Seasonal Flight Patterns

Why seasonal patterns repeat

U.S. flight performance has two predictable bad windows every year: late spring through summer (thunderstorm season) and late autumn through winter (snow and ice). According to the Bureau of Transportation Statistics, this seasonal rhythm shows up consistently across the five years of monthly On-Time Performance data (2020 through 2025) that underlies PlainFlights' database. The exact timing varies by region, but the underlying physics does not, convective weather peaks in the warm months across the eastern half of the country, and frozen precipitation peaks in the cold months at northern airports. See our methodology for how monthly BTS releases feed this database. Because these drivers repeat, the seasonal section on every airport page on PlainFlights is a useful forward-looking signal, not just a historical curiosity.

Summer thunderstorms: short, sharp delays

Convective weather, thunderstorms, lightning, hail, microbursts - tends to produce shorter but more frequent delays than winter storms. A line of summer thunderstorms can ground operations at a Southeast hub for thirty minutes to an hour before clearing through, then operations resume. The cumulative impact in the data shows up as elevated NAS delays and elevated weather delays during the May–August window at airports along the convective corridor (Atlanta, Dallas, Houston, Charlotte, the Florida hubs, the New York airports during the sea-breeze convection season).

Because thunderstorm cells move fast, recovery is usually quicker than after a winter storm, but the cascading late-aircraft impact can be significant. A two-hour ground stop in the morning at a major hub propagates through the rest of the day's schedule for every aircraft routed through that hub, which is why the late-aircraft delay category usually rises alongside weather delay during summer bad days.

Winter storms: long, network-wide delays

Snow and ice events are less frequent than summer thunderstorms but produce longer and broader delays. Airports need time to deice aircraft, clear runways, and re-sequence the entire arrival/departure flow. A major winter storm at a single Northeast hub can ripple through the national network for two days as displaced aircraft and crews are repositioned.

In the BTS data, winter storms tend to produce higher cancellation rates than summer thunderstorms, airlines proactively cancel rather than risk operating in conditions that would create longer delays and crew duty-time issues. December and January show elevated cancellation rates at northern airports for this reason.

The Sun Belt exception

Airports in the Pacific Coast and parts of the Sun Belt show much flatter seasonal patterns. Phoenix, San Diego, the inland California airports, and several Texas and New Mexico airports have neither a strong winter storm season nor a strong convective season at the airport itself, although Phoenix in July does see monsoon thunderstorms, and Texas hubs are exposed to summer convection. For travelers with destination flexibility, choosing a flatter seasonal pattern can mean materially fewer delays over the course of a year of travel.

How to use the seasonal section

Every airport page on PlainFlights includes a monthly performance breakdown derived from the BTS data. When you have date flexibility, the pattern is actionable: shifting a trip by two or three weeks can sometimes move you from the worst month at an origin or destination airport into a much better one. When you have route flexibility, choosing between two competing hubs for a connection, for example, the seasonal data can tilt the decision.

The pattern is statistical, not deterministic. A single month is not a forecast for next year. But over a full year of travel, avoiding consistently bad months at airports you transit through compounds into measurably fewer disrupted trips.

Monthly on-time performance, all U.S. airports

Computed live from BTS on-time performance data across all 382 tracked airports and five years (2020-2025), grouped by calendar month: Jul posts the lowest on-time rate at 87.6%, while Sep posts the highest at 92.5%.

A note on time-of-day patterns

The seasonal pattern is the largest predictable driver in the data, but the time-of-day pattern is also strong and is easier to act on because it does not require shifting your travel dates. Earlier flights consistently outperform later flights on the same route, because the aircraft has not yet absorbed any cascading delays from earlier disruptions. The first flight of the day on any aircraft is statistically the most reliable, and reliability degrades as the day progresses, especially at busy hub airports.