Turbulence — Page 104, Lesson 93

I want to walk you through the topic of turbulence. We'll start with the introduction and then get into windshear, the friction layer, and how all of this affects an aircraft in flight. A dictionary definition of turbulence is a 'disturbed state'. From the aviation point of view, that means disturbed or rough air. There are different ways this turbulence is caused, and it occurs in different parts of the atmosphere. Let's start with a key concept: windshear. Windshear is the sudden change in speed and/or direction of the wind, including vertical currents. These changes affect the energy of the aircraft, and that change in energy is what you feel inside the aircraft as turbulence. There are two specific types of windshear we need to distinguish. Vertical Windshear is a change in speed and/or direction with a change of height. It is measured in knots per 100 feet. Horizontal Windshear is a change in speed and/or direction in the horizontal plane, and it is measured in knots per 1000 feet. Now, where does turbulence occur? There are three main locations. First, in the friction layer. Second, in clouds — we'll discuss that in detail in the chapters on clouds and thunderstorms. Third, in clear air. Let's focus on the friction layer. The friction layer is the lower part of the atmosphere, extending from the surface up to a height of 2000 ft to 3000 ft above the surface. The depth of this layer depends on three factors. First, the roughness of the terrain. The rougher the surface, the greater the strength of the vertical deflection, and therefore the greater the height to which that deflection will penetrate. Second, the wind speed. The higher the speed, the greater the deflection. Third, the stability of the layer. Stable conditions will resist vertical movement and hence limit the depth. Within the friction layer, there are two sources of turbulence. One is convection from thermal currents. The other is frictional or mechanical turbulence. By day, the presence of thermal currents tends to reduce low-level stability and therefore increases the depth of the friction layer. At night, there is only mechanical turbulence, so the stability tends to increase because of surface cooling, and the depth of the friction layer will reduce. At night, surface cooling — particularly with clear skies — can lead to the formation of low-level inversions. Now, vertical mixing is inhibited, and the surface frictional effect is enhanced. This means that below an inversion, the wind speed will be light, with a significantly different direction compared to the much stronger wind above the inversion. Hence, windshear will occur at the inversion. An aircraft climbing or descending through that inversion will experience a rapid change — and that rapid change is the turbulence we've been describing.