Last updated: 10:54 AM ET, Mon August 08 2016

What Really Causes Weather Delays?

Airlines & Airports | Paul Thompson | August 08, 2016

What Really Causes Weather Delays?

Photo by Paul Thompson

DELAYED. Seeing that word on the departure board is every passenger’s dread. That single word can make or break your day, or even your vacation. Aircraft maintenance delays are easy to understand, but what about weather? Let’s look at what really happens during a weather delay.

Weather delays can be difficult to understand, especially if it’s sunny at your local airport. But local weather is far from the only factor. Weather along the way and weather at the origin or destination are also factors. Today, we’ll pretend we want to fly from New York JFK to Denver, and examine things that may delay our flight.

Lightning at any airport can delay flights. It’s safe for planes to fly in lighting, and they get struck all the time. But for the people working outside, that’s a different story. They’re required to seek shelter if there is lightning within three miles of the airport. If they’re unable to load the plane, the plane can’t depart.

It might surprise you to hear that rain doesn’t matter much. Planes can land in moderate to heavy rain, even when there is low visibility, thanks to modern guidance systems for the pilots. As long as most of the water is draining off the runway as it should, planes can land safely. Air Traffic Controllers sometimes decide to increase the separation distance between arriving and departing planes, which can lead to delays. Planes usually have to be at least three miles apart, to avoid a phenomenon known as wake vortex turbulence. But during bad weather or low visibility situations, that distance is often increased, which limits the rate at which planes may come and go.

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The most important and dangerous factor contributing to weather delays is wind. On a good day, JFK airport has four runways available for arrivals and departures, and Denver has six. Each airport has runways that run roughly north-south and east-west. Here in Denver, all six runways are usually in use. But on any given summer afternoon, thunderstorms pop up over the Rockies and find their way east, to or near the airport. When a strong wind comes in from those storms to the west, the airport will shut down the four north-south runways due to the potentially dangerous crosswinds.

A crosswind is when the prevailing wind direction blows across a runway. When planes take off and land, they usually fly into the wind, which creates more lift on the wings. Cutting the operation in Denver down to two runways greatly limits the number of planes that can arrive and depart, which contributes to delays. In addition, when the wind direction changes spontaneously, planes that were planning to arrive on a certain runway now have to be rearranged by Air Traffic Control to land on one of the available runways. ATC will often put planes into a holding pattern while they sort out the new arrivals sequence.

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Flying around in a holding pattern obviously causes arriving flights to be delayed. If your flight becomes low on fuel, it will have to divert to a nearby city to wait out the storm. In Denver, flights routinely divert to places like Albuquerque, Cheyenne, Colorado Springs, or Pueblo.

The most dangerous weather phenomenon for planes is wind shear, or microburst. In a microburst, the downdraft from a thunderstorm is cooled by raindrops then begins to sink through the thunderstorm, gaining speed while it falls. When it reaches the ground, wind speeds can easily top 60 miles per hour. If a plane is near the ground when this occurs, it can be slammed into the ground. The most famous case of a microburst incident in aviation is the crash of Delta flight 191 at DFW, in 1985.

This is all done in the name of safety. So the next time your flight has a weather delay, there’s no point in being upset at your airline, because airlines don’t control the weather, or the flow of air traffic. The best way to avoid weather delays as a passenger is to fly very early in the day, when weather is typically more calm, and delays haven’t had a chance to compound into each other, adding up to several hours. 


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