Lisbon has an Atlantic climate wearing Mediterranean clothes. It is warmer than northern Europe all year and cooler than inland Spain all summer, and that combination makes its shoulder seasons unusually long and unusually good.
The short version
- Best all-round: late March to May, and late September to October
- Hottest and fullest: July and August
- Wettest: roughly November to February
- Cheapest: January and February, with the caveat that you will get rain
Spring: the city at its best
From late March the jacaranda-lined squares start to fill with people rather than tour groups, the light turns hard and clear off the Tagus, and terrace season begins in earnest. Temperatures are comfortable for the hills, which matters more in Lisbon than in most cities. Sintra, up in its own cooler and mistier microclimate, is at its most atmospheric.
One caveat: Easter moves, and when it lands the city fills fast.