Wind Shear ( Lesson #3 )
Define temperature inversion
An increase in temperature with height`
What are the 2 requirements for wind shear
A change in wind direction and/or speed over some distance
Anticipate possible areas of turbulence caused by surface friction
Where wind speeds are >25kts
How does wind change direction and speed with height
Wind turns clockwise with height
What frontal temperature gradient produces moderate to severe turbulence
10°F in 50 NM
What is the approximate height / distance of a warm front?
1 NM 200 NM Distance
What conditions produce shear turbulence associated with nocturnal radiation inversions
1. Clear Sky 2. High Pressure
What conditions produce shear turbulence associated with terrain induced temperature inversions
1. High pressure system 2. Valley / sloping terrain
What are the two types of temperature inversions that produce shear turbulence
1. Nocturnal Radiation Inversion 2. Terrain Induced Inversion
What factors determine the intensity of turbulence associated with fronts
1. Speed of front 2. Temperature Gradient 3. Terrain
What are the 2 wind speeds and how do they affect the depth of the boundary layer?
1. Strong Wind - Thick Boundary Layer 2. Weak Wind - Shallow Boundary Layer
What are the probable location of shear turbulence
1. Surface Friction 2. Kelvin-Helmholtz Waves 3. Temperature Inversions 4. Fronts 5. Jet Streams
How can a pilot avoid effects of turbulence caused by surface friction
1. Takeoff with higher airspeed 2. Climb above boundary layer
What are common locations of clear air turbulence
1. Upper Fronts 2. Tropopause
Describe the progression of shear turbulence from a wave to Eddie
1. Wind shear induces the wave 2. Wave Break - At critical shear value 3. Leads into eddie turbulence
Calculate the final approach speed of an airplane based on gust factor
16G24KT Gust factor: 24-16 = 8 kts Approach Speed + ½ (Gust Factor)
What is the approximate height / distance of a cold front?
1NM Height 50 NM Distance
What is the shear value for moderate turbulence
5-6 kts per 1,000ft
What are characteristics of Kelvin-Helmholtz waves and their associated hazards?
Characteristics - Occurs in shallow layer - Form in crest of mountain wave Hazard - Strong turbulence with strong shear
Identify conditions that are conductive for nocturnal radiation inversions
During Day - Sun warms ground - Ground warms air - Temperature decreases with height During Night - Infrared Radiation Loss - Surface Cooling - Air aloft changes little --Temperature inversion
What frontal speed produces moderate to severe turbulence
Greater than 30kts
What are the conditions that produce moderate turbulence in the boundary layer
If surface winds > 30 kts
Define boundary layer
Layer in which friction affects wind
What is the shear value for severe turbulence
More than 6 knots per 1,000 ft
Define Jet Stream
Relatively strong winds concentrated within a narrow stream in the atmosphere
Define an Eddie
Shear induced rotation of air
What are Kelvin-Helmholtz Waves?
Shear induced wave that develops in a concentrated layer
What are areas of strong shear on constant pressure charts
Shear on Polar side of Jet
What is the relationship between wind shear and stability in producing turbulence
Stability suppresses Turbulence until shear becomes so great that it overcomes stability
Characterize the stability found within frontal zones
Strong Stability 50-150 miles
Define critical shear value
The amount of shear required to produce turbulence
Define Shear Gradient. What is it measured in?
The magnitude of shear - Knots per distance
Define Wind Shear Turbulence
Turbulence that results from wind shear
