Map from Google - Portugal

The climate of Portugal is temperate, being influenced by the Atlantic Ocean. In the north, the climate is cool and rainy, while moving south it becomes gradually warmer and sunnier; in the far south, the region of Algarve has a dry and sunny microclimate. In the interior, on the border with Spain, the climate becomes a little more continental.
In summer, Portugal is protected by the Azores High, so it's sunny everywhere, however a few weather fronts can pass through the northern part of the country. In the rest of the year, and especially from November to March, there is no shortage of rains, which are more frequent and abundant in the north. Therefore, the landscape in the north is very green, while it gradually becomes more arid towards the south, up to Algarve which has a fairly dry climate.
In fact, the annual precipitation exceeds 47 inches in Porto, while it drops to around 28 inches in the central area (see Lisbon), and goes down to about 20 inches in the south (see Faro). The rainiest season is winter.
Here is the average precipitation in the capital, Lisbon.
Average precipitation - Lisbon
Prec. (in)4.34.332.

Here are the average temperatures in Lisbon.
Average temperatures - Lisbon
Min (°F)464852545763646664595450
Max (°F)596166687279828281736459

Winter, from December to February, is mild on the coast, even in the northern part, since the average temperature in January goes from 48 °F in Porto, to 52 °F in Lisbon, to 53.5 °F in Faro.
In winter there are periods of good weather, because the Azores Anticyclone can move over the country even in this season, but at other times there are waves of bad weather, with rain and wind. Sometimes, gale force winds may blow from the ocean, especially in the north.
The position of the country, overlooking the ocean, provides good shelter from cold winds and night frosts, which in fact are very rare and not intense: the coldest records are around 28/30 °F.
In the interior, winter gets colder, not only for the greater distance from the sea, but also because the altitude increases, and this happens especially in the central and northern regions, east of Coimbra and Oporto (see Bragança), where at times it can snow.
Fatima is located north of Lisbon, about 20 miles away from the sea, so it's still mild, but it's located at 1,000 feet above sea level, and therefore it's slightly colder in winter, while it is equally warm and sunny in summer, with highs around 81/82 °F.
Here are the average temperatures of Porto.
Average temperatures - Porto
Min (°F)414146485255595957524641
Max (°F)555761646873777775706357

Summer, from June to mid-September, is sunny, and it's warm in the center and south, and even cool on the northern coast: in Porto the daily average in July is 68 °F, and daytime temperatures are around 75/77 °F. In Lisbon, the average in summer is higher, and reaches 74.5 °F in August, with highs around 82 °F. Along the southern coasts, the temperatures remain similar to those of Lisbon, because of the influence of the ocean.
Summer is generally not too hot, except during the brief periods of hot winds from Africa, when the temperatures can exceed 95 °F, especially in the center and south.
The heat becomes more frequent in inland areas, particularly in the south-central, on the plains and valleys at low altitudes (see Evora), where sometimes it can be scorching.


The sea temperature in the north is always very cool, if not cold, and reaches 66 °F in August.
Sea temperature - Porto
Sea (°F)575757575963646664636159

In the center (see Lisbon), the temperature of the sea is similar, just warmer, although still very cool for swimming, reaching 68 °F in July and August. In the south (see Faro) the sea is still cool, but it's a bit better for swimming, because it reaches 70 °F in August and September.
Sea temperature - Faro
Sea (°F)615961616366687070686463

When to go

The best months to visit Portugal are May, June and September, because in July and August it can sometimes be hot. However, the north (see Porto) can be visited from June to September, because hot days are rare even in mid-summer.
Winter is mild, but it can be very rainy and windy, especially in the north, and it can be cold in inland areas.
For swimming and sunbathing, the best months are those of summer, especially July and August (or even the first half of September in the south-central), however, as mentioned the temperature of the ocean is not high, and on the north coast it's definitely cool.

What to pack

In winter: for Lisbon and the south coast, spring/autumn clothes, sweater, coat, rain jacket; a bit heavier jacket and a hat for Porto and the northern coast. For inland and mountainous areas: warm clothes, down jacket, hat, raincoat or umbrella.
In summer: light clothes, sun hat, a light scarf for the sea breeze, a sweatshirt for the evening; sweatshirt and light jacket for the north and the mountains.