Winter delays are often attributed to runway conditions, but the more common constraint is the treatment applied to the aircraft itself. The reason is aerodynamic rather than procedural.
Why any contamination matters
A wing generates lift through the smooth flow of air over a carefully shaped surface, and that shape is engineered to close tolerances.
Frost, snow or ice disrupts the flow near the surface, reducing lift and increasing drag at the moment of takeoff when margins are smallest.
Even a thin, rough layer has a measurable effect, which is why the operating rule prohibits takeoff with contamination present rather than setting a permitted thickness.
What the two fluids do
The first treatment is a heated fluid applied to remove whatever is already adhering to the aircraft, working by both heat and chemistry.
The second is a thicker fluid applied afterward when precipitation continues, forming a protective layer that absorbs falling snow or freezing rain.
That layer is designed to shear off during the takeoff roll as airflow over the wing increases, leaving a clean surface at the moment lift is generated.
Why the holdover time is the real constraint
Protective fluid remains effective only for a limited period that depends on the precipitation type, its intensity and the temperature.
Once that window expires the aircraft must be treated again, which means returning to a pad and rejoining the departure queue.
Heavy wet snow shortens the window dramatically, so aircraft treated during a burst of heavy snowfall can expire in the queue before reaching the runway.
How capacity becomes the bottleneck
De-icing requires equipment, trained crews, fluid supply and dedicated pad space, and all four are sized for typical rather than extreme demand.
During a widespread event nearly every departing aircraft needs treatment at once, and the airport's departure rate falls to whatever the pads can process.
Fluid consumption during a prolonged storm can exhaust local supply, at which point resupply logistics rather than weather determine when departures resume.
Why the ground conditions add a second limit
Runway braking performance is assessed and reported, and reduced values require longer distances that can restrict aircraft weight or delay operations.
Clearing runways takes them out of service in sequence, cutting the number available and reducing the airport's capacity for the duration.
The two constraints compound: fewer usable runways while every departure requires a treatment step, which is why winter delays scale so sharply with snowfall intensity.