Turbulence Background, I
Most fluids exhibit a property called turbulence and its velocity seems chaotic and appears to be random. Because air is also considered a fluid and in the free atmosphere an aircraft and its occupants can and may experience turbulence due to these fluid (air) movements. But most of the time a flight is relatively free of turbulence.
The aircraft responds to these motions and the crew and passengers feel bumpiness during their flight. The amount of bumpiness is based on a number of factors, aircraft type and weight, altitude, terrain and weather conditions.
To minimize the effect the pilot must have a comprehension of turbulence, its background and the effects it may have on the flight. We will take a look how the pilot may visualize turbulence and how it is formed and the size of it.
Water, like air, is also subject to turbulence. In helping visualizing how the wind moves around buildings or other obstructions, think of it as water. Take a look at the beach observing waves and you can see how air in the atmosphere is pushed up and down by mountain creating similar waves and turbulence miles downwind.
Aviation weather reports use the term gusts to indicate wind speed variation of more than 10 knots between maximum and minimums. These gusts are a typical indication of turbulence.
This is an individual turbulent vortex embedded in the airflow. The same happens when a wing stalls and the airflow detaches from the top of the wing, this air is turbulent and filled with small eddies.
The size of these eddies can vary considerable. If an aircraft flies through a very large eddy it will experience an upward motion followed some time later by a downward motion. Like a sea wave action.
On a smaller scale, if two eddies next to each other produce a strong upward movement of air, the pilot will feel a jolt in stead of an up going motion. If the eddies are even smaller and the aircraft flies through them it feels like small chops, like driving on a rough road if you like.
Much like waves in the water these have certain dimensions: wavelength and amplitude. The effects on the aircraft and reports from pilot can be identical as discussed with eddies and gusts. Short waves can be experienced as chop and long waves as wave action. For a pilot it is difficult to see or feel the difference.
Any time a fluid moves over surface a boundary layer develops due to friction with that surface. It happens over a wing but also with the earth's surface. It is in this atmospheric friction or boundary layer where the most low level turbulence will occur.
The friction of this layer causes the wind to slow down and blow at an angle (typically some 30°) towards the low pressure area. At an altitude of around 2000 ft this effect is almost gone and the wind blows parallel with the isobars with the low pressure area to the left in the northern hemisphere. The low presssure area will be to the right in the southern hemisphere.
When taking a look at the vertical structure we see that the wind direction will be constant until 100 to 500 ft AGL, though the speed will increase as friction reduces. Above 500 ft we see the wind changing some 30° in its direction.
Boundary layer thickness will vary with surrounding terrain, mountains, presence of an anti-cyclone (high pressure area), stable or unstable conditions, day or night time and during summer it will also be thicker than in winter time.
In order to put turbulence into perspective and in relation to other similar phenomena, a scale was developed where horizontal dimensions are compared to the lifespan. That resulted into three classifications:
With horizontal dimensions between centimeters to a meter and a lifespan of seconds to an hour. Here you find gusts, building wake, dust devils, some CAT and shearing gravity waves. Really small but annoying stuff for pilots and their passengers.
Somewhat bigger phenomena sizing from a meter to a kilometer and with lifespans of a half hour to a week. This is the domain of thunderstorms, downbursts, rotors, lee waves, gust fronts, convective action, land and sea breezes, squall lines and Chinooks up to a hurricane and frontal systems.
From a thousand kilometers wide up to planetary scale and with life times from a day to even weeks. You will see cyclones, Jet streams and monsoons in this category.
You may conclude from this that the smaller the phenomena the shorter its lifespan and the less effect it has on the aircraft and occupants.