Where does turbulence come from and why must a paraglider pilot understand it? Part 1 covers its nature and four sources: obstacles, thermals, shear and rotors.
🌤️ Like many aspects of the weather, turbulence can not be readily seen but we can visualize its behavior. Knowing where turbulence lurks and when to expect it is a major step towards avoiding it. In this chapter, we will learn all about turbulence from the slightest texture to rodeo air.
🌀 What turbulence means
🌫️ If we could make the air visible by adding mist or dye in a large volume, we could actually see turbulence. What we would see would be swirls, whirls and roils of various sizes turning this way and that, interacting and breaking apart as they move along with the wind.
The Meaning of Turbulence
💧 You can readily witness a similar thing by watching the eddy action in a fast moving stream. The swirls or eddies are what we experience as gusts or turbulence when they pass by our body or wings. Here we see a passing eddy from a bird's eye view. A person standing at point A would first experience a gust from the left, then a lighter headwind, then a gust from the right and finally a return to the general wind direction as the eddy passes.
🧠 We thus can agree to a working definition of turbulence as the random chaotic swirling of the air. In truth, some forms of turbulence such as rotors and bumps caused by thermals can be somewhat organized, but randomness of the swirls is what characterizes most turbulence.
🛩️ What constitutes significant turbulence to the pilot involved depends on his perspective and choice of aircraft. To the man on the moon, the great swirls in our visible atmosphere are turbulence. On earth we experience these swirls merely as gradual weather changes. At the other extreme, a butterfly may experience extreme turbulence in a swirl that we feel as a gentle puff. As a rule of thumb, the smaller the aircraft and the lighter the wing loading, the more it will be affected by smaller eddies.
🪽 In the air turbulence affects our wings much like it affected our body in the illustration mentioned previously. The speed and direction of the relative wind will change as the eddy or swirl passes our wings. The effect depends on the intensity, size and orientation of the eddy. Small eddies will feel like rapid bumps, much like when a speed boat skims a choppy lake.
🎢 Eddies from several feet to about the span of your aircraft in diameter will be felt as larger bumps which may cause control problems or weightlessness if they are severe. Larger diameter eddies will be experienced as sudden lift, sink, rolling or yawing impetus. Finally, very large scale eddies will appear as wind speed or direction changes.
🚨 The dangers that turbulence poses to flying are several: A sudden gust may stall our craft which can have severe consequences close to the ground. Also loss of control can occur in turbulence when a wing gets lifted radically or pitch action occurs. Loss of control is also most dangerous close to the ground. In severe cases-most notably in rotors or strong thermals-rollovers or pitchovers can occur. Most of us prefer to do our flying right-side-up. Finally, severe turbulence can cause structural damage to our craft as it tweaks on the wings and rattles us around in our cage. Later we'll look at escape procedures to follow when turbulence greets you with a slap on the back.
Dust devil on the take off
⏳ The life cycle of a turbulent eddy begins when it is formed by one of the three causes discussed in the next section. It then moves with the general wind flow and breaks down into smaller and smaller but more numerous eddies. This process continues downwind until the eddies are so small their energy of motion is converted directly to heat due to the viscosity of the air (about 0.01 inch or 0.25 mm in diameter at sea level). Essentially what takes place is an exchange of energy from large-scale motion to movement on a smaller and smaller dimension. A moving mass of air will lose much of its initial momentum through this mechanism.
🔬 Smaller eddies do not necessarily have less effect on our wings, for they may contain most of the energy of the larger eddy they developed from. You can readily see this in turbulent water when small, fast spinning swirls move inside slower, larger whirlpools. It is only after time and space that normal turbulent eddies lose their punch (rotors never give up as we shall see).
🦠 Turbulence with its snarls and gusts tends to spread out all properties of the air. For example, heat, moisture and pollution are dispersed in all directions by swirling air. Also, wind differences are evened out by turbulence. What this means to flying is that gradients are reduced by turbulence but the turbulence itself may pose more problems than the wind gradient.
🔍 The causes of turbulence
Here we are going to separate the development of turbulence into three sources. They are: mechanical, thermal and shearing actions. Each cause of turbulence is distinct so we will discuss them separately. However, we should be aware that they can appear in any combination. For example, mechanical turbulence and thermal turbulence are often both present close to the ground on hot, windy days.
🧱 Mechanical turbulence
🚧 When a solid object obstructs the path of the wind-be it a mountain, forest, house or football lineman-the flow downwind from the object is disrupted. In very light flow the disruption may be gentle meanders, but as the flow velocity increases standing eddies may develop which give way to the random chaotic eddies of full-fledged turbulence. You can readily visualize this action by immersing your hand in water flowing at various velocities.
