Rebecca Priestley: What causes turbulence?

by Rebecca Priestley / 12 April, 2016
Photo/Getty Images 

What causes aircraft turbulence and do we have more of it in New Zealand than elsewhere in the world?
Earlier in the year, I enjoyed a smooth, uneventful two-hour flight from Portland to Los Angeles. The skies were mostly clear, there were views of mountains, forests and farmland, and the weather in LA was sunny and calm. As we approached LA Airport, the Alaska Airlines pilot apologised for the turbulent flight. I’m a nervous flyer and sensitive to turbulence, but there had been only a few tiny bumps during the trip. It made me wonder if what we endure flying in New Zealand, particularly in Wellington where I live, is more extreme than in most parts of the world.

I asked MetService aviation forecasters Tom Adams and Ravi Kandula what causes the turbulence that can make flying in New Zealand a bit of an adventure.

Jets typically fly at altitudes of up to 11km, but Adams advises that most turbulence is found in the lowest kilometre of the atmosphere. The kind of rough air you get close to land is called “mechanical turbulence”: the more wind and the rougher the ground surface – from buildings, vegetation and topography – the greater the turbulence. Eddies in the airflow create gusts and temporary changes in wind direction, as the air swirls around the obstacles. “Over a city, the gusts can be more than twice as strong as the mean wind speed,” says Adams.

So, why is landing at Wellington so notorious? “Wellington has a unique topo­graphy,” says Kandula. “It is close to a large body of water, and the ranges to the east of the city channel the wind strongly north-south. So you’re likely to get either a very strong headwind or a very strong tailwind.”

This channelling mostly affects surface air, says Adams. “So you might have a westerly a few thousand feet up, but a northerly at the surface. This dramatic change in wind – called directional shear – creates turbulence and also a sudden change in lift as planes come in to land.”

Roller coaster: Wellington’s unique topography causes tricky winds. Photo/Phillip Capper 

DANGEROUS DOWNDRAUGHTS


“Convective turbulence” is often experienced when flying through clouds. “You’ve possibly noticed a jolt as a plane comes in to land through a cumulus cloud,” wrote Adams in a 2015 MetService blog. “The jolt is due to the strong upward motion of air that was creating the cloud in the first place.”

In bigger convective clouds, such as towering cumulonimbus, which can reach 11km or so up into the sky – to the altitudes where jets are flying – these upward motions are much stronger and sometimes accom­panied by dangerous downdraughts.

“Pilots try to avoid flying through cumulo­nimbus clouds because of turbulence, but also due to the hazards of hail, lightning and liquid water at temperatures below freezing that can freeze onto the airframe,” wrote Adams.

As in all mid-latitude countries, New Zealand experiences high average wind speeds. But another cause of turbulence, says Kandula, is the mountain ranges of the South Island, and the ranges that stretch from Wellington to East Cape.

Lenticular clouds, such as those seen in the nor’west arch formed by airflow across the Southern Alps, can be an indicator of turbulence. Beneath these clouds, aircraft can encounter dangerous downdraughts, rotors and strong turbulence.

VISIBLE SIGNS

What about the unexpected jolts that can happen in clear skies over the Pacific? The cause of this is “clear-air turbulence”. Kandula says this sort of turbulence, which has no visible signs, can occur when flying through a jet stream or an air-mass boundary. Meteorologists can forecast it using satellite imagery and computer models and can alert pilots to give them the chance to avoid severe clear-air turbulence and warn passengers of any mild or moderate forms.

Darin Stringer, Air New Zealand’s general manager pilots, says flight-deck crew use information from feedback mechanisms, such as aircraft sensors, as well as weather reports to try to avoid turbulence. “Not only can turbulence feel uncomfortable for ­passengers and crew, it’s also inefficient for the aircraft because it needs to be flown slower than normal cruise speeds.”

Reassuringly, Stringer says “aircraft are manufactured to withstand the most severe turbulence and pilots and crew are well trained to deal with unexpected turbulence and manage passenger safety. The discomfort passengers feel in the aircraft cabin does not mean the aircraft is in any danger, and provided passengers and crew are seated with their seat belts securely fastened, there is very little risk.”

When the aircraft hits turbulence, the pilots “work to ensure the journey is as smooth and  comfortable as possible and that smoother air is reached quickly”.

ACCURATELY PREDICTED

Is what we endure in New Zealand more than is experienced in most parts of the world? Stringer doesn’t think so. Other mid-latitude countries have similar degrees of turbulence from wind flowing over terrain such as mountain ranges and hills. Although tropical cyclones, tornados and clear-air turbulence can be extreme, they do not occur regularly in New Zealand and can be reasonably accurately predicted and avoided.

As for my Portland to LA flight, although there could occasionally be a bit of convective turbulence along this route, Kandula says most of the turbulence associated with the mountain ranges that run parallel to the Pacific Coast would have been felt further inland on the leeward side of the ranges.

A comparable New Zealand flight, from Wellington to Dunedin flying east of the Southern Alps, where downdraughts, rotors and other forms of turbulence are more prevalent, is much more likely to be bumpy.

But even that’s nothing to worry about. “I don’t imagine there are too many pilots that would say they enjoy flying in turbulence,” says Stringer, but “extensive training means we’re well equipped to deal with it.”

More information at blog.metservice.com/node/1066

Read more: Air New Zealand chief executive Christopher Luxon profile

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