I think the best season to víit Hanoi is spring. Lasting from February to April, Hanoi’s spring is humid and wet with drizzles. The average temperature normally ranges from 15 to 20 Celcius degree. As this kind of weather is suitable for the plants to grow and flowers to blossom, spring will be the best time for tourists to witness the vitality of tropical nature. Spring is also the time for Tet holiday- the most essential and longest Vietnamese festival of the year.
Autumn (around September to November) and Spring (March and April) are two best seasons to visit Hanoi when the weather is pleasant with milder temperatures.In winter, Hanoi is quite cold when the temperatures can fall below 10ºC (50ºF) and getting hotter in the summer averaging around 28-35ºC (82º-95ºF). Hanoi has high humidity year-round.The rainy season in Hanoi is May to September. Heavy rains may happen sometimes in the summer, however, it is not consistent and does not affect your travel.