Although the United States' "shock and awe" bombing campaign seems to have subsided, anti-aircraft fire still flashes across the Baghdad skyline. Why are the rounds fired by Iraqi troops visible to the naked eye?
The glowing rounds seen on television are called tracers, which leave a luminescent trail to help Iraqi gunners aim. For Iraq's anti-aircraft weapons of choice, the U.S. Army says that about one out of every four rounds is commonly a tracer round. The last few rounds in a belt or magazine will occasionally be all tracers, to notify the shooter that he is running out of ammo.
The base of a tracer round contains a small cavity filled with a phosphoric compound. When someone pulls the trigger, it both fires the round and ignites the compound. Even though the tracer is moving at near-supersonic speeds, the burning phosphorus leaves enough of a streak in our fields of vision so we can follow its path. It's the same phenomenon that allows us to observe shooting stars.
Tracers made their first appearance in World War I, but the phosphoric compound was actually a drag on the bullet. The problem was fixed by World War II, when tracers were used by both the Axis and Allied powers. Throughout the Cold War, aluminum oxide or other elements were added to the phosphoric compound to make the tracers appear as different colors—the idea was to help soldiers decipher who their friends were while hunting in the dark. In Vietnam, North Vietnamese Army and Viet Cong tracers were designed to leave green trails while American and South Vietnamese tracers were designed to leave red ones. The New York Times reports that the color scheme is similar in Gulf War II: Americans, red; Iraqis, green.
While Iraqi troops may fill the sky with ack-ack, these days the U.S. Army rarely employs tracers to shoot down enemy planes. Most of its surface-to-air weaponry now consists of missiles, such as shoulder-launched Stingers or the increasingly famous Patriot.
Explainer thanks Major Amy Hannah and Jeff Brooks of the Department of the Army.