Climate

Vietnam has a tropical monsoon climate, with humidity averaging 84 % throughout the year. However, because of differences in latitude and the marked variety of topographical relief, the climate tends to vary considerably from place to place. During the winter or dry season, extending roughly from November to April, the monsoon winds usually blow from the northeast along the China coast and across the Gulf of Tonkin, picking up considerable moisture; consequently the winter season in most parts of the country is dry only by comparison with the rainy or summer season. During the southwesterly summer monsoon, occurring from May to October, the heated air of the Gobi Desert rises, far to the north, inducing moist air to flow inland from the sea and deposit heavy rainfall.

Annual rainfall is substantial in all regions and torrential in some, ranging from 1,200 to 3,000 millimeters (47.2 to 118.1 in). Nearly 90 % of the precipitation occurs during the summer. The average annual temperature is generally higher in the plains than in the mountains and plateaus. Temperatures range from a low of 5 °C (41 °F) in December and January, the coolest months, to more than 37 °C (98.6 °F) in April, the hottest month. Seasonal divisions are more clearly marked in the northern half than in the southern half of the country, where, except in some of the highlands, seasonal temperatures vary only a few degrees, usually in the 21–28 °C (69.8–82.4 °F) range.

Geography

Vietnam is approximately 331,688 km2 (128,065 sq mi) in area. The perimeter of the country running along its international boundaries is 4,639 km2 (1,791 sq mi). The topography consists of hills and densely forested mountains, with level land covering no more than 20%. Mountains account for 40% of the area, with smaller hills accounting for 40% and tropical forests 42%.

The northern part of the country consists mostly of highlands and the Red River Delta. Phan Xi Pang mountain in Sapa, located in Lao Cai province, is the highest mountain in Vietnam at 3,143 m (10,312 ft). The south is divided into coastal lowlands, Annamite Chain peaks, extensive forests, and poor soil. Comprising five relatively flat plateaus of basalt soil, the highlands account for 16% of the country's arable land and 22% of its total forested land.

The delta of the Red River (vietnamese: Song Hong), a flat, triangular region of 15,000 km2 (5,792 sq mi),[37] is smaller but more intensely developed and more densely populated than the Mekong River Delta. Once an inlet of the Gulf of Tonkin, it has been filled in by the enormous alluvial deposits of the rivers over a period of millennia, and it advances one 100 meters (328.1 ft) into the Gulf annually. The Mekong delta, covering about 40,000 km2 (15,444 sq mi), is a low-level plain no more than 3 meters (9.8 ft) above sea level at any point and criss-crossed by a maze of canals and rivers. So much sediment is carried by the Mekong's various branches and tributaries that the delta advances 60 to 80 meters (196.9 to 262.5 ft) into the sea every year.

Because of differences in latitude and the marked variety of topographical relief, the climate tends to vary considerably from place to place. During the winter or dry season, extending roughly from November to April, the monsoon winds usually blow from the northeast along the China coast and across the Gulf of Tonkin, picking up considerable moisture; consequently the winter season in most parts of the country is dry only by comparison with the rainy or summer season.

The average annual temperature is generally higher in the plains than in the mountains and plateaus and in the south than in the north. Temperatures in the southern plains (Ho Chi Minh City and the Mekong Delta) varies less, going between 21 and 28 °C (69.8 and 82.4 °F) over the course of a year. The seasons in the mountains and plateaus and in the north are much more dramatic, and temperatures may vary from 5 °C (41 °F) in December and January to 37 °C (98.6 °F) in July and August.