The best word to sum up Porto's weather is changeable, and it's not uncommon to feel as though you've experienced four seasons in one day. Typically, expect some rain during the autumn and winter months. Temperatures rarely dip below 40 F/5 C and average around 60 F/15 C during winter daytimes. When the wind picks up, however, it can feel bitterly cold, so do take a sweater and a warm coat if traveling in the cooler months.
Summers can get quite hot; despite average highs of around 75 F/24 C June-September, temperatures sometimes reach 85 F/30 C, making sightseeing a chore and boat trips or beach time more appealing. Even so, you'll need a scarf or thin sweater as evenings can be considerably cooler, averaging around 55 F/13 C May-October.
Ideal times to visit are May, June, September and October, especially those latter months if you want to experience the grape harvest in the Douro Valley, although there's a higher risk of rain by October.