In Flight Air Turbulence

One of the nagging worries with any aircraft flight is turbulence. It sometimes strikes without warning and can produce a frightening experience for even the most seasoned of fliers.

Mostly turbulence is only minor – a series of “bumps” that might last for a few seconds - but at the other end of the scale it can be far more serious. Major and abrupt vertical and horizontal changes in the position of the aircraft can throw loose objects and even people, around the cabin with injuries sometimes resulting. And at the extreme end of the spectrum turbulence can produce structural damage to the aircraft and in some very rare cases result in an aircraft accident.

Turbulence has four broad categories:

(1) Light: slight “bumpiness” but with little of no discomfort for passengers.

(2) Moderate: Passengers walking down the aisle will be moved off track; loose objects in the cabin will move about.

(3) Severe: Any standing passengers will be thrown off balance. Loose objects will pitch onto the floor.

(4) Extreme: Unsecured passengers thrown violently about – they may strike the roof or lateral areas of the cabin. Loose objects turn into interior missiles.

Cases three and four are quite rare but when they do occur can cause injury to passengers or cabin staff. In Australian airspace we see about between 2 and 3 reported significant turbulence encounters per month, or 24 to 36 per year. In terms of the number of flights and people carried during this time this represents only a very small percentage of flights.

In flight turbulence is caused by small scale currents or vortices in the air that produce “bumpiness” when the aircraft flies through it, similar in some ways to a car driving over a series of corrugations in the road. There are several ways turbulence is produced in the atmosphere.

Thermals: These are rising parcels of air generated by the atmosphere being in contact with a warm surface below (land or ocean). If there is sufficient moisture in the air, cloud will form over these thermals and form billowing cotton wool formations called cumulus cloud. An aircraft flying through this type of cloud will usually experience light to moderate turbulence.

Aircraft flying through cumulus clouds will normally experience turbulence because of the thermal activity. (Click to enlarge)

Thunderstorms: If rising air continues to be generated in a cumulus cloud it can grow to prodigious heights before it finally flattens out, typically at heights of 10 to 15 km above the ground. When the cloud grows to this height it contains regions of powerful “up and down” draughts in which there is a mixture of rain and hail, with lightning and thunder also added to the mix. This is a giant cumulus cloud, more properly called a cumulonimbus and is popularly known as a thunderstorm.

Pilots will avoid penetrating a cumulonimbus cloud because turbulence inside can be severe to even extreme. Fortunately these clouds are highly visible by day and also easily detectable on radar both day and night so are readily avoidable in a modern passenger aircraft.

Moderate to severe turbulence is often encountered within cumulonimbus clouds because of the violent mix of updrafts and downdrafts within. This diagram is not drawn to scale - in reality the aircraft would be less than the size of a pin head here. (Cloud image from Wikipedia Commons, click to enlarge)

An infamous case of thunderstorm penetration involving the loss of an aircraft occurred in November 1961 at Mascot airport in Sydney. See

http://passingparade-2009.blogspot.com.au/2009/06/into-storm-flight-of-tango-victor.html

Mechanical turbulence: As wind passes over the ground, air in contact with the surface is slowed by friction, while the air above continues at the unimpeded speed. This creates many thousands of rolling vortices, and depending on the wind speed and roughness of the terrain, these can extend upwards for a considerable height. Aircraft flying in these regions will encounter “bumpiness” called mechanical turbulence, that is usually only in the light to moderate range.

However in some cases, when strong winds blow over a mountain range, the air can organise itself into large organised waves that extend several kilometres downstream of the mountains. Very powerful rising and falling air currents are often present in these waves and organised bands of clouds sometimes form near the wave peaks. These waves are called lee waves or mountain waves and the clouds are called lenticular because of the characteristic “lens” shape they often adopt. Just as we see with waves on the ocean, mountain waves sometimes topple and break producing circulations called rotors.

Mountain waves (or lee waves) can produce severe turbulence. Clouds sometimes form at the peak of the waves. (Click to enlarge)

Turbulence produced by mountain waves can be moderate to severe, and in extreme cases, the actual destruction of an aircraft has been recorded. Perhaps the most infamous of these was the disintegration of a Boeing 707 jet, Flight 911 that was flying near Mt. Fuji in Japan on February 4^th 1966. It is believed that the aircraft flew into an area of powerful mountain waves that produced extreme turbulence and failure of the air-frame. A detailed analysis of this incident can be found at

http://en.wikipedia.org/wiki/BOAC_Flight_911

Wind shear:

Put simply, wind shear is the change of wind speed and direction with height or over a horizontal distance. There is always a degree of wind shear in the atmosphere, but when big changes take place over short distances, significant turbulence can result.

Wind shear turbulence can form when one layer of air slides across another, at a different speed or from a different direction, or both. This can generate wave like disturbances at the interface, called Kelvin Helmholtz waves, and if there is cloud present in this zone, the waves become visible.

Kelvin Helmholtz waves form when the winds above a certain layer (the dotted line here), are stronger than those below. (Image from Wikipedia Commons, click to enlarge)

But in many cases there is no cloud to alert the pilot that unusual atmospheric motion may be present and in these cases the phenomenon is referred to as “clear air turbulence” or CAT.

Jet streams, that are high-speed rivers of air that circulate the globe at approximately 10 to 15 km above the surface, are areas where CAT is often encountered. There are two main jet streams in each hemisphere, the polar jet and the subtropical jet and these meander and change direction as they circle the earth.

The main jet streams of the Earth. (Image from Wikipedia Commons)

Because jet streams are zones of high-speed winds running within the broad-scale atmospheric flow, there are surrounding regions where the wind speeds change quickly with both height and horizontal distance, and this can generate considerable wind shear and therefore turbulence. For more information on jet streams see

http://passingparade-2009.blogspot.com.au/2010/04/jet-streams-and-tropical-cloudbands.html

To get the issue of in flight turbulence into perspective it should be noted that even the most frequent fliers will never experience severe or extreme turbulence, and that the modern passenger jet is fantastically strong and well able to handle in flight turbulence in all its forms.

But for the passenger, by far the best protection against turbulence is to fasten your seatbelt. When not moving about the cabin keep the belt buckled up, and even if severe turbulence is encountered you will safely ride it out if properly secured.