Vietnam has a monsoon-influenced climate typical of that of mainland Southeast Asia. The diverse topography, long latitude (Vietnam spans over 15° of latitude), and influences from the South China Sea (locally known as the East Sea) lead to climatic conditions varying significantly between regions of Vietnam. The northern region experiences a monsoonal and humid subtropical climate with four distinct seasons (spring, summer, autumn, and winter) with winters typically dry and summers ranging from hot to mild. In southern and central areas, the climate is tropical monsoon with only two seasons (rainy and dry). In addition, a temperate climate exists in mountainous areas, which are found in Sa Pa and Da Lat, while a more continental climate exists in Lai Châu province and Sơn La province.

According to Indra Overland (2017), 20% of Vietnam's total surface area is low-elevation coastal area, making the country highly vulnerable to climate change effects and the rising sea levels in particular. From 1960 to 2011, high increases of 0.24 °C and 0.28 °C per decade are found at Vung Tau and Ca Mau stations, respectively, located in the South Coast. Most of the stations in the North - Central Coast show an increase of 0.15 °C to 0.19 °C per decade.Schmidt-Thomé, Philipp; Nguyen, Thi Ha; Pham, Thanh Long. Climate Change Adaptation Measures in Vietnam: Development and Implementation. Springer International Publishing, 2015. From 2007 to 2008, the flooding in the central provinces exceeded that of past 48 years; the northern part of Vietnam encountered an unprecedented cold wave, lasting for 38 days, resulting in 30 million US dollars Crop and livestock losses.Economics of Adaptation to Climate Change: 70272 Vietnam. Washington, DC. The World Bank Group. 2010.

Atmospheric circulation The atmospheric circulation influencing Vietnam is part of the Southeast Asian monsoon circulation that is characterized by 3 distinct features: Being, not only closely associated with the South Asian monsoon, especially in summer, but is also strongly influenced by the East Asian monsoon, especially in winter. In addition to being influenced by tropical, subtropical and temperate circulations from the Northern Hemisphere, the atmospheric circulation influencing Vietnam are closely associated with subtropical and tropical circulations from the Southern Hemisphere. Vietnam's climate is strongly influenced by its location to the adjacent sea in all seasons.

The two permanent atmospheric pressures that influence the atmospheric circulation of Vietnam are the equatorial low pressure, and the subtropical high pressure. Seasonal pressure centres that influence Vietnam include the Asian continental high pressure, the Aleutian Low, South Asian continental low-pressure centres, and oceanic continental high pressure centres. Across East Asia, the polar front moves southwards in winter, reaching down to 8°N in January as the southern limit while the northern limit of it is 25–27°N in July. Because all of Vietnam lies between the southern and northern limit of the polar front, Vietnam's climate are both influenced by polar air and tropical air (from the tropical convergent zone). In Vietnam, the monsoon circulation is a combination of both the South Asian and Northeast Asian monsoon systems. This leads to four distinct seasons of which Winter (November–October) and Summer (April–September) are the major ones while Spring (April) and Fall (October) are transitional ones.

Winter usually lasts from November until March. During winter, polar air originating from the Siberian High penetrate deeply into the low latitudes, facilitated by the eastern Tibetan Plateau that funnels the air southwards in a northeast direction (the cool air is a wind coming from the northeast). At the same time, a low pressure system over Australia strengthens that generates pressure gradients that intensify cold northeasterly winds. Many cold fronts can penetrate into Vietnam during winter of which there are 3-4 occurrences every month in northern Vietnam. This leads to cold temperatures where temperatures drop by . Cold weather, occasionally extreme cold can persist for a long time, being characterized by a long stretch of cloudless or partly cloudy days in the first half of winter or a long stretch of cloudy and drizzly conditions in the latter half of winter. Cold weather occurs more frequently in the north than in the south due to cold fronts penetrating the north more frequently.

The rainy season starts in late April/early May and lasts until October. In summer, the general wind pattern are southwesterly winds in the southern parts of Vietnam and southeasterly winds in northern Vietnam. The predominant air blocks in Vietnam are the equatorial and tropical blocks that originate from high pressure systems in the Southern Hemisphere, and a maritime tropical block originating from the subtropical high pressure system in the Pacific Ocean (Pacific subtropical high pressure). In addition, during summer, Vietnam is influenced by tropical air from the Bay of Bengal which occurs when a continental low pressure originating from South Asia (South Asian continental low) moves eastwards towards Vietnam, covering almost all of Vietnam and southern China; this causes hot, dry weather in the North Central Coast as westerly winds descend and warm adiabatically on the eastern slopes of the Annamite Range (Truong Son range). On average, 11 storms and tropical low pressures develop in the South China Sea during summer of which half are tropical cyclones that originate from the western Pacific. These storms and cyclones then move westwards towards Vietnam. On average, Vietnam is affected by 6–8 typhoons or tropical cyclones per year.

