Understanding Turbulence

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Turbulence is an irregular motion of the air resulting from the formation of eddies or vertical currents of air. This irregular motion often constitutes a major disruption of the mean wind flow. The aviation community is very concerned about the presence of turbulence which might cause disruptions ranging from mild discomfort of passengers to structural damage of aircraft. In particular, wind variations at low altitude have long been recognized as a serious hazard to aircraft during takeoff and landing.

Types and Causes of Turbulence

Mechanical Turbulence
Turbulence may result from friction between the air and the ground. This effect will generally occur in the lower one kilometre of the atmosphere which is referred to as the boundary layer. Mechanical turbulence will be most pronounced if the following factors are present:

Rough ground. The rougher the surface the greater the friction and the greater degree of turbulence. Plowed or rocky terrain will produce more friction than a smooth lake surface.
Hills and mountains. Similar to the turbulence caused by rough ground but the higher the obstacle, the higher will be the vertical extent of the turbulence. This is often referred to as Orographic Turbulence where hills and mountains can disturb the flow of air over them to thousands of metres above the earth's surface. Wind direction in relation to the barrier will also play a part with wind flowing more perpendicular resulting in more turbulence.
Strong winds. The stronger the wind the greater the friction and the greater the degree of turbulence. winds of 40 knots will produce significant mechanical turbulence as opposed to lighter winds.
Gusting winds. The gusts produce local irregularities in the wind flow thus creating increased turbulence in cases of strong gustiness. A steady wind speed of 30 knots will produce less turbulence than a wind speed of 20 knots with gusts to 30 knots.

Wind Shear
Wind shear turbulence is caused by a significant change of wind speed and/or direction with distance in the vertical or horizontal. This creates a type of friction between opposing air currents. An example of the presence of possible turbulence would be associated with a high level wind which has a large increase of wind speed within a short distance. wind shear turbulence is found within the boundary layer at the surface and in the free atmosphere as well.

Low Level Wind Shear
There are four criteria which are used to establish the presence of a strong LLWS hazardous to aviation:

A change of 25 knots of wind within 500 feet AGL
A change of 40 knots of wind within 1000 feet AGL
A change of 50 knots of wind within 1500 feet AGL
A reported loss or gain of indicated airspeed of 20 knots or more within 1500 feet AGL

LLWS can be particularly strong in the presence of thunderstorms, rotor clouds, low level jetstreams, precipitation or virga, baroclinic zones and other conditions such as katabatic winds, valley effect etc. Convective Turbulence
This results from the vertical motions associated with uneven heating of the earth's surface which causes air to rise unevenly. Summer afternoons are the most favourable times for convective turbulence since heating effects would be greatest at those times, thus creating stronger vertical currents.

Wake Turbulence
Wake turbulence is caused by the vortices from the wing tips of aircraft or the rotor-wash from helicopter rotors. The effect of wing-tip vortices can be severe and can extend a great distance behind an aircraft and off to the sides. The effect becomes less with distance and there is a strong relationship between the size of the aircraft and the amount of wake turbulence it creates.

Understanding Icing
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