While snow-lovers lament the late start to winter across much of the US, you'll find relief among at least one demographic: aviation operators.
Before we dive in, we'll also share explainers that we've written on airport capacity, delay propagation and traffic management initiatives - while not prerequisites exactly, they may provide some helpful background knowledge.
Like rain, snow can reduce airport capacity by way of lowered cloud bases and visibilities; when precipitation freezes, however, it introduces additional complexities (read: delays) in the way of snow removal and deicing. Let's tackle snow removal first. Whereas lowered ceilings/visibilities require increased spacing between arrivals for a runway (thus reducing runway capacity by relatively few slots), during the time that snow removal equipment is clearing a runway (and turnoffs), the runway is closed entirely. This duration can vary - from around 10 minutes for an urgent single pass to around 30 minutes for more routine, ongoing closures to upwards of 60 minutes for a full clean-up. During steadier and heavier snowfalls, closures are typically successive, such that upon completing one runway [and re-opening it], the snow removal team will move immediately to another runway [and close that one]. Though successive closures significantly reduce airport capacity, the monotony at least enables better planning.
At this point, perhaps readers are recalling a time when they transited a snowy hub and it felt anything but well-planned. Perhaps chaotic is the adjective that instead jumps to mind. Winter operations place an airport on a knife's edge (even more so than normal) and a couple poor braking action reports threaten to break that monotony. Allow us to back up: snow removal efforts ensure that pilots are able to decelerate the aircraft and maintain its directional control. To monitor the effectiveness of snow removal, the FAA tower will solicit braking action reports from arriving pilots, who grade braking performance on a scale of good, medium, poor and nil. A nil braking action report immediately closes a runway; consecutive poor reports triggers an assessment that often closes the runway. An unexpected closure quickly dissolves the orderliness of successive closures and can even close an airport to air traffic (if no runway is open). A poorly-timed driver turnover can similarly cause an airport to fall behind in its snow removal efforts.
While runway surfaces are given priority when clearing snow, the airport must also provide access to runways via taxiways. The airport may focus efforts on a relatively limited number of taxi routes - this can lead to congestion (or worse off, gridlock). We'll also mention that aircraft parking areas need to be cleared (to ensure aircraft are aligned properly and permit baggage equipment, jet bridge, etc. access to aircraft), which often requires some touch-up between a departure pushing back from a gate and the next flight arriving at that same gate. There's also the matter of what to do with all this snow being pushed around. Minneapolis-St. Paul Airport (MSP), for example, has nearly 650 acres of pavement (about 44,000 average driveways). Often the answer is large snow melters (and we mean large - like stretching to nearly 70 feet and melting more than 150 tons of snow per hour). If you're thinking that's nearly the size of an airplane, you're not wrong; and when you consider the size of the associated snow pile, it's no coincidence that melters are often set up where aircraft otherwise park. Unfortunately, the reduction in aircraft parking spots and gates can exacerbate taxiway congestion, as arriving flights are more likely to wait for an available gate.
Let's keep the effects of surface snow removal in mind while we transition to the topic of deicing, which is actually a two-stage process when frozen precipitation is falling. A heated, orange-colored liquid (called type I) is used first to clean the aircraft of already-accumulated "contaminant" then a viscous, green liquid (type IV) is applied to prevent accumulation. Similar to surface snow removal, the scale of deicing is pretty impressive. We'll lovingly pick on MSP again, where Delta Air Lines alone uses approximately 1 million gallons of deicing fluid each year. While critical for the safety of flight, the duration of the spray itself nevertheless represents delay in most cases. A separate queuing delay is also possible - like an imbalance between an airport's arrival capacity and arrival demand can result in a delay awaiting an arrival slot, departing flights can await a deicing "slot" if demand exceeds deicing throughput. That said, disruption resulting from deicing is likely manageable during light snowfall, even if steady.
Surface snow removal isn't the only team working on a knife's edge, however, and sleet can immobilize a deicing operation. Allow us to back up again: the type IV fluid used to prevent accumulation (i.e. anti-icing) is only effective for a certain length of time (called holdover time or HOT) and the timer starts when type IV application starts. Holdover time in light snow at 25 degrees should be at least 90 minutes - ample time to complete spraying the aircraft and for it to taxi to the runway. Near- or sub-zero temperatures, sleet (i.e. ice pellets) or freezing rain all conspire to reduce holdover times, however (and takeoff may be prohibited in heavy snow).
Let's take a HOT of 25 minutes, for example, which is plenty plausible given any of the above influences. Type IV application will likely take about 7 minutes, so we're down to 18 minutes by the time we're resuming or starting our taxi. Then there's the question of where did the spray take place - was the deicing location on the same side of the airport as the departure end of the runway? An uninterrupted taxi from even a conveniently located deicing bay is likely about 6 minutes, leaving 12 minutes of HOT. Recall, however, that snow removal often results in more aircraft occupying less pavement and it wouldn't take much congestion to consume our 12 minute buffer. If the timer expires prior to takeoff, the aircraft must be re-sprayed. Perhaps there's a small deicing operation set up at the departure end of the runway for this purpose, however throughput will not be sustainable in most cases. Otherwise, aircraft will be required to return to their original deicing location. The latter scenario can create a vicious circle wherein taxing back to the deicing location aggravates congestion and re-joining the deicing queue increases queueing delays.
Staffing is the last piece of the winter operations puzzle that warrants mention. Even at lower accumulations, airlines are still liable to deal with an uptick in call-outs from employees working outside (e.g. baggage handlers, fuelers). Higher snowfall amounts will hamper employee commutes, especially if public transit is interrupted. Airlines and airports will put-up employees at nearby hotels to mitigate staffing risks during more intense snowstorms; in the most extreme scenarios, airlines will actually temporarily reassign employees from unaffected airports.