Turbulence Types and Causes

I have explained elsewhere that disruption to the moving air is the cause of turbulence. I have flown in wind speeds of 150 miles an hour and the flight has been perfectly smooth, on other occasions I have been in winds of 4 or 5 miles an hour and it has been very bumpy.  

When air is moving across the earth’s surface it will be slowed down by contact with all the bits and pieces that make up our planet. Trees, mountains, cliffs, buildings, anything that is on the ground in fact. However, it only slows the bit of air that is within a few hundreds, possibly thousands of feet, of the ground. The air that’s higher up will be traveling faster and will probably be smoother too.  When a plane is near the ground then on a windy day it’s likely to be bumped around by the changing wind speed and direction. At Heathrow airport, for example, there’s a large airport building off to the left of the westerly runway, which, when the wind is in the right direction can make the moments before landing very bumpy. But don’t be alarmed, it’s not a problem any more than a bumpy road is when you drive your car.

The sort of turbulence I’ve described so far is the movement of the air caused by colliding with something. Either on the ground or by colliding with another air mass.  The biggest movement of air as I mentioned is the air moving from the equator and then colliding with other air masses. Near the equator, there are many instances of this occurring over smaller distances than I mentioned earlier. This pattern is constant between the equator and the tropics and within that area the weather pattern and behaviour is called the the inter-tropical convergence zone (ITCZ ) where thunderstorms are often present. 

Because there is a lot of air movement in this area it is likely to be turbulent BUT because thunderstorms are common it is quite often the case that most of the bumpiness is close to them which means that in between them the air can be quite smooth.

There are other areas of the world where there are regular ‘patterns’ of air movement associated with certain types of weather. For example, over the North Atlantic there is a constant stream of weather fronts passing from west to east. These weather fronts are associated with low and high-pressure air systems. A warm front is when the warm air slides gently up a mass of cold air causing flat extensive rainy clouds. 

By contrast, cold air pushing under a block of warm air will force it to rise quickly forming towering clouds that are often associated with turbulence and bumpy conditions.  At different heights these interactions of air masses cause different things to happen, so on your flight as you pass from one area or one height to another then the weather conditions will change and so will the likelihood of turbulence. Despite all this, it’s still difficult to say exactly when and where turbulence will occur. It’s only over very large areas where the conditions are fairly steady that areas of turbulence can be forecast, and it is this information that the pilots will receive in their pre-flight briefing.

I wonder if you have been on a very windy beach and used a windbreak to shelter yourself from the wind. If you have then I expect that you didn’t give the slightest thought to where the wind went after it had hit your windbreak.

For certain it didn’t just disappear … it went either around your windbreak or over it. This is important to know because I want you to think of what happens to the wind when it strikes a mountain range such as the Alps, the Rockies or the Andes. 

It is, as you’d expect pushed up over the hills and would certainly be a cause of turbulence. But that’s not all, we already know that air behaves like water and we know that in a fast flowing stream an obstruction in one place will cause ripples for some distance downstream. Well, that happens with air too. After the wind has hit a mountain range, it will go up and then fall again. When it hits the ground, it will bounce up again until it has lost all its energy. So quite often when I was flying from London to Rome I could feel the effects of the air being blown over the Alps hundreds of miles before I flew over them.

Most fearful flyers have heard of CAT Clear Air Turbulence and have great misgivings about it. For some reason, there are all sorts of myths and tales about CAT as if it were different from any other sort of turbulence. 

Clear Air Turbulence is most frequently encountered at high altitude (above 25000 feet) and is normally caused by a change of temperature in the air over a short distance and by colliding air masses. It has no special significance apart from being invisible to the older types of weather radar although there is progress on the radar that will detect CAT.

