FAQ: Weather Delay


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

Pie chart. Values discussed in following paragraph.
Causes of air traffic delay in the National Airspace System.

The largest cause of air traffic delay in the National Airspace System is the weather. The pie chart shows that weather caused 75.48 percent of system-impacting delays of greater than 15 minutes over the six years from June 2017 to May 2022, as recorded in the OPSNET standard "delay by cause" reports. While the weather is the largest cause of delay because of too much demand for the impacted resources, volume alone, caused by too much demand even with unconstrained resource capacity, also accounts for 14.54 percent of delays. Equipment failure creates 0.38 percent, runway unavailability 5.84 percent, and "other" factors the remaining 3.76 percent of delay. These delay statistics include air carriers, air taxis, general aviation, and military classes of aircraft.

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Which airports have the worst weather-related delay?

Bar chart. Values discussed in following paragraph.
These airports had the worst weather-related delay in 2019.

The bar chart shows that the combined delay at two of the biggest airports in the New York City area (Newark and LaGuardia) is the highest in the country, with more than 66,000 significant delays of more than 15 minutes in 2019. The other top delay airports are in Chicago (nearly 29,000 delays in 2019), San Francisco (almost 26,000), Boston (nearly 15,000), and Seattle (nearly 11,000).

These six airports with the worst weather-related delay experience many impacting weather events, but weather alone does not necessarily lead to huge delays.

If an airport has much excess capacity, many delayed planes can be shifted to non-weather periods without overloading the system. However, airports with the most weather delays also tend to operate close to capacity for large parts of the day. 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?

Area chart. Values discussed in following paragraph.
Weather-related delays compared to total delays at Newark, LaGuardia and Kennedy airports by month over the course of 2019.

The type of weather causing air traffic delay differs over a year, and also depends on the country's geographical area.

For example, the graph shows combined total delays (blue) and weather delays (dark yellow) by month in 2019 at Newark, LaGuardia, and Kennedy airports. Weather delays and total delays exhibit varying peaks, with weather delays peaking in the summer (nearly 7,800) and total delays peaking at nearly 11,500 in October. Overall, weather delays are most prevalent in the summer, with the highest values in April, May, and June (ranging from about 7,100-7,800).

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What happens when en route 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 en route flights always seek to go around them.

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

In case of a large-scale weather impact, a severe weather avoidance plan may be put into place to relocate demand to another part of the country. The planning team's strategic placement of airspace flow programs 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 are also used to temporarily hold aircraft at their departure airports to reduce the number of flights going into an impacted area.

Animated GIF of flights
Aircraft shown as dots avoid New York area weather-impacted airspace July 17, 2019. Trailing lines show their recent paths.

For example, on July 17, 2019, a low pressure ed to the southwest andmoved over the greater metropolitan area of New York, as a cold front approached from the north. The remnants of Tropical Storm Barry bring most shower and thunderstorm activity.

As seen in the animation, few flights could get through the weather-impacted airspace, many of which were routed around active regions to avoid the weather. Due to longer re-route times, flights bound for New York 'couldn't arrive on time.

In the Northeast region, one hundred-eight aircraft diverted to alternate airports, and 57 aircraft were assigned to a holding pattern. There were 565 departure cancellations, 535 arrival cancellations, and an accompanying 784 total delays.

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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 running out of fuel can become serious issues. Landing these arrivals safely becomes the top priority.

Controllers can use more of the available terminal routes for arrivals and fewer for departures. With fewer planes departing, terminal gates remain occupied, and airport gridlock can occur. Cases, where passengers were stranded for excessive periods, led the Department of Transportation to pass a rule prohibiting airlines from leaving planes parked for more than 3 hours without allowing passengers to disembark.

Animated GIF of flights
Flights shown as dots and their trailing lines showing their paths are forced into holding patterns during thunderstorms in the northeast region August 7, 2019. Oval patterns show aircraft that were held in the air.

If thunderstorms persist, holding aircraft will divert to alternate airports, wait out the bad weather, refuel, and fly again later to the original destination. Diversions are undesirable because of the magnitude of passenger delay and cost to airlines.

An example of thunderstorms temporarily preventing landing at airports occurred on August 7, 2019, as growing thunderstorms delayed several flights in the northeast region.

The animation shows that as storms approached the airports, aircraft were forced to hold around the airports until the weather cleared. In total, 171 airborne flights were held. Two hundred seventeen aircraft were diverted to alternate airports.

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

Line chart. Arrivals start at approximately 70 at midnight, drop to about 10 for 1am through 3am, then gradually rise to 95 by 8am. Arrivals then fall and rise in lulls and peaks from 9am to 11pm. Peaks include 127 at 1pm, 135 at 5pm, and 118 at 8pm. Lulls include 79 at 10am, 107 at 7pm, and 84 at 11pm.
Airborne flights arriving shown for Newark, LaGuardia and Kennedy on a day with no weather delay, September 19, 2019.

Unforeseen weather impacts on en route and terminal airspace can lead to long delays and ultimately be costly to the airlines and the traveling public. If weather impacts are either short-lived or local, they can be mitigated by effectively using available airspace. All airborne and scheduled flights can be handled with only minor re-routes.

However, as the weather impacts become longer lived, affect larger regions of the country, or both, management of the demand must be planned strategically. In weather events requiring moderate to aggressive 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 the managing short- and long-haul flights to control the demand effectively.

The line chart combines all arrivals into three New York airports – Newark, LaGuardia, and Kennedy – through an entire day with no weather delays.

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, 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 shorter flights.

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What is NextGen Weather providing to help reduce weather delay?

Aircraft wait in line for takeoff.
Aircraft wait in line for takeoff.

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

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

Tactical Traffic Flow Management

Managing traffic tactically uses available airspace resources to handle the normal traffic demand and requires accurate depictions of weather impacts in the 0-2 hour time frame. Traffic flow managers can take different steps, such as locally -re-routing traffic around weather, directing en route traffic to a weather-free arrival path near the destination airport, and delay arrivals by placing them in holding stacks until the weather clears. 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 8 hours to plan strategies such as traffic flow reroutes, flow rate restrictions, or both. They also need the shared situational awareness required for collaborative decision-making on strategic time scales. NextGen Weather provides the foundation for developing 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 fewer artifacts. For example, the NWP growth trend product updates every 25 seconds, and indicates where thunderstorms are actively growing and the airspace to avoid.

Additionally, Common Support Services – Weather (CSS-Wx) and the NWP Aviation Weather Display (AWD) enable access throughout the NAS to NWP products such as lightning and tornado detections. Broad and timely access to these products enhances awareness of serious ongoing safety hazards. Another safety improvement CSS-Wx and AWD provides is the display of NOAA aviation-oriented icing and turbulence products, providing users with an indication of where and when in-flight icing or turbulence may occur.

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Last updated: Thursday, November 10, 2022