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.
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 (such as Montjuic) and in the outer metropolitan area, further from the sea, 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 on 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. 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. Although at about the same latitude New Haven, Connecticut has values a little lower than Barcelona but still close.
Daylight
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 (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.
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", which can last several days and be enhanced when an Atlantic cutoff travels through the Straits of Gibraltar into the Alboran Sea. 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, brings rain or, in case of enough low temperatures, snow, as in November 1999. Especially in summer, when sea surface temperature (SST) is higher, convective precipitation can happen in the form of warm rain (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.
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 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. 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.