When is the best time to visit Portugal for Lisbon?

Portugal's weather service, IPMA, publishes 1991–2020 normals for the Lisboa/Gago Coutinho station, number 579, sitting at 103.9 meters above sea level. They describe a city with two quite different years stitched together. July runs a mean high of 83°F (28.3°C) and a mean low of 64°F (18.0°C) on 0.11 inches of rain for the entire month. August is hotter still — 84°F (29.0°C) by day, 65°F (18.5°C) overnight, 0.20 inches. Then the Atlantic reasserts itself: October brings 4.05 inches, November 4.51 inches, and the annual total reaches 28.4 inches.

September is the quiet winner in that table. A mean high of 80°F (26.8°C), a mean low of 63°F (17.4°C) and 1.41 inches of rain put it between the dry heat and the wet season, which is why the second half of the month reads better than the first half of October.

Portugal is a country of three distinct weather zones, and the national picture matters even when Lisbon is the reason you are going. IPMA maintains separate normals for Faro in the Algarve, Funchal on Madeira and Flores in the Azores, because the mainland south, the subtropical Atlantic islands and the mid-ocean Azores do not share a season. A single set of Lisbon figures will not predict any of them.

By late September the Tagus estuary light has lost its midsummer glare but the stone of the riverfront still holds heat into the evening, and with barely an inch and a half of rain across the whole month, the open ground at Belém stays dry underfoot.

The Monastery of the Hieronymites and Tower of Belém in Lisbon is one of 17 properties Portugal has inscribed on the UNESCO World Heritage List — and two of those 17 sit in the Atlantic, not on the mainland: the Laurisilva of Madeira and the Central Zone of the Town of Angra do Heroísmo in the Azores. If your mental map of Portugal is a train line from Porto to Faro, a third of the country's heritage is off it.

• April and May — Rainfall has dropped to 2.50 inches in April and 2.00 in May, and highs of 68°F (19.9°C) and 73°F (22.8°C) make long walking days comfortable rather than punishing.
• July and August — The driest weeks of the year, with under a quarter inch of rain a month, but also the hottest; shade and water matter more than an itinerary.
• October onward — The wet half begins in earnest at 4.05 inches, and November's 4.51 inches is the single rainiest month on the Lisbon record.

The US Centers for Disease Control and Prevention lists Portugal under its Global Measles notice at Level 1, Practice Usual Precautions, so vaccination status is worth settling before departure rather than at the airport. Weather-wise, the risk is not heat but water: the October-to-November block carries more than eight and a half inches between the two months, and the island properties add flights to an otherwise road-and-rail trip.

For the parts of the country those 17 properties scatter across, read the Portugal question in this group about what a first-timer should see beyond the obvious highlights.