In general, the summer season in Lisbon lasts about 6 months (from May to October), winter has a duration of 3 months (December, January and February) while the transitional months are March, April & November.
The average annual temperatures are between 13-14°C (at night) to 21-22°C (during the day). The rainiest period starts at the end of the transitional period and lasts during the winter months.
As a whole residents in Lisbon dress fairly informally and casually but a nice outfit or two are needed for a big night out at an elegant restaurant, opera, etc.
Lisbon is a steep city and full of slippery, cobblestone streets. That’s why comfortable flat shoes are a must. High heel shoes are not suitable unless for going out at an elegant restaurant, opera by taxi and door-to-door.
It is recommended that lightweight summer clothes (such as shorts and T-shirts, light skirts and sleeve-less shirts, etc.) should be taken. Flip flops and bathing suits are necessary even during the transitional periods as the average sea temperature is 20 °C.
Some light but warmer clothes (like a cotton sweater, a light jacket or a warm coat) are highly recommended for cooler evenings and nights, especially in the transitional period (and even in summer from time to time). There is no need to wear heavy clothing in winter.