When designing airport runways, engineers and other planners have many factors to consider. These include the type and volume of air traffic, the impact of noise, and restrictions due to nearby developments such as residential developments, tall buildings, radio towers, etc.
The orientation of the runways is of great importance. The orientation affects the traffic patterns of approaches and takeoffs, which must avoid any obstacles in the surrounding terrain and minimize inconvenience and danger to local residents as well as passengers. The local wind conditions also play a great role in this orientation. Planners must have detailed information about prevailing wind directions and speeds, as these affect the ability of an airplane to take off and land. Because the lift, or upwards pull, of an airplane depends upon the speed of the wind over the wings, pilots can take off and land in shorter distances and at lower ground speeds if they do so into the wind. So, for example, an airport with a single runway in an area where the wind is usually from the south will generally orient the runway from north to south instead of from east to west.
The topography of the area near an airport also affects the winds. Nearby mountains or bodies of water can affect flight wind conditions in ways not obvious from data of prevailing wind conditions. For example, dangerous rotor winds can develop in the vicinity of mountains. These are areas of wind that rotate around a horizontal axis. A plane encountering such a wind could experience sudden, dangerous changes in altitude, even in clear skies. Storm patterns must also be considered, with allowance made for winds common in bad-weather situations.
Many commercial airports have parallel runways. This allows a greater number of take-offs and landings in good weather conditions than does a single runway because planes can use the runways simultaneously. Other airports have triangular runways, which can be in the shape of a V that may be open at the bottom so that the runways do not intersect, or closed so that the runways do intersect. Triangular runways are more adaptable to a variety of wind conditions, as there are more possible directions for take-off and landing. In conditions where winds are light, both runways can be used simultaneously in the open-V triangular configuration.