Vietnam - Climate and when to go to Vietnam

Climate
The climate varies greatly from north to south.

Eastern Vietnam has a long coastline on the Gulf of Tonkin and the South China Sea. It has a tropical monsoon type of climate; from May-Sep the south monsoon sets in, and the country is dominated by south to southeasterly winds. From Oct-April, the north monsoon is dominant with northerly to northeasterly winds affecting the country. There is a transition period between each monsoon season when winds are light and variable.
The country is mountainous in the northwest and in the central highlands facing the South China Sea, with peaks reaching up to 8000ft (2450m) In the north around Hanoi and in the south around Ho Chi Minh City, there are extensive low-lying regions in the Red River delta and the Mekong delta respectively.

Vietnam has a single rainy season during the south monsoon (May-Sep). Rainfall is infrequent and light during the remainder of the year. Rainfall is abundant, with annual rainfall exceeding 1000mm almost everywhere. Annual rainfall is even higher in the hills, especially those facing the sea, in the range of 2000-2500mm.

Temperatures are high all year round for southern and central Vietnam; but northern Vietnam has a definite cooler season as the north monsoon occasionally advects cold air in from China. Frost and some snow may occur on the highest mountains in the north for a few days a year. In the southern Vietnam, the lowlands are sheltered from outbreaks of colder northerly air and the dry season is warm to hot with much sunshine.

 
When to go
The north has a cool and dry season from November to April and a hot rainy season from May to October. The central coast north of Nha Trang has a similar climate with the winter monsoon bringing cool, wet weather between December and February.
The south is hot and humid all year round, especially from February to May. The rainy season lasts from May to November. The central highlands has a similar climate to the south, but it is cooler and temperatures can be freezing in winter.
 
The official peak season in Vietnam is from September to April.