SFO

The spread between United and Southwest is 20 points, wide enough that it should decide the booking. On a hundred trips that is 20 more arrivals inside the on-time window.

The weak window here is the middle of the day, not the evening: 11am–2pm runs 72.0% on time against 92.2% for 5am–8am. That shape usually means a departure bank rather than an accumulating delay, so the recovery later in the afternoon is real rather than noise.

Delays peak in May, when the on-time rate falls to 70.1%. March is the strongest month at 84.0%. Moving a flexible trip across that boundary buys 14 points.

Across the week, Saturday is the strongest day at 82.6% on time and Sunday the weakest at 76.0%. 7 points is a smaller lever than departure hour, but it is free.

43% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 2.10 million reported delay minutes here, that single category outweighs weather by 14 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit.

The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 92.2% on time here.

Cancellations sit at 0.99% against a national 1.80%, which is ordinary. Diversions add another 0.31%.

7.2% of arrivals land an hour or more late, close to the national 8.2%. That is the figure that decides a connection, and it says a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route are the about an hour kind, which a normal layover absorbs.

145,890 flights sit behind these numbers, which is a large enough sample that the headline rate is not moving on noise. Where a sub-group falls below 30 flights the site shows a dash instead of a percentage.