Is Weather Really to Blame for Flight Delays?

Not as much as the official numbers suggest. US airlines report weather as the direct cause of just 6.1% of flight delay minutes — but that figure only counts extreme weather logged against a single flight. A lot of weather's real damage gets filed under a different cause code entirely, which means its true impact on delays is larger than the headline number implies.

• Weather is officially the cause of just 6.1% of delay minutes. Across 14.1 million US flights from June 2024 through May 2026, that's the smallest of the five categories except security.
• The other four categories: late aircraft 39.3%, air carrier 34.0%, National Aviation System (NAS) 20.4%, and security 0.1%.
• A meaningful share of that 20.4% NAS figure is itself weather-driven — ground stops, gate holds, and rerouting caused by storms, just logged under a different name.
• BTS doesn't publish a weather-vs-other split within NAS, so no one can calculate an exact "true" weather percentage — the data supports a direction, not a specific number.
• Practical result: a clear sky at your departure airport doesn't mean you're safe from a weather delay happening somewhere else in the network.

The five official causes of a delay — and where weather sits

The US Department of Transportation requires every airline to code each delay into one of five categories. Weighted by delay-minutes across our full dataset — 14.1 million flights from June 2024 through May 2026 — the national split is late aircraft 39.3%, air carrier 34.0%, National Aviation System (NAS) 20.4%, weather 6.1%, and security 0.1%. Each category is defined in detail in our full breakdown of why flights get delayed, and weather ranks fourth on that list by design — not because storms don't matter to travelers, but because of how narrowly the category itself is defined.

Why the 6.1% weather number undercounts weather's real effect

The "weather" category only counts extreme weather that an airline logs as directly delaying one specific flight — a plane held on the ground because a thunderstorm is sitting right over its own airport, for instance. Weather's bigger effect on the system runs through air traffic control instead, and that gets coded under a different name entirely.

When a storm shuts down arrivals at a major hub, the FAA can issue a ground stop or a traffic management program that slows flights across the whole network — sometimes for hours, at airports hundreds of miles away where the sky is perfectly clear. Those delay minutes get coded as National Aviation System (NAS) delays, a category that also folds in non-weather issues like equipment outages and staffing gaps, rather than as weather. A thunderstorm complex over Atlanta or a snowstorm bearing down on the New York airports can generate delay minutes at airports across the country that land in the NAS column, while the weather line itself barely moves.

So how much of all this is really weather's fault?

Nobody can say precisely, and we're not going to invent a number that sounds more exact than the data allows. BTS doesn't publish a split of how much of the 20.4% NAS share is weather-triggered versus pure congestion, equipment problems, or staffing, so there's no public data to compute a corrected "true" weather percentage. What the numbers do support is a direction: weather's real footprint on total delay minutes is meaningfully larger than the standalone 6.1% line, once you account for its indirect role inside NAS-coded slowdowns. It can ripple even further than that — an aircraft delayed by a weather-driven ground stop early in the day can run late for every leg it flies afterward, adding to the 39.3% late-aircraft category, which is the single largest cause of delays overall. The honest summary is "more than 6.1%, direction confirmed, exact size unpublished" — not a specific corrected figure.

What this means if you're trying to dodge a weather delay

Because weather's real effect is spread across categories instead of concentrated in one line, the national weather percentage isn't a very useful planning number on its own. A better approach is to check the actual on-time history of your airport, route, or specific flight — congestion-prone hubs tend to feel weather's ripple effects hardest, and our ranking of the most delayed major US airports is a reasonable starting point. Build in extra buffer during peak storm seasons too: summer afternoon thunderstorms cluster around Southern and Midwestern hubs, and winter systems hit the Northeast hardest, even when the forecast at your own airport looks clear. Our methodology page has the full detail on how these delay-cause percentages are computed from BTS data.

Frequently asked questions

What percentage of flight delays are caused by weather?

US airlines report weather as the direct cause of 6.1% of delay minutes, based on 14.1 million flights from June 2024 through May 2026. That figure only counts extreme weather logged against a single flight, so it understates weather's total effect once indirect, air-traffic-control-driven delays are considered.

Why is my flight delayed if the weather looks fine at my airport?

Storms elsewhere in the network can trigger ground stops or slower air-traffic flow that the FAA applies broadly, including at airports with clear skies. Those delays get coded as National Aviation System delays rather than weather, even though weather caused them in the first place.

What is a National Aviation System (NAS) delay?

NAS delays cover air-traffic control actions like ground stops, gate holds, and rerouting — caused by weather, congestion, equipment issues, or staffing. It's a separate BTS category from "weather," which only covers extreme weather logged directly against one flight. See our full breakdown of the five delay categories for definitions of all of them.

Does BTS publish how much of NAS delay is caused by weather?

No. BTS reports NAS as a single combined category without a weather-versus-non-weather split, so there's no public data available to calculate an exact "true" weather delay percentage.

What's the single biggest cause of flight delays overall?

Late-arriving aircraft, at 39.3% of delay minutes — a plane running behind schedule from an earlier leg. It's often itself downstream of an earlier weather-driven slowdown elsewhere in the system.

Can I avoid weather delays by choosing a different airport?

Not reliably. Because so much of weather's effect travels through network-wide air-traffic congestion and late-aircraft ripple effects rather than a single storm at a single airport, no airport is fully insulated. Checking that airport's or route's own on-time history is more useful than assuming a sunny-weather city is automatically safe.