twenty feet above sea level, the delta is covered with rice fields separated by earthen dikes. During the rainy season the paddies are marshy, making helicopter landings and vehicular troop transport extremely difficult. Hamlets straddle the rivers and canals, and larger villages (up to 10,000 people) and cities lie at tile junctions of the waterways. Bamboo brakes and tropical trees grow around the villages and usually extend from 50 to 300 meters back on either side of the canal or hamlet.
The entire country lies below the Tropic of Cancer, between the 8th and 17th parallels. The climate is generally hot and humid the year round. In winter the country lies under a high pressure system that causes a dry season in the south. In the summer, however, rains fall heavily, varying from torrential downpours to steady mists. The northern region of South Vietnam has the most rain, averaging 128 inches, while the Saigon region averages 80 inches. In the northern region and the Central Highlands, where most of the fighting by U.S. troops during the war occurred, dense fog and low clouds often grounded all aircraft. About ten times a year, usually between July and November, typhoons blow in from the South China Sea, soaking South Vietnam with heavy rains and lashing it with fierce winds.
Although the climate and terrain exacerbated the technical problems of medical evacuation by helicopter in South Vietnam, the air ambulance pilots who worked there worried as much or more about the dangers that stemmed from the enemy's frequent use of guerrilla tactics. The Viet Cong were wily, elusive, and intensely motivated. They usually had no respect for the red crosses on the doors of the air ambulance helicopters. Likely to be annihilated in a large-scale, head-on clash with the immense firepower of American troops', they usually struck only in raids and ambushes of American and South Vietnamese patrols. To perform their missions the air ambulance pilots often had to fly into areas subject to intense enemy small arms fire. Later in the war the pilots encountered more formidable obstacles, such as Russian- and Chinese-made ground-to-air missiles. No air ambulance pilot could depend on a ground commander's assurance that a pickup zone was secure. Mortar and small arms fire often found a zone just as the helicopter touched down. Enemy soldiers were known to patiently hide for hours around an ambushed patrol, looking for the inevitable rescue helicopter.
In these conditions the modern techniques of aeromedical evacuation developed and matured. The obstacles of mountain, jungle, and floodplain could be overcome only by helicopters. The frontless nature of the war also made necessary the helicopter for medical evacuation. Air ambulance units found ever wider employment as the helicopter-used both as a fighting machine and as a transport vehicle-came to dominate many phases of the war.
