What are the different types of turbulence and how can they affect your flight?

Know your mechanical from your mountain waves? Here's a handy guide to what makes planes go bump in the night

Pilots are trained to mitigate the effects of the many different types of turbulence. Photo: Vladimir Maravic

If you've been experiencing bumpier flights lately, then it's not in your imagination – turbulence is on the rise, according to the experts.

While much of it can be predictable and therefore avoided by experienced pilots, since the weather is getting evermore unpredictable, airlines are having to discover and utilise newer technology to keep track.

Here we outline all the different types of turbulence, and how uncomfortable they can realistically make a flight.

Clear air turbulence

This is the one that's made the headlines lately. Clear air turbulence is defined by the US Federal Aviation Administration as “sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft”.

This most commonly takes place at higher altitudes and is associated with the jet stream – fast-flowing air currents that move from west to east in the upper atmosphere.

A study earlier this year by the University of Reading found climate change had increased the temperature difference between the pole and the equator over the North Atlantic, intensifying the vertical wind shear of the jet stream. This means the change in the speed of west-to-east winds becomes more pronounced with altitude, thereby boosting the chance for clear air turbulence.

This type of turbulence is unpredictable. But Captain Michael Schreiber, chief of pilot technical operations at Emirates, tells The National the Dubai airline, for example, is investing heavily in new technologies to make predictions more accurate. The airline’s pilots also receive training on how to deal with it.

“Practical training around turbulence avoidance and management is conducted in the flight simulator,” he explains. “Pilots are instructed how to navigate around areas of turbulence and how to ensure the safety of our passengers and crew in cases where turbulence is encountered in-flight.”

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Wake turbulence

This type of turbulence is generated by aircraft vortices – circular patterns of rotating air left behind by a wing as it generates lift – or jetwash, which are gases expelled from the engine. It happens when one aircraft crosses paths with another, creating wing tip trailing vortices from the leading plane.

This is predictable and avoidable, and it's why planes have designated minimum separation distances from each other.

Thermal (convective) turbulence

Turbulence can occur on warm summer days, as the sun heats the earth's surface unevenly, causing the warm air to rise in columns and cooler air to descend. As planes fly in and out of these isolated convective currents, the flight experiences bumpy conditions. Rocky or sandy surfaces will also heat more rapidly than grassy fields or water.

Pilots therefore often prefer to fly in the early morning or evening when the thermal activity is not as severe.

It can be particularly prevalent when approaching a landing area, says weather.gov, since the moving convective currents vary in intensity, potentially causing an aircraft to veer from its glide path and over or undershoot the runway.

Frontal turbulence

Warm air is lifted by the sloping frontal surface of a cold air mass, causing friction between the two opposing masses and producing turbulence in the frontal zone. Frontal zones in aviation, also known as fronts, are regions of considerable weather activity.

Weather.gov says this type of turbulence is most marked when the warm air is moist and unstable, leading to a risk of thunderstorms, which would cause more severe bumpiness.

Mechanical turbulence

This is caused by friction between the air and the ground, generated by irregular landscapes or man-made objects (such as skyscrapers) found at low altitudes.

The obstacles cause the obstruction of airflow and the intensity of turbulence will depend on the strength of the surface wind, nature of the surface and the air stability. So, if the wind speed is strong, the terrain is rough and the air is unstable, this would create the greatest turbulence.

Mountain wave turbulence

Weather.gov also defines mountain wave turbulence as a form of mechanical turbulence, whereas other resources define this in its own category.

This occurs when strong eddies – whirls of air – are found downwind from mountain ridges. Mountain waves can produce some of the most severe turbulence associated with mechanical agencies.

It has also been known to cause structural damage to aircraft and result in a loss of control.

Thunderstorm turbulence

While planes can veer away from storm clouds, this is only the visible part of a turbulent region in a thunderstorm, and updrafts and downdrafts can often extend as much as 24 to 48 kilometres outside of the storm, with severe turbulence still possible.

As long as pilots know where these storm clouds are, however, it is possible to avoid this type of turbulence with up-to-date technologies and weather-predicting tools.

Wind shear

Where the FAA defines wind shear as an association with clear air turbulence, weather.gov separates it as its own type. This is the change in wind direction and/or wind speed over a specific horizontal or vertical distance.

Wind shears exist in atmospheric conditions such as areas of temperature inversions, along troughs (an area of low pressure) and around jet stream. When the change in speed or direction is strong, severe turbulence can be expected.

Temperature inversions

Again, weather.gov includes temperature inversions within the description for wind shear, whereas other resources may define it as its own type of turbulence.

In zones of strong stability, the stable low layer can be prevented from mixing with the warmer layer above and therefore lead to temperature inversions. “The greatest shear, and thus the greatest turbulence, is found at the tops of the inversion layer,” says weather.gov.

This type of turbulence can occur due to night-time cooling of the Earth's surface.