Why do planes fly in circles?
A holding pattern is a racetrack-shaped loop that keeps aircraft in an orderly queue when they cannot land yet.
What a hold is
When an airport is too busy, the weather is bad or a runway is blocked, air traffic control asks arriving aircraft to wait in the air. They fly a racetrack shape: a straight leg, a turn, another straight leg and a turn back, over and over.
Each aircraft flies its loop at its own height, so several can stack above the same spot without getting close to each other. A busy airport may have a stack of aircraft one above another.
Why it happens
Common reasons are heavy traffic, a runway closed for an inspection or an emergency, strong crosswinds, low cloud or fog, and a slow aircraft ahead on the approach. Pilots can also ask to hold to sort out a technical problem.
How long it lasts
Anything from one loop to half an hour or more. Fuel is planned in advance for this, and a pilot who is running low will say so and be given priority.
Watch for it
On the map, an aircraft that keeps flying the same oval at a steady height near an airport is holding. On AirLive it shows as Holding in the Inbound list.