When a cloud develops it is the result of rising air. The air will rise because it is warmer than the surrounding air, this may be when the sun’s heat warms a small area like a village or sometimes a cloud will form over an industrial site or power station. In these cases, the air will rise until it reaches a level where it is the same temperature as the surrounding air. Usually, air contains a lot of moisture and when, just like the steam on your bathroom tiles, it eventually cools down and releases all the water vapour which turn into tiny drops of water and makes a cloud … or in the bathroom, condenses into large drops of water. 

After a while, the cloud will decay and the air will cool become heavier and fall back to a lower level.

It’s when you fly through the air as it goes up or down … that you feel a bump!

Now then let’s think what would happen if the air didn’t cool down as it rose …suppose the air already in the sky stayed cooler than the rising air… the air would keep rising. In fact, it can start rising even faster and as it gathers momentum might suck extra air upwards. In this case, it’s likely to develop into a thunderstorm.  This will be making the surrounding air quite unstable so it’s likely to cause turbulence in the area of where the thunderstorm is developing. 

The rules of the air say that we may not fly closer than 20 miles to the centre of a storm … however, even at this distance, it can still be very bumpety bumpy!

Now it’s time to think about how the plane flies during turbulence. There is a video on this site where someone who is terrified of flying is actually flying a flight simulator in severe turbulence.

It’s interesting to note that because the pilot has a lot to be distracted by the level of fear is immediately lowered. The next thing is that because he is busy doing things … like strapping in and flying the plane his attention is diverted from the movement of the plane and all the sensations that would normally concern him. On a lighter note while he was securing his straps he forgot to keep flying the plane and as a result, the plane climbed a little … but sufficient to sound a warning. The important thing to understand from this is that despite the fact that no-one was flying the plane and it was in severe turbulence it still flew quite normally and only gained 300’ in height.  A wonderful demonstration of how stable a plane is and confirming the fact that it’s easy to fly a plane in turbulence, and that it doesn’t just drop out of the sky or go out of control. 

To fly a plane in turbulence, even in the most bumpy circumstances is undemanding. Firstly because it’s not moving as much as you think it is and of course the controls are designed to deal with anything that the plane is likely to encounter. 

You would be very surprised to see how little you have to move the controls to keep the plane flying on course.  The automatic pilot can do it even better than a human.  The plane doesn’t change height more than a few feet in turbulence even though while you are experiencing all the bumps you probably think it’s dropping thousands of feet. What you feel is not the same as what’s going on I promise you. 

It’s a perfectly understandable thought to have when you’re in turbulence … “is this plane capable of surviving?” Yes of course it is or it wouldn’t be flying as a passenger aircraft. It also has natural stability.

Let’s first think about the strength of the plane. The wings of any new aircraft are tested to and beyond their design limits. Before production starts the wings will be tested until they break. There are films on the internet showing this happening. The easiest way to understand how strong the wings are is this. Imagine that you are sitting next to the window in your plane looking out at the wings tips. Imagine that the wings started to bend upwards, more, and more, and more. Imagine that they bent upwards so far that even with your face pressed against the window you could not see the end of the wing. Even then the wings would not break. The wings could bend more and more before they would fail. That’s one point … however, the plane would have to fly at a speed so far beyond its maximum  AND  THEN pull up into a climb so steep to make the wings bend to that position that it would be an impossible thing for the pilots to do. The wings are not going to fall off … whatever it feels like to you.

The next thing about the plane I want you to think about is the way it’s designed to help the pilot to control it. We know that on a motor car the steering wheel always returns to the straight-ahead position after you’ve turned the car. If you’re using the correct steering technique you allow the wheel to return to the ahead position by keeping your hands in position and feeding it back to the ahead position. After a few months driving, we don’t bother with that … we just let go and then grip it again when the car’s going the way we want it to. A plane has similar qualities.

