All this week, travelers between Tucson and Phoenix may want to watch the skies -- particularly what’s falling out of them.

Over 600 competitors have gathered in Arizona to compete in the U.S. Parachute Association National Skydiving Championships, the largest annual skydiving competition in the world.

Held over 10 days at Skydive Arizona, the championships feature athletes vying for gold, silver and bronze medals in five disciplines: Formation Skydiving, Artistic Events (including Freestyle and Freeflying), Freefall Style and Accuracy Landing, Canopy Formation and Vertical Formation Skydiving.

U.S. team members will then go on to compete at the World Parachuting Championships in Dubai at the end of November, where 5,000 to 10,000 spectators are expected each day.

The sport of parachuting, according to U.S. Parachute Association Director of Competition Jim Hayhurst, dates back to the 1950s.

The first U.S. team was chosen in 1958, focusing on the two classic (and at the time, only) disciplines: Style and Accuracy.

Style, which Hayhurst jokingly refers to as “dizzy, dizzy, oh my god," involves an individual acrobatic sequence in free fall within a set amount of time.

The most recent discipline to arise, Vertical Formation Skydiving, began in the 1990s.

Around 80 percent of competitors are male and close to 85 percent are citizen athletes, i.e. competitors who have full-time jobs that they juggle while finding time for parachuting.

Hayhurst says he jumps on various days during the week or weekend at a facility about an hour from his home in western Pennsylvania while also balancing work, family life and swimming and weight workouts at his local Y.

For most competitors, practice isn’t cheap -- one jump can cost anywhere from $12 to $25, which adds up if an athlete is jumping four to five times a day, four times a week.

Some luckier teams live and work at drop zones, such as the four-way team Arizona Airspeed, which has made more than 800 jumps this year alone. Hayhurst says that Arizona Airspeed, which is competing at the World Championships, trains six to seven hours a day.

“Their challenge is finding time when not jumping to actually get out to the Y and work out,” Hayhurst says.

That’s because skydivers are also a very fit group. Hayhurst cites one of the best freestyle competitors, Tiffany Lamb, who’s just under 5 feet tall and “95 pounds soaking wet,” a frame that Hayhurst likens to a gymnast. Typical skydivers are between 140 and 180 pounds -- lean, fit and strong.

Cheryl Stearns holds the record for most jumps by a U.S. female. At just over 19,000 jumps and counting, the 5-foot-6, 127-pound captain with U.S. Airways will look to defend her national championship title in the Classics event in Arizona.

Stearns began jumping in high school and continued her love of flight while serving in the army for 28 years. She was the first woman on an army parachute team; following her retirement from the military, she kept jumping. At age 57, she shows no signs of stopping, even as she’s had to adapt with the sport’s evolution.

“I still love to jump -- it’s challenging and I love competing,” Stearns says. “I’m a pilot, so I get to use all my skills, from flying airplanes to flying my canopy like a glider to being a gymnast in the air, staying on a balance beam going 180 mph. It doesn’t get boring.”

Stearns also enjoys the individual element of Classics since most of the other disciplines are a team effort, including a cameraman jumper. For those events, much of the scoring ultimately depends on how well the free-fall camera flier captures and presents the performance.

Teams like veteran national champions Arizona Arsenal will compete in Vertical Formation Skydiving, a type of freeflying that involves four competitors building a series of formations in a mix of upright and head-down vertical body positions.

And while there is a potential for injuries, particularly in a sport where athletes are moving anywhere from 120 to 200 mph, Hayhurst says that skydiving is surprisingly safe.

“Most of us have broken a wrist or sprained an ankle on an exit or a bad landing,” Hayhurst says. “But our sport is no more injury-laden than riding a bicycle. Good athletes know that being safety-conscious and injury-free is critical.”

Perhaps some reassurance for those watching from below.

For a pictorial look at the championships, check out our accompanying Visuals post.

And follow all the action of the competition at the USPA site.