Vietnam is located in both a tropical and a temperate zone. It is characterized by strong monsoon influences, but has a considerable amount of sun, a high rate of rainfall, and high humidity. Regions located near the tropics and in the mountainous regions are endowed with a temperate climate.
The monsoon climate also influences to the changes of the tropical humidity. In general, in Vietnam there are two seasons, the cold season occurs from November to April and the hot season from May to October. The difference in temperature between the two seasons in southern is almost unnoticeable, averaging 3ºC. The most noticeable variations are found in the northern where differences of 12ºC have been observed. There are essentially four distinct seasons, which are most evident in the northern provinces(from Hai Van Pass toward to the north like Halong): Spring, Summer, Autumn, and Winter.
Every year there are 100 rainy days and the average rainfall is 1,500 to 2,000mm. The humidity ranges around 80%. The sunny hours are 1,500 to 2,000 and the average solar radiation of 100kcal/cm2 in a year.
Because Vietnam is affected by the monsoon, that why the average temperature is lower than the other countries which are located in the same longitude in Asia. The annual average temperatures range from 22oC to 27oC. In comparing with these countries, the temperature in winter is colder and in summer is less hotter in Vietnam. Under  influence of  monsoon, and further because of the complicated topography, the climate in Vietnam always changes in one year, between the years, or between the  areas   (from North to South and from low to high like Mekong). The climate in Vietnam is also under disadvantage of weather, such as typhoons (advantage there are 6-10 storms and tropical low atmosphere in year, floods and droughts are threaten the life and the agriculture of Vietnam). Vietnam Travel deals
Hanoi has a humid tropical climate, characterised by monsoons, like most of northern Vietnam. Summers, between May and September, are very hot with plenty of rain, while winters, from November to March, are cold and relatively dry. During the transition months of April and October anything is possible, and spring often brings light rain. The hottest month of the year is June. January is the coolest month, usually beset with a cold north-easterly wind.
Ho Chi Minh City is in the tropics, and very close to the sea, so its climate is steadily warm to hot all year round. Temperatures are slightly cooler between December and April, which is also the dry season. Rains begin in May and become heavy from June to August, but the showers are sudden and short, with the sun usually reappearing fairly quickly. There is a danger of typhoons from July to November.