Runways can be used in two opposite directions, and each direction is named with a two digit number based on the two different directions they face on a compass.

For example, a runway which runs from due east to due west would be called Runway 09/27, as east is at 90 degrees on a compass and west is at 270 degrees.

If airports have parallel runways, the two runways have the same numbers, but are identified by adding Left (L), Centre (C) or Right (R) after the runway number. For example Runway 09L/27R and Runway 09R/27L.

Air traffic controllers consider a number of things when deciding which runway to use. These can include:

• number and type of aircraft operating at the airport
• length of runway(s)
• weather conditions (both present and forecast) - including wind velocity and gradient, wind shear, wake turbulence effects and position of the sun
• availability of approach aids in poor visibility conditions
• location of other aircraft
• taxiing distances, including availability of taxiways
• braking conditions.

Some airports also have 'preferred runway' systems. This means that if wind conditions, workload and traffic conditions allow, a particular runway will be used to move traffic as efficiently as possible while reducing the noise impact over residential areas.

Wherever possible, aircraft take-off and land into the wind, so weather is one of the most important reasons in deciding which runway to use.

Larger airports often have more than one runway, so a runway is always available for different wind directions. Airports with one runway usually build the runway to align with the prevailing seasonal wind.

Air traffic controllers monitor wind and runway selection at all times, as weather conditions can quickly change. Pilots also monitor and report wind changes and can request a runway that is best suited to the capability of their aircraft.

Once the runway is chosen, it needs to be available for an extended time to allow pilots to plan their descent, approach and landing. As this involves predicting developing weather, aircraft may continue to land on a runway when local weather conditions no longer appear to require it.

In many Australian cities, prevailing winds vary by season. This means that one runway and flight paths may be used more in one season than in another. As a result, some communities may be affected more by noise from arriving aircraft in one season, and departing aircraft in another or some communities may notice flight paths are only used in some seasons.

A sudden change of wind direction when the wind is strong may mean aircraft planning to arrive on one runway need to divert to land in the opposite direction. This can require an immediate operational change. If this occurs, air traffic control will safely divert aircraft. This can lead to aircraft using flight paths over areas that normally experience few overflying aircraft.

Bureau of Meteorology (BoM) wind direction readings use a single instrument, located on the airfield, and are usually made at scheduled times throughout the day.

Our wind observations use multiple instruments, positioned at the end of every runway, and are provided minute-by-minute.