Looking out an airplane window, we’ve all seen the big, bold numbers (and sometimes letters) painted in white on the runways. At Pittsburgh International, you’ll see 28-L or 28-R, among others. Though these numbers may sound random, each designates one of the airport’s four runways.

From pilots to ground operators worldwide, taking off and landing a plane requires intense, coordinated communication, including this standardized system of naming runways.

For Jim Polachek, a retired pilot and current Airport Duty Manager, these numbers are part of an elaborate system that takes into account coordinates of the globe, prevailing winds and other weather conditions to ensure safe takeoffs and landings.

The best place to start is to think of a compass which points to the North Pole at 360 degrees, south at 180, east at 90 and west at 270. However, the direction the plane is traveling is key. On a north-south runway, for example, a plane enters at the south end in order take off at the north end. Thus, the south end of the runway is designated 36 for 360 degrees, the usual setting for true north, since that’s the direction the plane is heading.

Likewise, the north end would be designated 18, for 180 degrees, or true south. East and west flip flop from the traditional as well. That means either end of a runway, regardless of its orientation, is 18 digits apart, since they are 180 degrees from each other.

Three of the four runways at PIT are designated 28-10.

“Twenty-eight represents 281.1 degrees magnetic,” Polachek said. “Since three runways are aligned at the same angle, they are further named left, right and center to distinguish.”

The number is rounded to the nearest tenth of a degree, and the third digit is dropped for simplicity, he added.

Aircraft land in different directions on those runways due to the wind and other conditions at the time. Those decisions, he said, “can involve anticipated air traffic — both aircraft type and volume — historical weather such as prevailing winds, topography, adjacent land usage, land available and funds.”

Once an airport’s runways are set, pilots use maps of each airport, as well as verbal instructions from air traffic controllers, to identify the proper approach and which vectors, or legs, to turn toward for landing.

One factor that has a fundamental effect on runway designations is magnetic variation, or how much the poles deviate from one another, a phenomenon that is also used in navigation for ships and other vessels.

Magnetic variations can change due to the Earth’s core spinning at a different rate than the crust. When this happens, runways are renumbered, according to Kathleen Bergen, communications manager for the Federal Aviation Administration. Airports advise the FAA well in advance of needing to rename a runway, she added.

“The FAA then ensures that the charts and other documentation that pilots and airports use are updated to show the changes,” she said.

Just last year, John Wayne Airport in Orange County, California, had to change its numerical designations because of a magnetic variation.

“Changing the runway names is a costly, extensive process that happens to airports every so often,” said Polachek. “But with that said, magnetic variation has slowed in the recent years, so I do not foresee PIT’s runways being changed anytime soon.”

Oh, and PIT’s fourth runway? That’s 32-14.