1. Clear Air Turbulence

2. Clear Air Turbulence
Frictional stress caused by rapid changes of windspeed or wind direction (windshear) causes three dimensional eddies.
Near the tropopause.
Close to jet streams, particularly on the cold polar airmass side.
Jet stream CAT can be encountered above the tropopause boundary
anywhere the vertical change in wind speed exceeds 5 knots/1000 ft or the horizontal change exceeds 20 knots/60 nm.
In upper troughs and ridges, particularly where they are rapidly developing.
In the lee of mountains, under certain conditions - standing waves.

3. CAT
Defined as:
Air turbulence of a type other than that associated with airflow close to rough ground or that encountered in or near convective cloud.
It does not always occur in clear air and may be associated with thin cirrus.

5. Windshear

6. Definitions Windshear is defined as: Low Level Windshear is:
variations in vector wind along flight path which,
causes abrupt displacement from the intended flight path such that,
substatial control action is required.
Low Level Windshear is:
windshear encountered during final approach,
along the runway,
along take-off flight path,
along the initial climbout flight path.

7. Types of Windshear Vertical Windshear: Horizontal Windshear:
change of horizontal vector wind with vertical change of aircraft position.
Horizontal Windshear:
change of horizontal vector wind with horizontal change of a/c position.
Down-draught Windshear:
change in the vertical component of the wind with horizontal change in position.

8. Main Causes of Windshear
CB/TS Activity:
see Section 8 of Met Notes.
Frontal Surfaces:
Warning signs are:
Sharp changes in wind direction.
Temperature difference of 5°C or more across a front.
Frontal speed 30 kt or more.
Cold fronts windshear is just behind surface position of front.
Warm front windshear is just ahead of surface position of front.

9. Main Causes of Windshear (cont.)
Inversions:
A low level wind maximum occurs just below the top of a strong night time or frontal inversion.
The effect is sometimes to cause a decoupling of the surface wind from the prevailing gradient wind.
Thus air at the surface may be slow moving or stagnant and fast moving aloft giving a pronounced windshear across the interface.
See MET AIP 0-9, 0-10 insert at back of Met Practical notes.

10. Main Causes of Windshear (cont.)
Turbulent Boundary Layer:
Strong surface winds with gusts and lulls produce horizontal wind shear.
Thermal turbulence in hot countries due to intense surface heating gives strong up/down draughts.
Topographical Windshear:
“Wind shadow” caused my topographical or man-made objects,
ravine winds
rotors
lee side wind of hills, tree lines.

11. Windshear Warnings & Reporting.
Warnings reported in following ways:
Met warning,
ATS warning
Departure/ Approach ATIS,
Pilot warning,
On-board equipment.
NOTE: Pilots encountering windshear on approach or climb-out are required to make a report to ATC as soon as possible even if previously encountered or f/cast.

12. Windshear Alerts Issued when:
A marked temperature inversion of 10° or more exists between surface and 1000 ft. or
Mean surface wind exceeds 20 kt. or
Vector difference between mean surface wind and gradient wind at 2000’ exceeds 40 kt. or
TS or heavy showers are within 5 nm of airport.