AV21: Turbulence, Standing Waves, & Low Level Wind Shear - HS Flashcards
Turbulence
An irregular motion of the air resulting from the formation of eddies or vertical currents in the air.
What are the two categories of turbulence?
Friction and thermal
What are the three types of friction turbulence?
- Mechanical
- Shear
- Frontal
Mechanical Turbulence
The result of friction between the air and the ground; associated with low level eddies.
What factors affect the strength and vertical extent of mechanical turbulence?
- stability of the air
- roughness of the ground
- strength of the wind
How does the stability of the air affect turbulence?
The more unstable the air, the higher the turbulence will extend.
How does the roughness of the ground affect mechanical turbulence?
The rougher the terrain, the greater the friction and the greater the degree of turbulence.
How does the strength of the wind affect turbulence?
The stronger the wind, the greater the friction ad the greater the degree of turbulence.
What kind of air does BLSN indicate?
Unstable air
What kind of air does DRSN indicate?
Relatively stable air.
Shear turbulence
The result of friction between opposing air currents. Occurs when there is a strong wind shear and is common near a jet stream or with gust fronts ahead of a thunderstorm
Frontal turbulence
The result of friction between two opposing air masses near the frontal surface. Most marked when the lifted warm air is moist and unstable. Most commonly associated with cold fronts.
What are the two types of thermal turbulence?
Convective current and cold air advection
Convective currents
Turbulence caused by the unequal heating of the surface of the earth. Air stability, the degree of difference in temperature of the different surfaces, as well as diurnal and seasonal variations all affect how severe the turbulence is.
Cold air advection
When cold air moves over warmer water or land and is heated from below. The stronger the contrast between the air and surface temperature, the stronger the turbulence.
What are the five items that are used as a general rule for turbulence probability.
Expect turbulence when:
- Air is unstable
- There is a steep temperature lapse rate
- There are cumuliform cloud
- The precipitation is showery
- There is BLSN, BLDU, BLSA (vs DRSN, DU, SA)
What are the three turbulence intensities?
- LGT
- MDT
- SEV
Light turbulence
Momentary changes in altitude or attitude of the aircraft and passengers might be pressed slightly against their seatbelts.
Light chops
Slight, rapid, and somewhat rhythmic bumpiness with no appreciable change in altitude or attitude.
Moderate turbulence
The pilot is always in control of the aircraft, but the passangers are forced against their seatbelts and non-attached items can move around.
Moderate chops
There are rapid bumps or jolts without appreciable changes in altitude or attitude.
Severe turbulence
Causes large, abrupt changes in altitude and attitude and the aircraft may be momentarily out of control. Usually causes large variations in indicated airspeed. Passengers are violently forced against their seatbelts and a change of altitude or heading is required
Turbulence in and around cumuliform cloud:
MDT or SEV
Turbulence in and around stratiform cloud:
Generally LGT but can be MDT
Standing waves
Waves or ripples which form in the atmosphere caused by air encountering an obstruction or barrier at the surface of the earth.
What are the four conditions that must be met for a standing wave to be formed?
- Obstruction or barrier
- Minimum wind speed of 25kts or more perpendicular to the barrier
- Layer of stable air above the barrier
- Strong vertical wind shear above the barrier (jet stream)
What is the horizontal and vertical extent of a mountain wave system?
Vertical: Throughout the troposphere and sometimes into the stratosphere; can be many times the height of the barrier
Horizontal: The mountain wave system can extend 80-240km on the lee side
The wavelength varies from 3-30km. (The greater the speed, the longer the wavelength)
What can help to visually identify mountain waves?
When sufficient moisture is present, characteristic cloud formations and patterns are present.
What are the three types of clouds associated with mountain waves?
- cap cloud
- rotor cloud
- lenticular cloud
Cap cloud
Found on the tops of the range and often down the leeward slope followed by circular eddies down the leeward slope
Rotor clouds
Found in the lower levels of the mountain wave system immediately below the crests of waves. Powerful circular eddies are present both in and below rotor clouds.
Lenticular clouds
Almond or lens shaped clouds that form on the wave crest aloft and may extend vertically 40,000ft and horizontally several hundred km.
What is the visual indicator for lower level turbulence from a mountain wave system?
Rotor clouds
What is the visual indicator for higher level turbulence from a mountain wave system?
Ragged edges on lenticular clouds
What are six weather conditions which could signal the presence of significant low level wind shear?
- Thunderstorms
- Rotor clouds
- Low level jet (inversions)
- Frontal zones
- Convective currents
- Local winds
What are the five different types of local winds that can produce LLWS?
- Valley effect
- Funnel effect
- Katabatic winds
- Steep slope effect
- Sea and land breezes.
Pages 47-52
Review for visuals of different causes of LLWS
What is the most turbulent area in the mountain wave system?
Within the rotor cloud is the worst, but it extends outside and below.
Where are downdrafts most severe in a mountain wave system?
Near the mountain at about the same height as the mountain top.
LLWS
Low Level Wind Shear: A variation in wind direction/speed over a short distance in the low levels of the troposphere (generally the first 1500ft)
Increase performance shear
Happens when an AC abruptly enters an increased headwind or decreased tailwind. The airspeed will increase, also increasing lift. Can be a change in wind speed or direction.
Decrease performance shear
Occurs when an AC abruptly enters a decreased headwind or increased tailwind. The airspeed will decrease, also decreasing lift. Can be a change in wind speed or direction.
Which AC characteristics increase the danger of LLWS?
Heavier AC, faster AC, or higher engine power because it takes longer to return to desired airspeed and glide path.
What are six weather conditions that are favourable for LLWS?
- Thunderstorms
- Rotor clouds
- Low level jet (inversions)
- Frontal Zones
- Convective currents
- Local winds
