Vietnam is stretching length and diverse topography allows it to span several different climate zones. The climate of Vietnam varies considerably from region to region, from the sub-tropics in the North to the tropics in the South. The weather varies greatly from North to South, depending on elevation and type of landscape.
The humidity is high, with average ranging around 84% a year, the sunny hours are 1,500 to 2,000 a year. There are about 120 rainy days a year and the average rainfall is 1,500 to 2,000mm.
Moderated by two monsoons, the southwest monsoon from April to September and the northeast monsoon from October to late March or early April. There is a transition period between each monsoon season when winds are light and variable. During the north monsoon, northern Vietnam has cloudy days with occasional light rain, while southern Vietnam tends to be dry and sunny. The average temperature (from 22 degrees to 27 degrees) is lower than other countries also in the equatorial regions.
People often equate monsoons with rain, but that is only partly the case in Vietnam, as a number of regional variations affect the rain. What is worth remembering though is the southwest monsoon is warm and wet and the northeast monsoon cool and not so wet.
The climate in Vietnam is also under disadvantage of weather, such as typhoons. From July to November, violent and unpredictable typhoons hit central and northern Vietnam, which can dampen the spirits of even the most enthusiastic traveller (advantage there are about 10 storms and tropical low atmosphere in year, floods and droughts are threaten the life and the agriculture of Vietnam).
As far as planning goes, the easiest way to get your head around it is to break up the country into three areas — the north, the centre and the south.
Northern Vietnam (from Hue up North) has four seasons with a cold winter and hot summer. The temperature ranges from 5-6C in winter to up to 35C in summer. The further North it is, the colder it gets. In mountainous regions in the far North (such as Sapa) the temperature some times drops below zero. Rainy months are August, October, November. February and March have persistent, light, drizzling rain.
Temperatures are high all year round. The temperature tends to be warmer and more humid than the South in the middle of the year and cooler during the dry season from November to April. Flooding may occur from October to December.
Southern Vietnam has a distinct dry season (from November to April) and wet season (from May to October). Average temperature ranges from 25-30C. During wet months, heavy and short downpours may occur in mid-afternoons.