Older generations of Portuguese citizens tend to dress up more than their counterparts in the United States or the United Kingdom. That said, attitudes toward clothes have become more relaxed in recent years among the younger generations.
Jeans, however, are generally still paired with a collared shirt and, if necessary, a sweater or jacket. Dressier outfits are needed for more expensive restaurants, nightclubs, and fado houses, though, and people still frown on shorts in churches.
Sightseeing calls for casual, comfortable clothing (well-broken-in low-heel shoes, for example). Away from beaches, wearing bathing suits on the street or in restaurants and shops is not considered good taste.
Summer can be brutally hot; spring and fall, mild to chilly; and winter, cold and rainy. Sunscreen and sunglasses are a good idea any time of the year, since the sun in Portugal is very bright.