Seasonal Route Reliability: When a Summer Route Becomes a Bet

A route's annual on-time rate is an average across twelve months that may not resemble any single month you actually plan to fly. Across all reporting carriers in the U.S. DOT on-time filings (Marketing Carrier On-Time Performance, TranStats table 236, June 2023 through June 2026), July arrivals came in at 71.3 percent on time. October arrivals ran at 83.8 percent. That is a 12.5-point gap between two months on the same routes — and an annual average somewhere in the middle tells you almost nothing about which one you are booking into.

Why the annual rate misleads

The federal on-time definition is fixed: a flight is on time if it arrives fewer than 15 minutes after its published arrival time, measured at the gate (14 CFR § 234.2). What changes month to month is not the rule — it is the weather and traffic environment that routes operate inside.

Some routes pass through airport pairs where one season is genuinely benign and another is not. A route touching a Gulf Coast airport in August runs through afternoon convective storms that do not exist in November. A route between two northern cities in January runs through snow and ice that disappears by May. When an annual rate blends all twelve months together, the good months and the bad months cancel each other out, and the result is a number that accurately describes no particular month.

The DOT filing records every departure and arrival for every reporting carrier, so the seasonal signal is in the data — it just requires looking at months individually rather than accepting the annual summary.

What the system-wide seasonality looks like

Using the full June 2023 through June 2026 record from the BTS Marketing Carrier On-Time Performance filing, the month-by-month arrival on-time rates across all reporting carriers show a clear pattern:

• January: 78.8%
• February: 81.3%
• March: 78.6%
• April: 80.5%
• May: 76.4%
• June: 73.7%
• July: 71.3%
• August: 77.7%
• September: 83.3%
• October: 83.8%
• November: 83.8%
• December: 78.7%

June and July are the lowest months in the record — 73.7 and 71.3 percent respectively. September, October, and November are the highest, all at or above 83 percent. The difference between the worst month (July, 71.3%) and the best months (October and November, both 83.8%) is 12.5 percentage points across the same network of routes.

This is a system-wide average. Individual routes, particularly those touching airports with concentrated weather exposure, can show swings that are larger still.

The misleading annual rate: a worked example

Consider what happens when you look at a route touching Orlando (MCO). MCO's overall on-time departure rate for the trailing 12 months (July 2025 through June 2026) was 73.7 percent — already one of the lower figures among large airports in the BTS filing. But that 73.7 percent includes every month of the year, from the relatively calm winter months to the convective-storm-heavy Florida summer.

If you book a June or July departure from MCO, the system-wide data above shows you are already operating in the two worst months in the record (73.7% and 71.3% nationally). A route-level annual rate that looks passable on its face — say, somewhere in the mid-70s — could be pulled toward 80 percent or higher by the calm October-through-February stretch, while the summer months that actually match your travel date run materially lower.

The same logic applies in reverse for winter-sensitive routes. A route between, say, Chicago O'Hare (ORD, 72.7% overall for the trailing 12 months) and Minneapolis (MSP, 80.3% overall) will carry more winter weather risk than its annual rate suggests if you are booking in January — and less if you are booking in September.

How to spot seasonal hidden risk

The annual on-time rate on any route is a starting point, not a conclusion. To read seasonal risk out of it:

Step 1 — Identify the weather season for your travel month. Summer (June–August) is thunderstorm and convective-storm season for most of the central and southeastern U.S. Winter (December–February) is snow and ice season for northern and mountain airports. Both seasons show up clearly in the DOT delay-cause records.

Step 2 — Compare the system-wide monthly rate for your travel month to the annual average. The data above shows July at 71.3% versus October at 83.8%. If a route's annual rate is 78%, its July performance is likely below that, and its October performance likely above it.

Step 3 — Check both endpoints. A route is only as weather-stable as its most exposed airport. A flight from Los Angeles (LAX, 80.0% overall) to Dallas-Fort Worth (DFW, 71.0% overall) inherits DFW's summer convective risk even though LAX itself is relatively weather-stable year-round.

Step 4 — Run the route in the Route Reliability Check. The tool pulls on-time arrival rates, typical and worst-case delays, and cancellation figures from the DOT on-time filings for every airline operating a specific U.S. route. Comparing carriers on the same route in the same month gives you a much sharper picture than any annual headline rate.

For a broader look at how hub dynamics affect the same carrier's performance on different routes, the guide on hub airports vs. non-hubs explains why the same airline can look very different depending on which end of the route it calls home.

What the delay-cause data adds

The DOT filing breaks delays into five causes: carrier, late-arriving aircraft, National Aviation System (NAS), weather, and security. Weather delays are the most directly seasonal, but NAS delays — which include air traffic control volume and system-wide congestion — also spike in summer because convective weather forces reroutes that slow traffic across the entire network, not just at the storm-affected airport. The guide on National Aviation System delays covers why NAS delays are the hardest category to predict from a route's historical rate alone.

The point is that a summer route's on-time drag comes from more than direct weather hits. The network effects mean even routes that are not in the storm's path can absorb delays from it.

What this data does not do

The figures in this guide are historical base rates from the DOT on-time filings — they describe what happened over a defined period, not what will happen on any specific future flight. This site does not provide live tracking or predictions. U.S. domestic operations of reporting carriers only; non-U.S. routes are not covered. Read cancellation rates alongside on-time rates: a carrier can improve its on-time percentage by canceling flights that would have run late.

This guide is for informational purposes only and is not professional travel or financial advice.

Frequently asked questions

Does a route's annual on-time rate reflect what I'll see in July?

Not reliably. The annual rate blends all twelve months, and July is the lowest-performing month in the DOT record across the June 2023–June 2026 window, at 71.3% on-time arrivals system-wide. A route that looks solid at 78% annually may be running several points lower in July and several points higher in October.

Which months are the safest for on-time performance?

Based on the BTS Marketing Carrier On-Time Performance filing (June 2023 through June 2026), September, October, and November each reached 83.3%, 83.8%, and 83.8% respectively — the three highest months in the record. June (73.7%) and July (71.3%) are the lowest.

Why does summer perform worse even on routes that aren't near storms?

Convective weather forces air traffic control to reroute flights across the network, creating NAS delays that affect routes well outside the storm's immediate footprint. A flight from Seattle to Salt Lake City can run late because storms over Texas have compressed traffic into the same altitude bands it needs to use.

How do I check seasonal risk for a specific route?

Start with the system-wide monthly rates in this guide to understand which travel months carry the most network-level risk, then run your specific origin-destination pair through the Route Reliability Check to see carrier-level on-time rates, cancellation figures, and delay profiles from the DOT filings.

Should I look at both airports on a route?

Yes. A route inherits the weather exposure of its most vulnerable endpoint. An airport like LAX that is relatively stable year-round does not protect a flight from the summer convective risk at its destination. Check the on-time rate for both the origin and the destination before treating an annual route figure as representative of your travel month.