💨 The illustration shows the turbulence caused by a solid object in the air flow. Note the difference at different wind velocities. Not only does the stronger wind produce more turbulence but it is more intense and travels further downstream. You will also see the presence of standing eddies in the figure. These are more or less stable swirls that stay in one place and are set up by the shape of the solid. In the atmosphere we call them rotors.
Mechanical Turbulence
🌬️ Occasionally, these standing eddies or rotors may break away and move downstream to be replaced by a new eddy. Generally, though, they are stable and remain in place as long as the flow remains steady. Once the flow increases above a certain value, however, they are broken up and replaced with general turbulence.
✋ The force that a solid object imparts to the air is equal and opposite to the force the air imparts to the object as Newton would have told us. This force can be felt by holding your hand out the window of a moving car. You can't see it but youcan bet your hand is leaving a trail of turbulence. The force is caused by drag due to pressure differences on the front and back surfaces of your hand. Any solid object in the air flow works the same way. Most of the energy acquired by the air from the drag forces on your moving hand goes directly into the creation of turbulence.
📐 Besides the wind velocity, the shape of an object disrupting the flow is an important factor in determining whether or not the air is turbulent. If sharp edges or curves are present on the object, the air will have a difficult time moving uniformly over the entire surface due to the inertia of the air molecules. The illustration shows a variety of shapes and how they affect the flow of air. The first drawing has a cross section which presents the least resistance to the wind and therefore causes the least amount of turbulence. Airplane stabilizers and struts, boat hulls and even trees growing in a steady wind are often of this configuration. The other drawings illustrate how sharp edges or curves can initiate turbulence.
⛰️ Now we have a fairly good idea how irregularities and obstructions from the size of mountains on down that lie in the path of the wind cause turbulence. Since solid objects other than birds and aircraft exist entirely on the earth's surface, turbulence from this source is usually limited to a layer below 1,500 feet (500m) above the highest object. We call this layer the friction layer as indicated in the chart. In this layer some disruption of laminar or smooth flow is expected.
📏 The size of the objects blocking the air's flow is called the roughness of the surface. The roughness determines the initial size of the turbulent eddies. Larger obstructions tend to create larger eddies, but these may quickly divide into smaller cells. Typically, an object creates an initial eddy from 1/10 to 1/7 its size.
📊 Roughness values and eddy size by terrain type
1️⃣ City or Forest: 6.6 ft / 2m
2️⃣ Suburban Homes: 1.6 ft / 50 cm
3️⃣ Farm Crops Sage Brush: 4.0 in / 10 cm
4️⃣ Mown Grass: 0.4 in / 1 cm
5️⃣ Ocean, Large Lakes: 0.012 in / 0.3 mm
🌪️ Once again, the actual effect roughness of a surface has on the wind is determined not only by the size of the obstructions but also the mean wind velocity. In light winds little or no turbulence may occur. In slightly higher winds turbulent eddies may form and the wind direction will become quite variable. In stronger winds (over 20 mph or 32 km/h) the turbulent eddies may become very intense and smaller and travel well downstream before they break up. In this case the variation in speed will be great but the meandering changes in direction is lessened.
The Effect of Shape on Turbulence
💥 The force of the wind and the energy in turbulent eddies increase with the square of the velocity. Thus a wind blowing twice as hard as before will exert four times the force. A turbulent eddy in such an increased wind will be similarly more vigorous.
⚡ RULE: The strength of turbulence increases with the square of the wind velocity.
☀️ Thermal turbulence
🌡️ The second source of turbulence in the atmosphere is convection currents or thermals. When a thermal penetrates upward it disrupts the air it's passing through to form turbulent eddies and other velocity changes. Thermals themselves are usually organized with a mass of rolling lift in the center surrounded by sink around the perimeter. The act of flying through such a mass of air acting in concert often presents one with healthy sink followed abruptly by lift then another abrupt change to sink. Thus we have described the traditional "air pocket" by a bygone era.
Turbulence Caused by Thermals
🎢 The edges of some thermals are turbulent by anyone's standards. Extremely virulent thermals in hot desert areas can exert enough force to roll or pitch a small aircraft over on its back if it is caught with part of the wing in the up air and part in the down air. Fortunately thermals with such a bad attitude are quite rare and sport aviators generally fly safely in most thermal conditions.
Thermal Disruption of the Airflow
🌪️ Thermal turbulence is usually strongest in the lower 2 to 4 thousand feet (600 to 1300 m), but may reach to tens of thousands of feet in desert or in thunderstorm conditions. When thermal turbulence is added to mechanical turbulence in a flowing wind the results can be quite chaotic as shown in the illustration.
