I wrote an app called FlyOrDelay™ (it's in the Apple App Store) which reports on airport delays. It doesn't report on individual flight delays, but on delays at airports. For the last 3+ months, it has had nothing interesting to report. Why is that? Simply put, the scheduled arrival and departures at all the US airports was below even the bad weather capacity of the airports.
An example will make this evident:
Chicago O'Hare, or ORD to the FAA, has a fair weather capacity of something like 110 arrivals an hour. Airlines used to schedule that many, or even more, arrivals in peak hours. When the weather would get bad, the capacity of ORD would drop to something like 60 to 80 arrivals an hour and, voila!, we'd have an airport delay! That is why every time thunderstorms or snow squalls come through ORD, you see delays that can only be cleared out over time.
For the last three months, however, the scheduled number of arrivals (and departures) must have been below even the bad weather capacity at US airports, because even when bad weather came through, I did not see delays. That means the airports were operating at 50-60% of total capacity. That is bad.
So I was excited to see the delay at Philadelphia airport yesterday. It was a short delay (under 30 minutes) and didn't take long to clear, but it meant that PHL had, at least briefly, gotten back to the point that the total number of planes arriving and departing was above its bad weather capacity. No one likes delays, but I was happy to have this be one more piece of evidence that we are getting back toward normal.