although in this month the sea temperature is still mild: around 17–18 °C (63–64 °F). The summer season ends in October. Over the summer season (4 months), the average temperatures is 26–29 °C (79–84 °F) during the day and the average sea temperature is 23 °C (73 °F). 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.^[37]

Climate in other places in Iberian Peninsula:

• Climate of Valencia
• Climate of Madrid
• Climate of Bilbao
• Climate of Lisbon
• Climate of Porto
• Climate of Spain
• Climate of Gibraltar