It is designed so that if the wing goes down then it gets extra ‘lift’ to bring it up again. If the nose goes down or up then the tailplane at the back of the plane will apply a force to get it back to the position where the pilot had set it. The pilot can set the nose into a position and then use the tailplane adjustment to keep it there. So the natural balance of the plane will bring it back.  Also at the back of the plane is the upright bit where the airline usually identifies itself. The bit (the Fin and Rudder) acts like a weather vane. If the plane slips sideways then the tail will point it back into the direction it was traveling. 

Finally, in the steadiness of a plane, we need to think about the size and speed of it. The laws of motion say that if something is moving it’ll keep moving unless there’s a force to slow it down, speed it up or change its direction AND the faster something’s going the bigger the force needs to be to change it. So a plane will tend to keep going the way it’s going. And because of that Law of motion, you can’t get things thrown around inside a plane in turbulence they just move in relation to the plane. 

Once again you can confirm that by watching your drink on your table in front of you. You’d think that the water/contents would spill everywhere but almost always it just bounces around and stays in the glass.

It’s quite understandable that someone who is anxious about flying and who knows very little about planes will think that the plane may not be able to withstand all the forces on it and that the wings might ‘fall off’.

First, the wings can’t fall off because they’re made in one piece … they’re not attached to the side of the cabin … the wing goes all the way through to connect with the wing on the other side. Although to you it seems as if the plane is falling and out of control, in reality, it’s very different. In fact, it might be a good idea to have a look at the section on sensations and physiology in the library available on this site. It is a fact that the feeling of falling is more noticeable than the feeling of going up so it’s not surprising that most people feel that the plane is falling during turbulence. It’s worth saying that one of the first things that a pilot has to learn when flying the plane by instruments is to ignore everything that he feels … you have to believe the instruments because they are correct and your feelings are always wrong. 

If you happen to have a drink on your table when you are in turbulence just see how little the contents move around…you really will be amazed.

Turbulence is the movement of the aircraft caused by the movement of the air through which the plane is flying.

Here are some definitions of turbulence levels.

Different intensities of turbulence

Light turbulence – briefly causes slight, erratic changes in altitude and/or attitude.

Light chop – slight, rapid and somewhat rhythmic bumpiness without noticeable changes in altitude or attitude.

Moderate turbulence – similar to light turbulence, but greater intensity. Changes in altitude/attitude occur. Aircraft remains in control at all times. Variations in indicated airspeed.

Moderate chop – similar to light chop, but greater intensity. Rapid bumps or jolts without obvious changes in altitude or attitude.

Severe turbulence – large, abrupt changes in altitude/attitude. Large variation in indicated airspeed. Aircraft may be temporarily out of control.

It is worth noting that the statement temporarily out of control means for a few seconds and not immediately responding to the requirements and control inputs of the pilot NOT that it is out of control and falling out of the sky. The natural stability of the plane will ensure that it flies within certain limits.

what happens inside aircraft vary from the occupants feeling slight strain against their seat belts where unsecured items are slightly displaced, through to occupants being forced violently against seat-belts, and unsecured items being moved about.

A jet stream is a flow of air caused by differences in temperature they usually run east-west and west-east across the planet. When present in the atmosphere the routes taken by aircraft are selected to take advantage of these fast flowing streams of air. At the boundaries of them, there is often an area of clear air turbulence. 

When you stand by the side of the road, waiting to cross you can often feel the blast of wind that accompanies a passing vehicle. The bigger the vehicle and the faster it’s moving will determine how big the blast is. It’s the same with planes. As they go through the air they leave a trail of disturbed air behind them. This air has been forced over the top of the wing and had the pressure reduced, some of the air passed under the wing and had its pressure greatly increased and the rest of the plane would have upset the air anyway. The result of all this air coming back together again is an enormous swirling and mixing of the air. If you happen to fly through this air … you’ll feel a really short sharp shock.  That’s wake turbulence. Its effect was unknown prior to the large jets like the jumbo but once they came into service it was discovered that any plane flying through their slipstream behind them would feel the turbulence.

