How Much of Each Season’s Lateness Is the Weather

Summary

Summer was 10.2 points below fall in on-time arrival (72.9% against 83.1%). 8.9 points of that, 87%, was lost in normal-weather hours, and only 6.2% of summer flights left in bad weather. Winter is where weather counts most, 1.5 points of 3.7 points.

Fall (September to November) was the best season, with 83.1% of flights arriving on time. Summer (June to August) was the worst, at 72.9%, 10.2 points lower. 6.2% of summer flights left in bad weather at the departure airport, against 6.3% in fall. Of the 10.2 points, 8.9 points (87%) came from flights in normal weather arriving on time less often than they did in fall, 0.9 points from flights in bad weather doing worse, 0.0 points from the mix of weather hours and 0.4 points from the hours between. On-time arrival in normal weather was 84.1% in fall and 73.7% in summer.

The whole network, every year in the data together. The four columns after the gap add to the gap in points of on-time rate against the best season. A part is positive when it lowered the rate. Mix of weather hours is the effect of the season having a different share of flights in each kind of weather; the next three are the effect of a lower on-time rate within each kind, weighted by that kind’s share of the season’s flights. Hours between are marginal weather and flights the weather record could not place.

All airlines and airports, every year in the data for the month together, so a month that appears in more years has more flights. On time is the share of completed flights that arrived under 15 minutes late; flights in bad weather is their share of completed flights.

Winter was 3.7 points below fall: 1.8 points in normal-weather hours, 0.7 points in bad-weather hours, 0.8 points from the mix of weather and 0.4 points from the hours between. Spring was 5.4 points below fall: 4.0 points in normal-weather hours, 0.6 points in bad-weather hours, 0.4 points from the mix of weather and 0.4 points from the hours between. Summer was 10.2 points below fall: 8.9 points in normal-weather hours, 0.9 points in bad-weather hours, 0.0 points from the mix of weather and 0.4 points from the hours between. Winter is where weather accounts for the most: the mix and the bad-weather hours together are 1.5 points of its 3.7 points.

Airlines with at least 10,000 completed flights in each of the two seasons and at least 120 in each kind of weather hour in both. Points lost is fall’s on-time rate minus summer’s.

Across 15 airlines the share of the summer loss that falls in normal-weather hours runs from 83% at Endeavor Air to 93% at Envoy Air. For 15 of them it is half or more. The airlines lost between 2.4 points and 13.3 points of on-time rate against fall, and the share of it in normal-weather hours averaged 88% across the airlines.

Weather is measured at the departure airport in the hour a flight was scheduled to leave. A thunderstorm along the route or at the destination, a storm that cleared an hour earlier, and weather at the airport where the aircraft came from are all in the normal-weather hours of this report, so the normal-weather share is not the share of lateness that has nothing to do with weather. What it shows is how much of each season’s loss shows up in hours the departure airport reports as clear. The data does not say what delayed those flights. The seasons pool the months of every year in the data, and the years are not equally represented in each: some months appear in more years than others, and airlines enter and leave the data. The best season is the best of these four, so every other season is measured against the most favorable one.