The Hai Van Pass, the ridge that divides Vietnam's weather

Vietnam is long and thin, an S-shape stretched along the coast of the South China Sea for well over a thousand kilometres. For much of its length, the Annamite mountains run parallel to the coast, a few dozen kilometres inland. In one place, between the old imperial capital of Hue and the port city of Da Nang, a spur of those mountains turns east and runs straight into the sea. The road between the two cities has no choice but to climb over it. This is the Hai Van Pass, whose name means "sea cloud pass", after the clouds that so often sit on its summit while the sea glitters below.

A frontier for centuries

The ridge has long been a natural barrier. For centuries it marked roughly the border between the Vietnamese kingdom of Dai Viet to the north and the kingdom of Champa to the south, a Hindu-influenced civilisation whose red-brick temples still stand in central Vietnam. As the Vietnamese pushed south, the frontier moved past it, but the pass kept its military importance. A fortified gate at the summit was built under the Nguyen emperors in the early nineteenth century. The French and later the Americans and South Vietnamese built bunkers on the ridge, some of which still stand, overgrown and crumbling, beside the old gate.

The ridge that divides the weather

The pass also marks a real climatic boundary. In winter, cold, damp winds blow down from the north-east across northern Vietnam, bringing grey skies, drizzle and surprisingly low temperatures to Hanoi. The Hai Van ridge blocks much of that air. South of it, winters are generally warmer, so the pass divides the cooler north from the warm south.

The rainfall, too, differs on either side. Hue, just north of the pass, is one of the wettest cities in Vietnam, with heavy rains in the autumn and early winter, and its Perfume River can flood. Central Vietnam as a whole is also in the path of typhoons from about September to November. So the most reliable time to visit the stretch around the pass, Hue, Da Nang and Hoi An, is from about February to August, when it is drier and, in the later months, very hot.

The road over the top

Since a long road tunnel opened under the mountain in 2005, most trucks and buses avoid the pass entirely, and the old road has become a scenic route. It winds up for about twenty kilometres in a series of curves, with views back over the curving bay of Lang Co and its lagoon to the north, and down over the wide sweep of Da Nang Bay to the south. On a clear day it is one of the finest coastal drives in South-east Asia. On a cloudy day you climb into the cloud and come out the other side into a different weather. A 2008 British television motoring special showed it to an international audience, and it is now popular with motorcyclists and cyclists.

Da Nang and the tours

South of the pass, Da Nang has grown into one of Vietnam's largest cities, with a long beach, My Khe, which was a rest-and-recreation beach for American soldiers during the war. The Marble Mountains nearby are a cluster of limestone and marble hills riddled with caves and Buddhist shrines. The Cham Museum holds the world's largest collection of Cham sculpture, carved sandstone gods, dancers and animals from the temples of Champa.

Explore Worldwide's Inside Vietnam, thirteen nights, includes the pass, which it lists as the Ocean Cloud Pass. Its other places include Hanoi, Cat Ba Island and Lan Ha Bay, Hue, the Cham Museum, My Khe Beach, the Marble Mountains, Hoi An and My Son, the Mekong Delta and Tam Hiep Island.

Explore Worldwide's Inside Vietnam and Cambodia Highlights, nineteen nights, includes the Hai Van Pass on the same route through Vietnam, as well as Da Nang and Ho Chi Minh City, and adds Cambodia, as its name says.

Few places in the world mark the line between two climates so clearly. Standing at the summit, with the clouds on one side and the sun on the other, it is easy to see why.