The visible length of the streak of lightning depends on the terrain and can vary greatly. In mountainous areas where clouds are low, the flash can be as short as 300 yards (273 meters); whereas in flat terrain, where clouds are high, the bolt can measure as long as 4 miles (6.5 kilometers). The usual length is about 1 mile (1.6 kilometers), but streaks of lightning up to 20 miles (32 kilometers) have been recorded. The stroke channel is very narrow—perhaps as little as half an inch (1.27 centimeters). It is surrounded by a “corona envelope” or a glowing discharge that can be as wide as 10 to 20 feet (3 to 6 meters) in diameter. The speed of lightning can vary from 100 to 1,000 miles (161 to 1,610 kilometers) per second for the downward leader track; the return stroke is 87,000 miles (140,070 kilometers) per second (almost half the speed of light).