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FAQ: Weather Delay

What is the largest cause of delay in the National Airspace System?

Which airports have the worst weather-related delay?

What type of weather causes the most delay?

What happens when enroute flights encounter thunderstorms?

What happens if thunderstorms prevent landing at an airport?

How far in advance do traffic flow planners need weather predictions?

What is NextGen Weather providing to help reduce weather delay?

What is the largest cause of delay in the National Airspace System?

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Causes of air traffic delay in the National Airspace System.

By far, the largest cause of air traffic delay in the National Airspace System is weather. The pie chart shows that weather caused 69% of system impacting delays (> 15 minutes) over the six years from 2008 to 2013, as recorded in the OPSNET Standard "Delay by Cause" Reports.

While weather is the largest cause of delay (i.e., too much demand for the impacted resources), volume alone (too much demand, even with unconstrained resource capacity) also accounts for 19% of delay. Equipment failure (1%) and runway unavailability (6%) account for smaller portions of the delay, and the final category "Other" accounts for the remaining 5%. These delay statistics include Air Carrier, Air Taxi, General Aviation, and Military classes of aircraft.

The portion of delay due to weather represented nearly 10 million minutes of delay in 2013. Delays translate into real costs for the operators and passengers. Currently, the cost to the aircarrier operators for an hour of delay ranges from about $1,400 to $4,500, depending on the class of aircraft, and whether the delay is taken on the ground or in the air. If the value of passenger time is included, the cost goes up an additional $35 per hour (personal travel) or $63 per hour (business travel) for every person on board.

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Which airports have the worst weather-related delay?

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Airports with the worst weather-related delay in 2013.

The bar chart shows that the combined delay at the three biggest airports in the New York City area (Newark, LaGuardia and Kennedy) is the highest in the country, with over 57,000 significant delays (delays > 15 minutes) in 2013. The other top delay airports are in Chicago (nearly 26,000 significant delays in 2013), Philadelphia (almost 18,000), San Francisco (16,000) and Atlanta (nearly 12,000).

These seven airports with the worst weather-related delay certainly experience a large number of impacting weather events. But weather alone does not necessarily lead to huge delays.

If an airport has a lot of excess capacity, even a large number of delayed planes can be shifted to non-weather periods without overloading the system. However, the airports with the most weather delays also tend to operate very near capacity for significant parts of the day. The system-impacting weather, combined with excess demand, means that delayed flights may have to wait hours to land or depart.

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What type of weather causes the most delay?

Chart showing total number of weather-related delays compared to the total number of delays for the combined three major airports in NY by month in 2013. The chart shows corresponding sharp increases in both types of delays in the summer and lesser increases through late fall and early winter. The least amount of both types of delays occur in February and September. The weather-related delays range approximately from 50-80% of the total delays over the year.

Weather-related delays compared to total delays at Newark, LaGuardia and Kennedy airports by month over the course of a year (2013).

The type of weather causing air traffic delay differs over the course of a year, and is also dependent on the geographical area of the country.

For example, the graph shows the combined total delays (dark orange) and weather delays (light orange) by month in 2013 at Newark, LaGuardia, and Kennedy airports. Both weather delays and total delays peak in May, June and July (weather delays ranging from nearly 6,700 to over 7,800; total delays ranging from 9,000-9,500), but weather delays are also considerable in March, April, October, November, and December (ranging from about 3,500 to 5,400).

The pie charts below show the type of weather leading to this combined Newark, LaGuardia, Kennedy 2013 delay. In the winter months (October through March), the combination of airport surface winds and low ceiling and visibility (C&V) conditions account for approximately 75% of delays; convective weather, winter weather, and a small amount of other weather conditions contribute to the remaining 25%.

However, during the summer months (April through September) when delays peak, most of the delay affecting airport arrivals (over 40%) is due to convective weather (rain, thunderstorms). Low C&V conditions (~30%), airport winds (~20%), and other (~10%) also contribute to summer delay.

