Winter lasts from November to April in northern Vietnam, says Bamboo Travel, with “temperatures averaging 16C, little heavy rainfall but a fair bit of drizzle.” Summer begins in May and lasts until October, with an average temperature of 30C, heavy rainfall and the occasional typhoon. “During the summer months northern Vietnam is generally hot and muggy, but in the winter, it can get downright cold -- especially in Halong Bay and most definitely up in the mountains around Sapa,”explains Smiling Albino, so “dress warmly if you’re coming then.”