On the eastern side of a single mountain range, annual rainfall can exceed 3,800 millimeters. Just a few dozen kilometers to the west, beyond the same ridgeline, rainfall drops dramatically.
The dividing line is neither an ocean nor a great distance. It is a narrow mountain chain stretching from north to south: the Annam Mountains, known to researchers as the Annam Cordillera.
According to a 2007 study by Haiming Xu and colleagues, published in Meteorology and Atmospheric Physics, the Annam Cordillera runs along the Viet Nam-Laos border. The research was conducted by scientists from the International Pacific Research Center at the University of Hawaii and Nanjing University of Information Science and Technology.
The mountain range ends just north of Ho Chi Minh City. It has an average elevation of more than 500 meters, while remaining less than 200 kilometers wide.
One Mountain Range, Two Contrasting Climates
According to the study, when moisture-laden southwest monsoon winds strike the mountain range, the air is forced upward on the windward side. This process triggers cloud formation and heavy rainfall. On the opposite side, the air descends and suppresses cloud development.
This phenomenon is known as the rain shadow effect.
"The rising motion creates a windward rain band and the subsequent subsidence produces a rain shadow on the lee," the researchers wrote.
Viet Nam's side, which faces the South China Sea, receives abundant moisture carried inland from the sea. As a result, it forms the wetter side of the mountain range, with annual rainfall exceeding 3,800 millimeters, according to the One Earth ecoregion database affiliated with WWF.
Laos, on the leeward side of the mountains, loses much of that moisture before the air reaches its territory. As a result, it is considerably drier and experiences a shorter rainy season than its eastern neighbor.
The climatic contrast is also visible on the ground. Viet Nam's side is covered by dense tropical rainforest throughout the year. Across the mountains into Laos, the vegetation gradually becomes more open and seasonal, reflecting a much more limited water supply.
The Mountain Range That Even Shapes Ocean Currents
The influence of the Annam Cordillera does not end on land.
The same study found that the mountain range generates a strong low-level wind jet off the southern coast of Viet Nam. The jet forms as monsoon winds accelerate around the southern tip of the mountains.
This wind jet cools sea surface temperatures over the South China Sea during midsummer. It also helps shape the region's characteristic double-gyre ocean circulation pattern.
The Same Pattern, Repeated Elsewhere
It is important to note that the Annam Cordillera extends only along the Viet Nam-Laos border. It does not directly reach Thailand.
However, Xu and his colleagues found that rain shadow patterns like this are not unique to the Annam Mountains.
"Such orographic rain bands are also observed on the west coasts of Myanmar, Thailand, Cambodia, and the Philippines," they wrote, referring to other mountain ranges across the region that produce similar rainfall patterns along their respective coastlines.
The broader impact is even more evident inland.
The dry zone on the leeward side of the Annam Cordillera merges climatically with the Khorat Plateau, which stretches from southern Laos into northeastern Thailand (Isan). The plateau forms a transition zone between the wetter highlands to the north and the much drier plains farther south.
In other words, a single narrow mountain range along the border of two countries helps determine how far moist air can penetrate into the interior of mainland Southeast Asia. Its influence extends well beyond the mountains themselves.
