Ghadames sits at the edge of the Sahara, and the climate is extreme. Summer temperatures regularly exceed 40°C and can reach 50°C; visiting between June and August is strongly discouraged and potentially dangerous. The old town's covered alleyways and thick mud-brick walls moderate the heat somewhat — the traditional architecture was specifically designed for this climate — but the surrounding desert is brutal in summer.
The best time to visit is between October and March, when daytime temperatures are warm and pleasant (20–30°C) and the nights are cool. Spring (March–April) can bring sandstorms, and winter nights can be surprisingly cold, dropping close to freezing. October and November are particularly good: the date harvest is underway, the light is golden, and the town hosts its annual cultural festival showcasing Tuareg and Berber music, dance, and traditional dress.
Note that Libya's political situation since 2011 has significantly disrupted tourism. Before planning any visit, consult current government travel advisories from your country. The town itself is relatively removed from the main zones of conflict, but access requires travelling through parts of the country where conditions vary. When Ghadames was fully open to tourists, the late autumn months were the most popular and logistically easiest time to visit.