Guide
US airports with the biggest gap between their best and worst month
At Dallas (DAL), the on-time rate swung 26.3 points between January (82.8%) and June (56.5%). Here is every major airport's best and worst month.
An airport's annual on-time rate hides its calendar. Two airports with the same yearly figure can behave completely differently: one steady all year, the other excellent for eight months and poor for four. If you know when you are flying, the month matters more than the year.
This guide takes the 50 busiest US airports with a page on this site and compares each one's best and worst month from July 2025 to June 2026. The difference between the two is the airport's seasonal swing.
Every airport's best and worst month
Each airport links to its full record, and each month links to that airport's page for the month, with the carriers, routes, departure times and delay causes for those four weeks.
The largest swing belongs to Dallas (DAL). Its best month, January, ran 82.8% on time; its worst, June, ran 56.5%. That is a gap of 26.3 points at a single airport. The smallest swing is at Sacramento (SMF), 7.3 points, and the median across all 50 airports is 16.4.
Summer airports and winter airports
The month an airport does worst in says a lot about what disrupts it:
- 35 airports had their worst month in June, July or August. Summer problems usually come from afternoon thunderstorms, which close arrival routes for hours at a time, combined with the fullest schedules of the year.
- 12 airports had their worst month in December, January or February. Winter problems come from snow and ice, de-icing queues and the occasional storm that shuts a hub for a day.
- The rest peaked in spring or autumn, often because of a single disruptive event rather than a seasonal pattern.
This is where the month pages earn their place. A summer-weak airport can be one of the best places to connect in winter, and the reverse is true for airports in the snow belt.
Why some airports swing so much
A large swing usually has one of three causes.
The first is exposure to a specific weather pattern. Airports in thunderstorm country lose afternoons all summer. Airports in the northern hubs lose whole days in winter. Airports in mild climates have neither and keep a flatter record.
The second is how close the airport runs to its capacity. An airport with spare runway and gate space absorbs a bad afternoon. One scheduled close to its limit spreads the same afternoon into the evening and the next morning.
The third is the schedule itself. Airports that add large numbers of seasonal flights, for ski traffic in winter or leisure traffic in summer, carry a much heavier schedule in their peak months, and a heavier schedule is harder to keep on time.
How to use this when planning
If you have flexibility about when to travel, compare your airports' best and worst months in the table, then open the month pages for the months you are considering. A swing of 20 points or more is large enough to change how much connection time you should leave.
If your dates are fixed, the month page is still the most useful page for your trip. It shows which departure times held up best that month, which carriers ran most punctually there, and whether delays were driven by weather, the airspace system or the airlines themselves.
For connections, combine this with the connection risk calculator, which uses a route's full-year arrival record. In an airport's worst month, add a margin on top of what the calculator suggests.
Month pages also list the busiest routes from the airport that month and how each ran. If your route is on that list, its figure for the month is a closer guide than the airport's annual rate or even the route's own twelve-month record.
An important caveat
Each month in this data is one calendar month from one year. A month that looks bad may have been bad because of a single storm that will not repeat, and a month that looks good may simply have been lucky. That is why the table shows the swing rather than a forecast.
The pattern across many airports is more reliable than any single airport's figure. When most airports in a region share the same worst month, the season is the cause. When one airport stands out on its own, look at its month page for the day or week that did the damage.
Common questions
Is the worst month the same every year? Not always. Seasonal patterns repeat, but a single storm or an airline's bad week can move the worst month from one year to the next. That is why the summer and winter counts matter more than any one airport's result.
Should I avoid flying through an airport in its worst month? If you have a choice of connecting airport, it is worth considering. If you do not, leave a longer connection, prefer earlier departures and check the airport's month page for which carriers held up best.
Why does a small swing not mean an airport is good? A small swing means the airport is consistent, not that it is punctual. An airport can be steadily average all year. The best-month and worst-month columns show both the swing and the level.
How are the months counted? Each month is a calendar month of departures from the airport, counted only when it had at least 1,000 flights. The same on-time definition applies as everywhere else on the site.