Graeme Clode of Dunedin asks :-

Why does it take an hour longer to fly from Los Angeles to Auckland than it does to fly from Auckland to Los Angeles?

Gavin Fisher, an atmospheric physicist with the National Institute for Water and Atmosphere, Auckland, responded.

Like many things in our lives, it is all to do with the weather.

Aeroplanes move relative to the air. If the air is moving also (which it almost always is), then this affects how the aeroplane moves relative to the ground. Pilots talk in terms of 'airspeed' (the speed with which they move through the air), and 'groundspeed' (the speed relative to the ground) and the two are different.

The way the air moves (ie winds) can vary enormously. Because of the way the world turns on its axis, the atmosphere generally circulates in a westerly direction. This is not true everywhere. In the tropics it is more easterly and not at all times, but on average, in both the northern and southern hemispheres, we get mostly westerlies. The so called 'zonal' average westerly at the latitude of Auckland is about 15 km/hr at the surface of the earth, rising to about 70 km/hr at the height at which aeroplanes fly. Within this, it is common to have 'jet streams' which can contain winds of up to 150 km/hr.

So if a high altitude aeroplane gets into a jet stream this can add 150 km/hr to its ground speed if its going with the jet stream, but slow it down by 150 km/hr if it is travelling against it.

Looking at the Auckland to Los Angeles flight, the Boeing 747s often used have an airspeed of about 800 km/hr. The distance over the ground is about 10,400 km hence the flight time should be 13 hours. But on the Auckland to LA leg, the flight is going to encounter a lot of westerlies (on average), helping it along, so it might have a groundspeed more than 800 km/hr, perhaps as much as 900 km/hr giving a flight time as short as 11 hours 30 minutes. On the LA to Auckland leg it is going to be flying into the westerlies, with an average groundspeed of perhaps as little as 750 km/hr - giving a flight time as long as 14 hours!