Forest Service smokejumpers throughout the United States have jumped with circular parachute canopies since 1939, although square canopies were developed in the 1960s and '70s and are currently used by smokejumpers who work for the Bureau of Land Management. Square-shaped canopies behave more like a wing, requiring forward speed in order to stay aloft. Circular canopies rely primarily on drag, making it easier for a jumper to drop straight down. There are pros and cons to both designs: While circular canopies can descend at a nearly vertical trajectory, they become difficult to control when the wind blows faster than 10 mph. The BLM's square models, sometimes called "ram-air" chutes because they behave like airfoils, remain maneuverable at twice that speed.