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Types of weather contributing to delays at Newark, LaGuardia and Kennedy in winter vs summer (2013).

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What happens when enroute flights encounter thunderstorms?

Jet aircraft can safely fly over thunderstorms only if their flight altitude is well above the turbulent cloud tops. The most intense and turbulent storms are often the tallest storms, so enroute flights always seek to go around them.

If a busy jet route becomes blocked by intense thunderstorms, traffic will reroute into the neighboring airspace, which can become overcrowded if the flow is not managed (see animation). In these cases, the Planning Team (consisting of FAA personnel at the Air Traffic Control System Command Center who coordinate with the Centers, select Terminals, the airlines, NAVCANADA, general aviation organizations and the military) has several options, some of which are discussed below.

In the case of a large scale weather impact, a Severe Weather Avoidance Plan (SWAP) may be put in place to completely relocate demand to another part of the country. The Planning team's strategic placement of Airspace Flow Programs (AFPs) with reduced hourly flow rates allows airlines to prioritize and plan which of their scheduled flights they will route through the restricted airspace. Ground Delay Programs (GDPs) are also used to temporarily hold aircraft at their departure airports to reduce the number of flights coming into an impacted area.

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Flights avoiding New York area weather-impacted airspace on September 11, 2013.

As an example, on September 11, 2013, an approaching cold front caused a broad region of rapid storm development in the New York Center airspace. An Airspace Flow Program was set about one hour before the weather impact quickly increased, as a way of reducing flow, but west coast traffic bound for New York was already enroute.

As can be seen in the flight path animation (aircraft are shown as dots; the trailing lines show their recent paths), very few flights could get through the weather-impacted airspace, and many of them were routed northward to avoid the weather. However, the weather continued to progress northward, making for increasingly long reroutes. Even though the New York airports remained clear of weather, the flights bound for New York couldn't get there on time.

There were 69 diversions (meaning aircraft had to land at alternate airports), and 72 taxi-backs (meaning the aircraft pushed back from the gate, ready to take off, but there was no available airspace and they had to return). In addition, there were 55 airborne holding events, and almost 600 departure and arrival cancellations.

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What happens if thunderstorms prevent landing at an airport?

As the arriving aircraft approaches its destination airport, the pilot will usually be asked to slow down or enter a holding pattern until the thunderstorms in and around the airport have cleared. As more planes arrive and holding continues, over-crowded airspace and low fuel conditions can become serious issues. Landing these arrivals safely becomes top priority.

Controllers can opt to use more of the available terminal routes for arrivals and fewer for departures. But with fewer planes departing, gates are not freed up and airport "grid lock" can occur. Cases where passengers have been stranded for excessively long periods of time led the Department of Transportation (DOT) to pass a rule prohibiting airlines from leaving planes parked for more than three hours without allowing passengers to disembark.

If the thunderstorms persist, the holding aircraft will eventually have to divert to alternate airports, wait out the bad weather, refuel, and fly again (much later) to the original destination. Diversions are very undesirable because of the large passenger delay and high cost to the airlines.

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Flights forced into holding patterns during thunderstorms on July 2, 2014.

An example of thunderstorms temporarily preventing landing at airports occurred on July 2, 2014, as growing thunderstorms moving eastward delayed several flights destined for the New York airports.

The flight path animation (dots being aircraft, and the trailing lines showing their recent paths) shows that as the storms neared the airports, aircraft were forced to hold (seen as oval patterns) in various locations south, west, north and east of the airports until the weather cleared. In total, 177 flights were held because of this weather event, representing over 5,000 minutes of delay. There were also 97 aircraft that had to divert to alternate airports.

Once the storms began to dissipate and move away, the regular flow of traffic resumed.

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How far in advance do traffic flow planners need weather predictions?

