April, May and October are the best months, with afternoon highs of about 20 to 25 degrees. July and August are the hottest, with highs above 34 degrees, and the stone lanes give little shade. Winter is mild by day, but nights fall to between 1 and 3 degrees in January and February.
Highs rise from about 18 degrees in March to 25 in May, with five or six rain days a month. The lanes are comfortable at midday and the views are clear.
What to wear: Bring layers and comfortable shoes, since mornings are cool and the cobbles are uneven.
Summer
18 to 34 °C · about 2 rainy days a month
Afternoons reach 31 to 35 degrees from June to August, with almost no rain. Plan monuments for the morning and rest indoors in the early afternoon.
What to wear: Wear light clothes, a hat and walking shoes with grip, and carry water.
Autumn
10 to 23 °C · about 5 rainy days a month
September is still hot, near 29 degrees, while October settles near 23 and November near 16. October and November have around six rain days a month.
What to wear: Bring a light jacket for the evenings and something waterproof for November.
Winter
2 to 13 °C · about 5 rainy days a month
Days are around 12 to 14 degrees and nights drop close to freezing. Most monuments close early, between 17:00 and 18:30, so the day is short.
What to wear: Wear a warm coat, a scarf and shoes with grip, because wet cobbles get slippery.
If it rains
Rain makes the steep lanes slick, so slow down on the slopes. The Cathedral, the Army Museum, the Museo del Greco, the Sephardic Museum and the churches are all indoors, which makes a wet day easy to fill. Save the Tagus viewpoints for a dry afternoon.
Summer afternoons are hot and the old town has little shade, so keep outdoor walking for the morning and the hours after 18:00. Churches and museums work as cool breaks in the middle of the day. Carry water and wear a hat.