By Daniel Silliman
The storms don't all come in exactly the same.
The wind picks up and the rain starts to fall, but there's an element of unpredictability, forcing the air traffic controllers in the tower at Hartsfield-Jackson Atlanta International Airport to deal with a storm, moment by moment.
"A storm four or five miles away changes the wind on the surface [of the runway] drastically," said Marshall Mowery, manager of system operations for air traffic control. "That changes everything for us at the airport, because they're going to land and take off into the wind, and if that wind shifts around, so a head wind becomes a tail wind for a short time, well, then the aircraft are limited ... the wind is the biggest issue for us."
According to Federal Aviation Administration and Hartsfield-Jackson officials, weather is the No.1 cause of flight delays at the airport. Though the addition of the fifth runway in 2006, has caused the number of delays to plummet, air traffic controllers still find themselves fighting weather, such as the sever thunderstorms on Saturday, March 15.
On Friday night, Mowery said, the storms that struck the city were relatively small and didn't arrive until after the evening rush of landings, so even though hail pounded parts of Clayton and Henry counties, only a few flights were delayed.
On Saturday, though, there was a "whole train" of thunderstorms, which filled up the airport's air space and left the air traffic controllers delaying more than 700 flights, according to Mowery.
"Pilots won't take their airplanes through a thunderstorm," he said. "So it changes what our normal flight patterns are, into and out of the airport. When we change those patterns just slightly, it has a ripple effect."
Often, the severity of the storm is not as significant to air traffic as the location. On a clear day, the tower directs 126 airplanes per hour to land, and another 126 airplanes per hour to take off. Those airplanes land and take off from four directions, from the four corners of the airport.
If a storm, even a small storm, moves through a "fix," one of the 10-mile wide, 40-mile long airspace lanes, then air traffic space is occupied by the storm and the traffic patterns are disrupted.
When we get weather that's moving in and through some of those 'fixes,' it really constrains us on whether we can move airplanes. We have to decide to hold them or reroute them around to where we're not having to deal with the weather," Mowery said.
The air traffic control tower receives specific forecasts for those airspace lanes, as the FAA's Hampton based meteorologists analyze Doppler radar images.
Constrained by space in the air, the airplanes are also constrained by space on the ground, Mowery said. At northern airports, there are places dedicated to the elaborate process of de-icing airplanes. But at a southern airport like Atlanta, where there are only a few de-icing incidents every winter, that parking space just isn't available.
"If we're only departing 40 airplanes an hour, and we have conditions where we could still land 120 an hour," Mowery said, "there really is not any space for us to put those other planes ... the weather really isn't what's causing us to have delays, it's the constraints of available space."