It is clear that unforeseen weather impacts on both enroute and terminal airspace can lead to large delays and ultimately be very costly to the airlines and the travelling public. If the weather impacts are either short-lived or local, they can be mitigated by effectively using the available airspace. All the airborne and scheduled flights can be handled with only minor reroutes.

However, as the weather impacts become longer lived and/or affect larger regions of the country, management of the demand must be planned strategically. In weather events requiring moderate to aggressive strategic management, many scheduled flights will require new flight plans that do not intersect the weather impacted areas.

Some flights through the impacted airspace may originate at nearby airports, with only short intervals from departure to arrival, whereas other flights may cross the country and be airborne for hours. A severe long-lived weather impact will require management of short- and long-haul flights in order to effectively control the demand.

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Airborne time of flights arriving at Newark, LaGuardia and Kennedy on a day with no weather delay (September 19, 2014).

For example, the bar chart combines all arrivals into the three New York airports (Newark, LaGuardia, and Kennedy) through the course of an entire day with no weather delays, and categorizes them by their airborne time interval. Most arriving flights have been airborne 1–3 hours (blue), with approximately 45–60 flights arriving each hour from 8 am to 9 pm local time. There are about 10–25 arrivals per hour with airborne time intervals less than one hour (green) throughout most of the day (6 am to midnight), reflecting the steady traffic between New York, Boston, Washington and other nearby cities. Finally, a very large number of New York arrivals have been airborne over three hours (red), with 30–40 flights per hour landing daily between noon and 9 pm. The noon–9 pm time period happens to coincide exactly with the time period most thunderstorms occur in the summer months.

Strategic traffic flow managers must plan hours in advance to influence long haul flights. If the time needed for pre-departure planning and filing of amended flight plans is added to the airborne time intervals, it becomes clear that predictions of convective weather impacts on airspace capacity are needed 4-8 hours in advance to influence long haul flights and 2-6 hours in advance to influence flights with shorter airborne time intervals.

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What is NextGen Weather providing to help reduce weather delay?

NextGen Weather provides aviation weather products that support both tactical and strategic management of air traffic during weather events, helping to minimize passenger delays as well as improve aviation safety.

Tactical Traffic Flow Management

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Aircraft waiting in line for takeoff.

Managing traffic tactically uses available airspace resources to handle the normal traffic demand, and requires very accurate depictions of weather impacts in the 0-2 hour time frame. Traffic flow managers can locally reroute traffic around weather, direct enroute traffic to a weather-free arrival path when nearing the destination airport, delay arrivals by placing them in holding stacks until the weather clears, etc. Many of NextGen Weather's improved products support tactical traffic flow management.

Strategic Traffic Flow Management

To support strategic traffic flow management, operational planners require clear, high confidence predictions of weather impacts on airspace capacity out to eight hours so they can plan strategies such as traffic flow re-routes and/or flow rate restrictions. They also need the shared situational awareness that is required for collaborative decision making on strategic time scales. NextGen Weather provides the foundation for development of critically-needed traffic flow management tools.

Detailed information is provided in the section on Support for Strategic Traffic Flow Management.

Aviation Safety

Although improved efficiency of the National Airspace System (NAS) is NextGen Weather's primary benefit, the program also enhances aviation safety in several ways. The NextGen Weather Processor (NWP) provides aviation weather products with improved coverage, faster product update rates, and reduced artifacts. For example, the NWP Growth Trend product updates every 30 seconds, and indicates where thunderstorms are actively growing -- clearly airspace to be avoided. Additionally, Common Support Services – Weather (CSS-Wx) and the NWP Aviation Weather Display (AWD) enable access throughout the NAS to NWP products such as microburst and wind shear detections. Broad and timely access to these products enhances awareness of serious ongoing terminal safety hazards. Another safety improvement provided by CSS-Wx and AWD is the display of NOAA aviation-oriented Icing and Turbulence products, providing users with an indication of where and when in-flight icing and/or turbulence may occur.

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This page was originally published at: https://www.faa.gov/nextgen/programs/weather/faq/