Weather Share of Delay, Winter and Summer

Summary

39 of 62 large airports put a bigger share of their delay minutes down to weather in December to February than in June to August; 23 put a bigger share down in summer. The median difference is +1.3 points. Pooled across the airports, weather is 6.6% of attributed delay minutes in winter and 5.7% in summer.

Delay minutes in the federal data come with a stated cause for every flight that arrived 15 minutes or more late. This report takes the share of those minutes the data files under weather, at each of 62 large airports, and compares December to February with June to August. It is a share of the delay that is attributed, not of all flights, so an airport with little delay in a season can show a high share on few minutes.

Sorted from the largest winter lead to the largest summer lead. Weather share is weather delay minutes over all attributed delay minutes, the sum of the five cause categories the data reports, for flights that left the airport in the months named, added up before dividing. Difference is the winter share less the summer share.

The data’s weather category covers extreme weather only. Ordinary weather that slows the air traffic system is filed under that system, so every share here is lower than the weather’s true part, and by how much differs by airport and season. A share can fall because another cause grew, so a lower weather share in one season does not by itself mean fewer weather minutes. The table gives shares only; it does not give minutes. Airports are included when they have flights in all three months of both seasons and at least 5,000 completed flights in each season; 2 did not and are left out.