Altitude levels of aircraft in flight are difficult to gauge while standing on the ground. However, the MAC makes altitude information available through its FlightTracker tool. Aircraft may appear to be lower than they actually are because their large sizes make them look closer. Aircraft arriving at an airport will generally fly at a steady angle of descent, approximately three degrees, while aligned with the runway centerline. Aircraft departing an airport will climb out at varied rates due to aircraft performance, weather conditions, air traffic control direction, weight of the aircraft and departure procedure being followed. When the airspace is crowded, aircraft may spend time flying a holding pattern at relatively low altitude in order to ensure a suitable flow of traffic.