Taking off over Dulles today and getting a good look at its runway configuration (as I have dozens of times before), it occurred to me that the wide runway centerline spacing likely helps to keep delays at IAD a minimum even in inclement weather, as the runways are almost certainly far enough apart to allow parallel instrument approaches.

Thinking about other major hub airports, they do seem to categorize themselves into two categories: generally "reliable" and generally "unreliable."

Generally reliable would be (IME) airports that have consistently good weather, the ability to handle operations smoothly in inclement weather, runway/taxiway/terminal designs that contribute to good flow and a minimum of congestion, and/or other considerations that help to keep delays at a minimum the majority of the time.

Compare LAX vs SFO. Ignoring the current runway construction at LAX, LAX is generally pretty reliable: clear weather nearly all of the time and a runway setup that allows simultaneous parallel instrument approaches in the relatively rare event of low visibility. LAX is a massively busy airport, but I cannot remember the last time I've flown to LAX and had a ground hold preventing us from departing or having to enter a holding pattern for flow control. (The worst I've ever seen has been maybe a few minutes of waiting for congestion in the alleys between terminals.)

SFO, on the other hand, regularly experiences fog, which, due to its narrow runway spacing and inability to handle simultaneous instrument approaches, cuts the arrival rate by more than half, leading to rippling delays and flight cancellations.

In my experience, I've rarely been delayed at IAD due to weather or ATC (usually my delays have been due to late-arriving aircraft from other United hubs). Their three north-south runways are spread super far apart, so they can accommodate three simultaneous instrument approaches even in poor visibility. Newark, on the other hand, is a miserable pit of delays. They can only accommodate one approach at a time in their frequent bad weather (they suffer from both incessant summer thunderstorms and bad winter blizzards, whereas IAD experiences iffy winter weather much more rarely).

IAH does have summertime weather issues, but their five runways consist of three that are spaced far enough apart to accommodate three simultaneous approaches. DFW's runway configuration isn't quite as good--they can only accommodate two simultaneous instrument approaches--and they seem to have a greater incidence of both summertime thunderstorms and truly horrific wintertime ice storms (fewer hurricanes, though).

ATL gets backed up but the airport layout is pretty efficient (including some nice touches like Taxiway Victor) and, with the completion of Runway 10-28, allows three simultaneous approaches. CLT, too, allows three simultaneous approaches and is quite a bit less busy than ATL, meaning that recovery from delays is easier.

You'd think that MSP would be miserable in the winter, but the airport's used to this thing called snow that most other airports seem to act like they've never seen before and handles it very well. I've flown through many times in winter and have never once had a weather delay.

ORD is in its own category of year-round bad. JFK and LGA aren't much better.

SLC and DEN are both usually OK; DEN occasionally has issues with snow and thunderstorm delays, but its widely-spaced runways and efficient airport and taxiway design helps. PHX has, of course, reliably clear weather.

SEA's very good--although it rains a lot there, they can now (with the opening of Runway 16R-34L) accommodate two simultaneous instrument approaches, enough to easily accommodate its current activity. Plus, it's not really "rain," it's more of a "falling mist" originating from higher clouds that allow visual approaches once arriving aircraft break through the relatively high ceiling.

In any case, as I thought about this, I thought it would be interesting to have a discussion here about the topic (I'm sure there's a lot of considerations that I've missed) and perhaps even craft a wiki with a list of generally reliable vs. unreliable airports that may even help folks make their travel plans to minimize the possibility of IRROPS.