As a year round destination, Lisbon has a mild and equable climate, with a mean annual temperature of 63º F (17º C), which makes the city pleasant to visit at any time of the year. The summer days can become very warm indeed in the city centre, but the evenings are marvellously cool. Further afield, cool sea breezes keep the heat at bay and the unique scenery a blaze of colour.

Winter temperatures are considerably better than the UK , Germany and other countries in northern Europe. However, on occasion it does snow in the Lisbon region and the rain is certainly a more prominent feature than during the summer months.

Spring is an time ideal to visit the monuments and the blooming gardens, as the temperatures are always nice for a stroll. Summer is the beach season ‘par excellence’ and it’s the best time of the year to experience the wonderful beaches along the Lisbon Coast.

Autumn, when the leaves begin to fall, is a wonderful time for you to discover the quiet, romantic, and narrow streets of the old bairros (quarters) like Alfama and Bairro Alto. You can see an annual climate chart of Lisbon by clicking here.

Source: http://www.holiday-weather.com/lisbon/index.html