Bad weather is by far the number one reason why airports slow down. When it hits, ATC (Air Traffic Control) will implement measures such as ground stops or flow controls.

If it's snowing heavily in Montreal for instance, departures from Toronto may be instructed to remain on the tarmac (ground stop) or be given a specific "wheels up" takeoff time (flow control).

At a high traffic airport, you might see an aircraft sitting conspicuously on the tarmac, patiently waiting for their turn to depart. Toronto Pearson has aircraft parked at the deice facility, whereas Chicago O'Hare has a "penalty box" where aircraft wait for a gate to open.

Airborne flights meanwhile can be assigned holding patterns, or asked to fly in a specific direction or reduce their speed to allow controllers to bide time.

This article originally appeared in the February 2018 edition of Air Canada’s enRoute Magazine as Takeoff with Captain Doug.