Smaller aircraft following a jumbo when it has taken off have a two-minute delay imposed on them before they can start their take off.  On approach, the distance a small aircraft has to be behind a jumbo is greater than if another large aircraft were following it.

A big part of this turbulence is the spinning air that comes off the ends of the wings. This is reduced by the upright mini wings at the ends of the wings of most modern commercial jets.

Another important subject which worries some travelers is microburst. This is something that happens normally in the vicinity of thunderstorms but which has a very simple means of detection.

When a thunderstorm is fully developed it will have sucked thousands of tonnes of air into its making. The air carries moisture with it and the cooling of the air can be a trigger for the cloud to grow and grow. Eventually, however, the cloud must start to decay. It is during this period that microburst can occur. When the cloud is collapsing it is unable to hold the air inside it. This air having cooled now becomes heavier and descends. This gathers momentum as it falls and from the bottom of the cloud an enormous mass of air spills out. Because this happens near the ground and involves such a large mass of air it hits the ground and spreads out in every direction. This is a microburst.

Imagine pouring a bucket of water onto the ground in front of you. Some will spill on to your legs some will go sideways and some will splash away from you. I want you to now to think of that water being air and instead of spilling over a small area affecting an area of several miles. 

Think of a plane flying towards this microburst area. As the air suddenly spreads out the plane will encounter a wind blowing towards it this increases its speed and because of that the wings get more lift and the plane starts to climb. The pilots would control the speed by reducing power, just like you would take your foot off the accelerator (gas pedal) in your car if you were going too fast.  After a few moments, the plane would enter the area where the air is just falling and this would mean that the plane would descend. This will be made worse because the pilot has just reduced the power to control the speed. Suddenly it would be necessary to increase power considerably to reduce the descent rate.

Then having passed through the area of falling air the wind would now be blowing from behind and this would temporarily reduce the speed of the plane so even more power would be needed.

Now, none of this is difficult … or at least it wouldn’t be if it didn’t take so long for the aircraft to respond to these changes in speed.  

From the pilot’s point of view, the difficulty is in anticipating what is going to happen and having enough time to deal with it. It is common practice now to train pilots to deal with this sort of experience. There is also equipment on board to detect this.

But it is always better to avoid a problem in the first place than to deal with it when it happens. It took only a little research to devise a way of predicting this phenomenon. If the direction of the wind over a particular area was not steady, but in fact radiating from a point, then it could be predicted with great accuracy that there was a microburst taking place. If a simple observation showed that a thunderstorm was nearby then a microburst and wind shear was a certainty.

I have not mentioned wind shear previously because I wanted you to concentrate on the features of the microburst. 

The effect that a changing wind speed has on an aircraft is a difficult concept. The easiest way to think of it is like this. Imagine you are cycling on a calm day. You know that it takes a certain effort to travel at a particular speed. You know also that at that speed the wind is blowing at you at a particular amount. If you’re cycling on a very gusty and windy day you know that when there’s a sudden gust in front of you that you’ll have to peddle a bit harder to keep going. If the gust comes from behind then you don’t have to peddle so hard to keep the same speed.

The problem with a plane is that it needs a certain amount of wind over the wings for them to work. During the approach to land, we set the speed at the correct value and the speed control system will keep it there. If there’s a sudden gust like on your bike the plane will pick up speed and start to go up. The pilot will counteract this by reducing the power and ‘lowering the nose’ .  However, the next bit of the microburst is where the plane enters descending air and now the pilot has to do the opposite of what he’s just done. More power required and nose up. This may happen in just a few seconds. The next thing that the plane experiences is a sudden wind from behind the plane. This means that the amount of air going over the wings is reduced and so the pilot needs even more power and more nose up to keep flying … sometimes there hasn’t been enough time or power or speed for the plane to do this … with disastrous results.

That’s why landings in these conditions are not allowed and never undertaken. 

Sometimes in flight, you’ll experience very very small versions of this … and as you can imagine would be a bit of a bumpy ride!