Vietnam is a country of Southeast Asia, rather larger than Britain and about half the size of the state of Texas. Extending between 9° and 23°N, it lies entirely within the tropics. It has a long coastline on the Gulf of Tongking and the South China Sea.
On the landward side it borders China in the north and Laos and Cambodia in the west. Like other countries of the region, it has a tropical monsoon type of climate dominated by south to southeasterly winds from May until September and northerly to northeasterly winds between October and April. There is a twice-yearly period of variable winds at the time of transition from the north to south monsoon.
There are considerable areas of high land rising to over 2,450 m/8,000 ft, particularly in the northwest and in the central highlands facing the South China Sea. In the north around Hanoi and in the south around Ho Chi Minh City (formerly Saigon) there are extensive low-lying regions in the Red River delta and the Mekong delta respectively.
These two lowlands contain a large proportion of the population and the productive rice-growing areas.
The general features of the climate of Vietnam also apply to the two adjoining countries, Laos and Cambodia; all three countries are often grouped together under the name Indochina. Over most of Indochina there is a single rainy season at the time of the south monsoon between May and September.
During the rest of the year rainfall is infrequent and light. Annual rainfall is almost everywhere above 1,000 mm/40 in, and rises to between 2,000 mm/80 in and 2,500 mm/100 in on the hills, particularly those facing the sea.
On the coast and in those parts of the central highlands which face northeast the season of maximum rainfall is between September and January (see the table for Da Nang). This area often receives heavy rain from typhoons, or severe tropical storms, which develop in the western Pacific at this time of year.
This is also a time of much cloud and frequent drizzle (called locally the crachin). In the north of Vietnam there are more cloudy days with occasional light rain during the period of the northeast monsoon. The south of the country is more likely to be dry and sunny at this time. Compare the tables for Hanoi and Ho Chi Minh City.
In the southern and central parts of Indochina, temperatures remain high around the year, but in the north there is a definite cooler season as the north monsoon brings colder air from central China from time to time.
Frost and occasional snow only occur on the highest mountains in the north for a few days a year. In the south of Vietnam, and in Cambodia and Laos, the lowlands are sheltered from any such outbreaks of colder northerly air and the dry season is warm to hot with much sunshine.
The weather of Vietnam, Laos, and Cambodia is rather sultry and oppressive during the rainy season and the humidity is high at this time.
On the coast and in the hills the frequent cloud and high humidity combine with lack of sunshine to make this time of the year rather unpleasant in spite of the reduction of temperatures with height.
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