Boulder has over 300 days of sunshine per year and a relatively dry climate. Summers (June-August) are warm and sunny with afternoon thunderstorms; temperatures typically reach the mid-30s°C but drop pleasantly in the evenings. This is the busiest season, particularly on the trails.
Spring (April-May) and autumn (September-October) are excellent for hiking — mild temperatures, thinner crowds, and beautiful light on the Flatirons. The aspens in the mountains turn gold in late September to early October, one of the best natural displays in Colorado.
Winter brings snow and cold (-5 to 10°C), but Boulder typically sees good stretches of sunny winter days. The ski resorts nearby are in full operation from December through March or April.
Avoid visiting during University of Colorado home football games (autumn) if you want affordable accommodation — the city fills completely.