Vietnam has a tropical monsoon climate, dominated b the south or southwesterly monsoon from May to September and the northeast monsoon from October to April. There are marked differences according to altitude and latitude; temperatures in the south remain equable all year round, while the north experiences distinct seasonal variations.
In southern Vietnam the dry season lasts from December to late April or May, and the rains from May through November. Since most rain falls during brief afternoon downpours, monsoon season should not prevent one from traveling during that time. Daytime temperatures in the regions rarely drop below 20 Celsius, occasionally hitting 40 during the hottest months (March, April and May). The climate of the central highlands generally follows the same pattern, though temperatures are cooler, especially at night.
Along the central coast the rainfall pattern reverses under the influence of the northeast monsoon. Around Nha Trang the wet season starts with a flourish in November and continues to December. Further north, near Hue and Da Nang, the rains last a bit longer. If possible it pays to visit these two cities in the spring (February to May), just before the rains break in September or as they begin to fizzle out in November. Temperatures reach their maximum (upper 30’s) from June to August. The northern stretches of this coastal region experience a more extreme climate, with a shorter rainy season (peaking in September and October) and a hot dry summer.
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Northern Vietnam is generally warm and sunny from October to December, after which cold winter weather sets in, accompanied by fine persistent mists which can last for several days. Temperatures begin to rise again in March, building to summer maximums that occasionally reach 40C between May and August, though average temperatures in Hanoi hover around a more reasonable 30C. The northern mountains share the same basic climate, though temperatures are considerably cooler and higher regions see ground frosts, or even a rare snowfall, during the winter (December to February).
Overall, autumn (September to December) and spring (March and April) are probably the most favorable seasons if you’re covering the whole country.