🌬️ Even when the general wind is still, thermals can create ground turbulence as they suck in air from all directions when they lift off. The illustration illustrates the effect of thermals on the surface air movement. When thermals rise they send air down from aloft to take their place. If a wind is blowing aloft, this downward moving air will be moving horizontally as well as vertically and will be felt as gusts on the surface. This is the source of "cat's paws" on water and the rush of air you can see in trees or fields of grass on windy thermaly days. In the air these gusts and packets of cool air are felt as moderate to strong turbulence.
Wind Velocity Variation Due To Thermals
✂️ Shear turbulence
🌬️ The third and final cause of turbulence in nature is through the mechanism of wind shear. The word shear means a cutting or tearing and when two layers of air lying next to each other move with different velocities (speed or direction), a shearing action takes place. In this case the boundary between the two layers becomes turbulent due to the friction of the opposing action as shown in the illustration.
Turbulence Caused by Shear
🌪️ In truth, all turbulence is caused by shearing action, for mechanical turbulence is created by the roughness of the terrain creating a velocity gradient and thus a shearing action as depicted earlier in the illustration. A thermal penetrating aloft also creates a shearing action as it pushes its mass through the ambient air. However, we ignore these details and focus on shear turbulence as that produced between two layers or volumes of air rubbing each other the wrong way.
🌡️ It has been said that it is impossible to separate the velocity distribution of the air from the temperature distribution. That is to say that when we have layers of air with different temperatures they also exhibit different velocities so that the presence of shear turbulence between the layers should be expected. We have seen in previous chapters that heating and cooling effects as well as high pressure systems create temperature inversions and jets at various levels. These are typical phenomena related to shear turbulence.
⛰️ Indeed, the most likely place you will encounter shear turbulence, is near an inversion layer. This layer may be thousands of feet aloft as formed by the sinking air in a high pressure system, or it may be close to the ground at night when a low layer of air is rapidly cooled by the cooling ground. In the first case the inversion layer may stop the rise of thermals so that their general turbulence is added to the mixture. The illustration shows several situations where inversions and shear turbulence is common. In the last drawing we see how a valley can trap a pool of cool, dense air that slips off the slopes then stagnates in the valley while the warmer winds aloft keep blowing to produce strong shear at the warm and cool air interface.
Causes of Shear Layers
🌒 Also in high mountain areas the sudden cooling of the slopes as evening falls can result in a downslope breeze that thrusts out into the general valley air mass to create strong shear turbulence. This event occurs most frequently on an eastward facing slope with deep canyons at the end of a hot day when the sun suddenly slips below the crest to throw shadow across the entire east face.
🌊 Other places where shear turbulence frequently appears are fronts –cold, warm and seabreeze fronts. This is pictured in the illustration. Note that the strength of the shear turbulence is determined by the relative velocities of the two air masses. Because seabreeze fronts can be quite vigorous, shear turbulence in their vicinity is often strong.
Shear Caused by Downslope Winds
⏳ Shear turbulence tends to persist for a long time for the layers that produce it are often stable. Certain fronts can create shearing layers that last for days. Air layers of different temperatures and thus densities do not mix very readily. They are like oil and water. Thus they maintain their separate identity for an extended period as they rub against one another and mix in a narrow band.
Shear Turbulence at Fronts
🛩️ Over the years shear turbulence has not been particularly hard on sport aviation. For the most part it is avoidable if one shuns inclement weather or uses prudent judgement when flying in the above described conditions. Shear turbulence can, however, be strong enough to pluck the feathers off an airplane but this generally only occurs near the upper level jet stream that frequents the higher latitudes.
🌪️ Vortex turbulence
🪽 We have reviewed the three natural causes of turbulence above. There is a man-made cause which we should mention for the sake of completeness. This is wing tip vortices which are powerful swirls emanating from the tips of all wings. Because these swirls are so uniform they possess a lot of energy and can be readily felt in the air.
⚠️ The vortices from another aircraft about your same size will feel like a couple quick bumps or a force lifting your wing depending on how you hit them. This you can live with. Vortices from aircraft larger than you can cause you more serious control problems or even structural damage. Avoid them to preserve your health. The heavier loaded, the less aerodynamically sleek and the higher an angle of attack an aircraft flies, the more violent are the vortices and general wake turbulence.
Illustration of vortex turbulence forming in an airflow.
⚠️ Rotors
🌀 In certain conditions around sharp terrain features standing eddies or rotors can exist and persist as long as the wind blows. We shall see examples of rotors in later illustrations. Rotors appear most readily in stable conditions with light to moderate wind. In unstable conditions, thermals tend to break up the rotors and give them an erratic existence or eliminate them entirely. In stronger winds rotors usually get blown apart by the general turbulence that rages through their area of residence.
🚨 Rotors should be avoided in flight because of the strong sink and control problems they offer. Flight along the rotor axis could roll you over. Rotors that exist below waves have broken airplanes.
