An airfield traffic pattern is a standard path followed by aircraft when taking off or landing while maintaining visual contact with the airfield. At an airport, the pattern (or circuit) is a standard path for coordinating air traffic. It differs from "straight-in approaches" and "direct climb-outs" in that aircraft using a traffic pattern remain close to the airport. Patterns are usually employed at small general aviation (GA) airfields and military airbases. Many large controlled airports avoid the system unless there is GA activity as well as commercial flights. However, some kind of a pattern may be used at airports in some cases such as when an aircraft is required to go around, but this kind of pattern at controlled airports may be very different in form, shape, and purpose to the standard traffic pattern as used at GA airports. The use of a pattern at airfields is for aviation safety. By using a consistent flight pattern, pilots will know from where to expect other air traffic and be able to see and avoid it. Pilots flying under visual flight rules (VFR) may not be separated by air traffic control, so this consistent predictable pattern is a vital way to keep things orderly. At tower-controlled airports, air traffic control (ATC) may provide traffic advisories for VFR flights on a work-load permitting basis. Wind direction Pilots prefer to take off and land facing into the wind. This has the effect of reducing the aircraft's speed over the ground (for a given airspeed), thus reducing the length of runway required to perform either maneuver. An exception to this rule is at airports where the runway is on a severe slope, such as alpine airports (altiports). In these instances, takeoffs are usually made downhill and landings uphill regardless of wind direction with the slope aiding in acceleration and deceleration. Another exception is at airports with mountains at one end. Many airfields have runways facing a variety of directions. The purpose of this is to provide arriving aircraft with the best runway to land on according to the wind direction. Runway orientation is determined from historical data of the prevailing winds in the area. This is especially important for single-runway airports that do not have the option of a second runway pointed in an alternative direction. A common scenario is to have two runways arranged at or close to 90 degrees to one another, so