Spring and Fall are transitional seasons. The atmospheric circulation in these seasons represent a transition between winter–summer and summer–winter respectively. In general, the northern parts of the country have four seasons: winter, spring, summer, and fall (autumn). In the south, only two seasons are present: a dry and a wet season.

Temperature Mean annual temperatures in the country, based on meteorological data from weather stations range from in Hoàng Liên Sơn. At the highest altitudes in the Hoang Lien Son range, mean annual temperatures is only . As temperatures vary by altitude, temperatures decrease by for every increase in altitude. The lowest mean annual temperatures are found in the mountainous areas, where the altitude is higher, and in northern areas, because of their higher latitudes. Because Vietnam is strongly influenced by the monsoon, the mean temperatures in Vietnam are lower than other countries located at the same latitude in Asia.

In winter, mean temperatures range from , which decreases from south to north, and/or as one climbs up its mountains, and vice versa. In the coolest month, mean temperatures range from in the northern highlands to in the southern highlands. Generally, mean winter temperatures are below in many northern locations. In addition to lower insolation in winter, the Northeast Monsoon contributes to colder conditions. Many mountainous areas in the north have experienced subzero conditions. In contrast, temperatures in the Spratly Islands never falls below . In summer, mean temperatures vary between . The highest temperatures normally occurs in March–May in the south and May–July in the north. This is due to the fact that in the north, drizzle predominates leading to a slight temperature increases in February and March before increasing from April–August while in the south, the temperature increase (from December–February/March) is much larger. Consequentially, the south reaches their highest temperatures in late winter while in the north, they occur in July and August due to this. Temperatures in summer are relatively equal among the northern and southern parts of the country with differences being mostly due to altitude (the decrease in temperature is predominantly due to altitude).

In May 2023, the country experienced an unprecedented heatwave where sections of the country registered temperatures over 40 °C. One recording saw the weather reach a record high 44.1 °C at the Hoi Xuan station, breaking the 43.4 °C record set in 2019 according to the National Centre for Hydro Meteorological Forecasting.

Precipitation

Mean annual rainfall in the country ranges from although most places in Vietnam receive between . Bạch Mã National Park is the wettest place in Vietnam, which annual precipitation is and up to at the 1,448-metres tall summit. Two cities Phan Rang and Phan Rí receive the least amount of rainfall, with and each year, respectively. The majority of rainfall occurs during the rainy season, which is responsible for 80%–90% of the annual precipitation. Generally, northern parts of the country receive more rainfall than southern parts of the country. Islands located in the north general receive less rainfall than the adjacent mainland while in the south, this is reversed where islands such as Phú Quốc receive more rainfall than the adjacent mainland.

The annual average number of rainy days ranges from 60 to 200 in which most days have rainfall averaging less than . The amount of rainy days in a month usually corresponds with the mean monthly precipitation although in the north and north central coast, drizzle is common in winter (despite being the drier season), leading to higher amounts of rain days. For example, there are more rainy days in the drier season during winter in Yên Bái Province due to drizzle than there are rainy days in the main rainy season. Drizzle is a weather phenomenon that is characteristic of the weather in winter in the north and north central coast. Days with thunderstorms occur 20–80 days per year, which are more common in the south and north, and more common in mountainous areas than coastal delta. Thunderstorms can occur year-round although they are the most common during the rainy season. In the highest peaks in the north in Sa Pa, Tam Dao, and Hoang Lien Son, snowfall can occur.

Depending on the region, the onset of the rainy season (defined as when the monthly average precipitation exceeds ) differs: In the North West and North East, the rainy season beings in April–May with a peak in July–August and ends in September and October. In the Red River Delta (North Delta), the rainy season beings in April–May with a peak in July–August and ends in October and November. On the North Central Coast, the rainy season begins in August and September, reaching a peak in October and November before ending in November and December. For the South Central Coast, the rainy season begins in August and September, reaching a peak in October and November before ending in December. In the Central Highlands, the rainy season begins in April and May that peaks in August before ending in October and November. Finally, the South has its rainy season beginning in May.

Regional climate

Based on geographic and climatic conditions, there are 7 different climatic regions in Vietnam: Northwest (Region 1), Northeast (Region 2), North Delta (Red River Delta/Region 3), North Central (North Central Coast/Region 4), South Central (South Central Coast/region 5), Central Highlands (region 6), and the South (region 7). These seven regions are widely accepted within the Vietnamese climatological community. Generally, they are grouped into two macroregions: the Septentrional (regions 1–4) and the Meridional (regions 5–7), separated by the Bạch Mã Range, approximately at the 16.1st parallel. These climatic regions are based on time of rainy season and other climatic elements such as insolation, sunshine, temperature, precipitation, and humidity. The diverse topography, wide range of latitudes (Vietnam spans over 15° of latitude), and influences from the South China Sea lead to climatic conditions varying significantly between regions.

Northwest