Lisbon is the capital and largest city of Portugal. It has a mediterranean climate with mild, rainy winters and warm to hot, dry summers. The average annual temperature is 17.4 °C (63.3 °F): 21.3 °C (70.3 °F) during the day and 13.5 °C (56.3 °F) at night. In September, the daily mean temperature is 22.1 ºC (71.8 ºF).
You can also check the official tourism website, Visit Lisboa, to find some more information on what you can do.
