This week's background information is available in Chapters 11-12.

You should already know some basic knowledge about turbulence from last week.

WIND SHEAR is a change in wind direction and/or speed with height over a very short distance. We can think of wind shear as a gradient in wind velocity that can be interpreted in the same way as a pressure or temperature gradient. Wind shear can be broken up into two parts:

• Horizontal wind shear - A change in the wind over a horizontal distance
• Vertical wind shear - A change in the wind over a vertical differene

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Wind shear may be associated with a thunderstorm, a low-level temperature inversion, a jet stream or a frontal zone. It is important to understand some of thse main sources of wind shear.

• Microbursts - A concentrated, severe downdraft that induces an outward burst of damaging winds at the ground with a horizontal dimensions of 2.2 nautical miles (4 km) or less. Microbursts have a strong core of cool, dense air descending from the base of a convective cloud. As the air spreads out horizontally, it forms a vortex ring which rolls upward along its outer boundary. Once it reaches the ground, the lifetime of a microburst ranges from 5 to 30 minutes. The average outflow speed is about 25 knots.

• Fronts and shallow lows - Remember that a front is a boundary between two air masses. Since cold air is denser than warm air, it will always wedge under the warm air, creating a sloping frontal zone that contains horizontal and vertical shear.

• Airmass wind shear - This type of wind shear ocurrs at night under fair weather conditions in the absence of strong fronts and/or strong surface pressure gradients. We know with radiational cooling, the ground will cool faster than the overlying airmass. If the cooling is strong enough, a nocturnal inversion results.

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TURBULENCE - Any irregular or disturbed flow in the atmosphere. In aviation, it refers to bumpines in flight.

We have already discussed Turbulence and CAT in lesson 10. Turbulence is classified by its intensity ranging from light to extreme.

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• Low-level turbulence (LLT) - Turbulence that occurs primarily within the atmospheric boundary layer, where surface heating and friction are significant. For operational purposes, it is defined as turbulence below 15,000 ft MSL. LLT includes several subcategories of turbulence.
  • Mechanical Turbulence - LLT that results when airflow is hindered by surface friction or an obstruction. It occurs because friction slows the wind in the lowest layers causing the air to turn over in turbulent eddies which can cause fluctuations in winds and vertical velocities.

  • Thermal Turbulence - LLT produced by dry convection. Typically a daytime phenomenon, it occurs over land under fair weather conditions. As solar radiation heats the ground during the morning and afternoon generating thermals that move away from the ground, creating chaotic patterns that produce LLT.

  • Turbulence in Fronts - LLT produced when moving frontal boundaries are combine with convection and strong winds.

  • Wake Turbulence - Turbulence found to the rear of a solid body in motion relative to a fluid. In aviation, it is the turbulence caused by a moving aircraft. Wake turbulence is somewhat more predictable since all aircraft generate lift, a requirement for wake turbulence.

• Turbulence in and near thunderstorms (TNT) - Turbulence which occurs within developing convective clouds and thunderstorms, in the vicinity of thunderstorm tops and wakes, downbursts, and gust fronts.
  • Turbulence within thunderstorms - The most frequent and, typically, the most intense TNT, is caused by the strong updrafts and downdrafts. It is found within the cloud and made worse because it occurs in conditions with heavy rain, lightning, and possibly hail and icing.

  • Turbulence below thunderstorms - The primary areas for turbulence below thunderstorms include downdrafts, downbursts, and microbursts described in chapter 11. These phenomena can produce intense turbulence as well as wind shear. Due to the combination of turbulent areas below thunderstorms along with wind shear, heavy precipitation, low ceilings and visibility, the area below a thunderstorm is very dangerous.

  • Turbulence around thunderstorms - Referring to the area outside the main region of convection, turbulence around thunderstorms includes turbulence in clear and cloudy air next to the main cumulonimbus cloud and turbulence in and over the anvil cloud. Turbulence is produced around the thunderstorm when the cell acts as a barrier to the large scale airflow. Since multicell and supercell thunderstorms move slower than the winds at upper levels, part of the airflow is diverted around the storm, producing turbulent eddies. Stronger thunderstorms produce greater effects.

• Clear air turbulence (CAT) - CAT is turbulence which occurs in the free atmosphere away from any visible convective activity. CAT was discussed in more detail in lesson 10.

• Mountain wave turbulence (MWT) - Turbulence produced in conjunction with lee waves, which are gravity waves that occur when stable air flows over a mountain barrier. MWT is responsible for some of the most violent turbulence encountered away from thunderstorms. The intensity of MWT depends on the wind speed near the mountain peaks.
  • Lee wave region - The upper layer of a lee wave system (within 5,000 ft. of the tropopause) where smooth wave flow dominates and microscale turbulence occasionally occurs. As you approach the tropopause, winds reach maximum speeds with vertical shears above and below. Lee wave activity strengthens the shear, which promotes the development of shearing-gravity waves, especially near stable layers. MWT and CAT are more favorable when a jet stream is present over a mountainous area.

  • Lower turbulent zone - The lower layer of the lee wave system, extending from ground leel to just above the mountaintop. It is characterized by turbulence due to strong winds and rotors (mesoscale circulation around a horizontal axis located parallel to a mountain ridge on the downwind side of the mountain). Widespread turbulence is produced in this region by strong winds and wind shear. Typically, the worst turbulence occurs along the lee slopes of the mountain. The greatest MWT occurs in the lower turbulent zone in rotor circulations which are found under the lee wave crests.

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