Fianarantsoa sits at around 1,200 metres and has a pleasant temperate climate by Malagasy standards. The dry season (April to October) is the practical time to visit: cool mornings, warm days, and reliable roads. The rainy season (November to March) brings heavy showers in the late afternoon — manageable but the train to Manakara is more likely to be cancelled when the line gets washed out.
Bring layers regardless of season. Highland mornings can be cold enough that fires are lit in the cheaper guesthouses, and the temperature can swing 20°C between dawn and midday.