Trujillo sits in a coastal desert and benefits from the "garúa" — a persistent coastal mist that keeps temperatures mild year-round. Unlike Lima to the south, Trujillo sees actual sunshine through much of the year, making it a pleasant place to visit at any time.
The warmest and sunniest months are December to April, when temperatures reach 25-27°C and the beaches at Huanchaco are at their best. The cooler months (June to October) see more mist and grey skies, but temperatures remain comfortable (18-22°C) and the archaeological sites are perfectly good to visit.
The El Niño phenomenon, which cycles irregularly every few years, can bring heavy rains and flash flooding to this coastal zone. When a strong El Niño is forecast, adobe ruins like Chan Chan are at real risk of damage from water.