The climate of Barcelona is Mediterranean, with mild and relatively rainy winters, and hot and sunny summers.
The average temperature of January is 9 °C (48 °F), that of August is 24.5 °C (76 °F).
Here are the average temperatures.
Average temperatures - Barcelona

Barcelona	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Barcelona	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	5	5	7	9	13	17	20	20	17	14	9	6
Max (°C)	14	14	16	18	21	25	28	29	26	22	17	14
Min (°F)	41	41	45	48	55	63	68	68	63	57	48	43
Max (°F)	57	57	61	64	70	77	82	84	79	72	63	57

Rainfall is not abundant, due to the city's location on the east coast of Spain, in fact the flow of Atlantic weather fronts comes from the west, so when they reach the city, they have already released moisture on western and inland regions: for this reason the inhabitants of central and southern Catalonia are accustomed to reduce water consumption. However, the rains are more frequent in autumn and spring, that is, in the seasons in which the southerly currents, which can pick up moisture from the sea, are more frequent. In a typical year, 620 millimetres (24.5 inches) of rain fall, with a maximum of 100 mm (4 in) in October, when it rains on average for 7 days. July is the driest month, with just 20 mm (0.8 in); in summer the rains are rare, but sometimes thunderstorms can erupt in the afternoon, reaching the coast afert forming in the interior.
Here is the average precipitation.
Average precipitation - Barcelona

Barcelona	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Barcelona	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	40	40	35	40	55	30	20	65	85	100	65	40	620
Prec. (in)	1.6	1.6	1.4	1.6	2.2	1.2	0.8	2.6	3.3	3.9	2.6	1.6	24.4
Days	4	4	5	5	5	4	2	5	5	7	5	5	56

Winter, from December to February, is mild, although you may encounter different weather situations. When the Azores High moves over Spain, there can be several mild and sunny days, with highs around 15 °C (59 °F). On the other hand, Atlantic fronts can bring clouds and rains for a few days. Sometimes, even cold and windy days can occur, when cold air masses from eastern Europe arrive: in such cases it may even snow in the hills behind the city (such as Tibidabo). The lowest temperatures in Barcelona are not particularly cold: typically the temperature drops around freezing or slightly below on colder nights; only in January 1985, which has been an historical winter in various regions of Europe, it dropped down to -7 °C (19 °F).

Summer, from mid-June to August, is hot and sunny, with highs close to 30 °C (86 °F); the heat is sultry due to humidity, but sea breezes make it more bearable. The city is somewhat protected from heat waves that can affect other parts of Spain, but we can not exclude some particularly hot days, with peaks of 35/36 °C (95/97 °F).

The intermediate seasons, spring and autumn, are mild and relatively rainy, however, there's no shortage of sunny days also in these periods.

The amount of sunshine in Barcelona is never bad, because some sunny day can occur all year round, although it is better, as happens in Mediterranean climates, in late spring and summer, especially from May to August, when sunny days prevail. The average goes from 4.5 hours of sunshine per day in December, to 10 hours in July.
Sunshine - Barcelona

Barcelona	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Barcelona	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	5	6	7	8	8	9	10	9	6	6	5	5

The temperature of the sea is warm enough for swimming from July to September, peaking in August, when it reaches 25 °C (77 °F). In June, the sea is still cool, around 20 °C (68 °F).
Sea temperature - Barcelona

Barcelona	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Barcelona	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	13	13	13	14	17	20	23	25	23	20	17	15
Sea (°F)	55	55	55	57	63	68	73	77	73	68	63	59

The best time to visit Barcelona is spring, from mid-April to mid-June. September, too, is a good month, but it is quite rainy. Summer, from mid-June to August, can be hot and muggy, though it is the best time for a beach holiday. Winter is usually mild, but sometimes it can get a bit cold and it can rain.

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