Durham has a classic New England climate: cold winters with significant snowfall, warm humid summers, and spectacular foliage in autumn.
Fall (September–October) is the best time to visit. The academic year is in full swing, the town has energy, and the surrounding forests turn brilliant red and orange in October. Temperatures are comfortable — highs in the 60s Fahrenheit — and the campus looks its best.
Summer (June–August) is quiet. Most students leave and the town drops into a slower gear. Temperatures reach the high 70s and 80s, which is pleasant, and the nearby seacoast beaches at Hampton and Rye are at their best. If you're visiting UNH specifically, check whether summer sessions are running.
Winter (December–March) brings real cold — temperatures regularly dip below freezing, and Durham typically gets 60–70 inches of snow per season. The Whittemore Center arena hosts UNH hockey games, which are well-attended and a good reason to visit in winter if you follow the sport.
Spring (April–May) is mud season in New England — unpredictable weather, wet trails, and the campus still finding its rhythm after winter. The lilac collection on campus peaks in May and is worth timing a visit around if you can.