Tlemcen sits at about 800 metres above sea level on a north-facing hillside, which means the weather is noticeably cooler and pleasanter than coastal Algeria most of the year. Spring (March to May) is the best season: warm but not hot, the surrounding hills green and dusted with wildflowers, and the surrounding plateaus walkable. Autumn (September to early November) is the second-best choice and avoids the worst of the summer crowds.
Summer (June–August) is hot — daytime temperatures regularly into the high thirties — and many local visitors descend on the city at weekends from the coast. Winter is cold for North Africa: snow is not unknown in January, and evenings on the Lalla Setti plateau can be bitter. December and January also bring the city's main religious celebrations around the feast of Sidi Bou Medien, which is worth timing a trip for if you can.