Best time to fly Seattle to Ontario
Every month pooled across the years on record, so "July" means July generally rather than one particular July. Full SEA to ONT record →
Best month
September
91.1% on time · 213 flights
Worst month
December
72.2% on time · 200 flights
| Month | Flights | On time | Cancelled | When late, by |
|---|---|---|---|---|
| January | 157 | 85.3% | 0.64% | 52 min |
| February | 135 | 84.4% | 0% | 37 min |
| March | 177 | 86.2% | 1.69% | 67 min |
| April | 200 | 89.5% | 0% | 54 min |
| May | 249 | 86.2% | 0.8% | 42 min |
| June | 240 | 85.6% | 1.67% | 46 min |
| July | 302 | 72.3% | 3.31% | 59 min |
| August | 266 | 77.7% | 0.38% | 39 min |
| September | 213 | 91.1% | 0% | 47 min |
| October | 200 | 87.9% | 1% | 69 min |
| November | 211 | 82.8% | 3.32% | 42 min |
| December | 200 | 72.2% | 1% | 51 min |
What a seasonal pattern can and cannot tell you
Months differ for reasons that repeat — summer thunderstorms in the American interior, winter fog and de-icing in northern Europe, school holidays filling every seat and leaving no slack in the schedule. Those are real and they come back each year.
What the pattern cannot do is forecast a particular flight. A quiet month has late days and a bad month has punctual ones. Treat this as a way to choose between two dates you are indifferent about, not as a promise.