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September is Porto's month: harvest in the Douro, warm days, and the river fog burns off by ten. Bring a layer for the Gaia side after sunset — it drops faster than you'd expect for the temperature shown.

Regulars are specific that this month runs genuinely warm by Portuguese standards, a real step up from the cooler spring months just prior, even as rain stays a realistic possibility rather than a rare event. Our row backs that mild, still-showery read, 20.7C over 12.3C nights, with 79.6mm of rain across 9.7 days and humidity at 77 percent. Regulars are consistent this remains a comfortable month for walking the Ribeira and crossing the Dom Luis bridge without summer's crowds or heat, a light layer for cooler mornings and evenings the standard advice. The sea sits at a cold 15.3C, still too cool for most for a proper swim this early. The record: 11.9 hours of sun, a 17.4 kph wind and AQI 45.

June here is the month the board discusses one date. Sao Joao runs through the night of the 23rd into the 24th: the hotels sell out, the streets stay full until dawn, and the customs are specific - sardines and caldo verde eaten outdoors, paper balloons sent up over the city, fireworks by the river at midnight, and everybody hitting everybody else with squeaky plastic hammers, which is affection rather than assault. Their weather summary, about 23C by day and 14C at night, is almost exactly our row at 22.3C over 14.3C, so this is a mild month rather than a hot one - 55.7mm of rain across 8.4 days, a steady 16.8 kph off the Atlantic, and a sea still at 18.3C. The record: humidity 79 percent, 12.7 hours of sun and AQI 47.

Regulars make two comparisons that decide the month. Against Lisbon: Porto is cooler and wetter, high seventies here against low eighties there - our row's 24.9C is that figure exactly - which in an August heatwave is the advantage. And against its own year: the city is not quite on the coast, its residents leave on holiday, and the crowds go to the beach towns, so August can be quieter here than other months even while the coast fills. The queues that remain are at the famous bookshop and the clock tower. The sea is Atlantic-cold at 16.2C. Rain is almost absent, 11mm across 2.2 days. The record: 15.3C nights, 12.3 hours of sun and AQI 41.

Forum posters call March a genuinely lovely month to visit despite the rain risk - days are getting longer, spring blooms are starting, and a great sunny weekend is entirely possible alongside the more typical grey, foggy stretches. Porto is consistently described as rainier and foggier than Lisbon and the south, so the odds of a wet day are real, but posters are clear that a full week of solid rain is uncommon anywhere in Portugal, even in the north. Nearly everything touristy stays open year-round except the longer river cruises, so the advice is simply to have an indoor plan B rather than avoid the month.