San Sebastián (Donostia) weather in December

Regulars are honest this month genuinely swings between extremes, one visitor's mid-June trip landing perfect with mid-70s Fahrenheit and no rain at all, another describing late June as unbroken cloud with neither real sun nor rain, temperatures around 21 to 22C still reading as pleasant. Our row backs that mild, genuinely wet read, 22.7C over 16.3C nights feeling 17C, with a substantial 129.4mm of rain across 12.6 days and humidity at 83 percent. Regulars are consistent a warm jacket and an umbrella both belong in the bag regardless, some years bringing genuinely hot spells into the high 30s Celsius while others stay cool and rainy throughout. The sea sits at 21.5C. The record: 10.6 hours of sun, a 15.9 kph wind and AQI 41.

May in San Sebastian is the unstable month - June is the reliable one, and late May is where it starts to settle. Regulars will tell you it can be 30C and sunny or 14C and raining, sunny in the morning and pouring by afternoon, and they quote the Spanish line about not taking your coat off until the fortieth of May. The local word for the default is sirimiri, the fine constant Basque drizzle that is neither shower nor storm; our row's 116mm across 12.7 precipitation days is mostly that. Their packing answer is layers, waterproofs and a swimming costume anyway. The record: 19.6C days, 13C nights, humidity 80 percent, 10.7 hours of sun and a 16.9C sea.

July in San Sebastian is the month regulars refuse to promise. The averages they quote run 14 to 29C and their honest summary is that it can rain every day at 16C or sit dry and sunny in the mid-30s, because rain falls here on something like 190 days a year whatever the season. Mornings often open with chirimiri, the fine Basque drizzle-mist, which usually disperses as the day goes on. The sea is the Bay of Biscay and gets described as bracing rather than warm - fine for a dip, not for staying in. July is busy but it is not August: Semana Grande, which fills the city and puts a fireworks competition over La Concha every night, runs 12 to 19 August. The record: 23.9C days, 18.3C nights, 11.7 precipitation days carrying 82.8mm, 11.2 hours of sun, humidity 81 percent and a 22.8C sea.

September is the month Basque Coast forum regulars quietly favor, and the trade-off it carries is well described in the threads. The sea stays near its annual peak - about 23C in our record, a fraction off August's high - and the July-August crowd pressure eases off. The counterweight is the coast's Atlantic temperament: a Bilbao-area destination expert describes this shoreline as 'not the Mediterranean, but the rough Atlantic... expect a similar weather to that in Ireland,' green, mountainous and 'rainy any time,' with July and August the only stretch he would call dependably mild. September keeps genuinely mild days, with highs around 23C, but our record shows the coast's showery pattern back in force - about 11 rainy days and 133mm spread across the month. The practical read from the threads: plan around changeable days rather than guaranteed beach weather, and count the warm sea and thinner sands as what makes the showers worth it.