The north of Vietnam, including Hanoi, Halong Bay, Cuc Phuong, Mai Chau, Ninh Binh, Sa Pa, Ha Giang and Bac Ha, is a beautiful hilly and mountainous area that gets rainy season during spring through mid-autumn. The rainy season brings lots of wet weather and though trekking is possible all year round, it might hinder a bit your experience. The wettest months are July to September. In these months it can sometimes rain for several days in a row.
Heavy downpours usually occur in the afternoons, and low-lying areas often get flooded. Summers in Northern Vietnam are hot and humid, but it rains less often as you move closer to the center of the country. The central part stays drier due to the mountains.