8.22.4 Describe the operational characteristics of the cloud sensor used in Automatic Weather Stations and reported in METAR AUTO reports

Modern cloud ceilometers fire a low powered beam upwards, which bounces off the base of a cloud layer back to the ground unit and through simple mathematics calculates the height of the layer

A portion of the beam will also pass through the lowest layer and bounce off a higher layer – so multiple layers can be scanned

The ceilometers operate day and night

The sensor measures the time it records a base versus the time it doesn’t for the same height which equates to number of oktars. Immediately above the cloud sensor the cloud base readings are very accurate

The limitations include:

 – sensor only measure above – so distant clouds are missed

 – a small patch of stationary cloud immediately above the sensor may be reported as overcast when this is not the case

 – Approaching low cloud will be missed until immediately above the sensor

 – An overcast layer with a hole immediately above the sensor may be reported as no cloud detected

 – sensor is unable to differentiate between cloud types – thus /// is included after every cloud group to indicate the cloud could be TCu       or Cb

Describe the basic cloud formation process.

All cloud found in the atmosphere is formed when air is lifted and cooled. 

Air is lifted by one of four mechanisms in the atmosphere ie: orographic (hills), mechanical turbulence, convection and slow widespread ascent

Clouds are basically suspended water in liquid or solid form (ice)

The condensation process during which water vapour turns to liquid, requires condensation nuclei.

There is an abundance of these nuclei in the atmosphere; eg salt particles, dust, smoke, soot, etc. 

Condensation can therefore occur prematurely ie before the relative humidity reaches 100%. 

When there is a lack of condensation nuclei , such as in very clean air, water vapour remains in the vapour stage beyond 100% relative humidity – a condition known a “super saturation”

When the temperature of the air is very cold it is possible for water vapour to turn directly to ice crystals through deposition – relatively rare.

Describe the flight conditions in the presence of inversions.

Inversions have some good aspects and some not so good for VFR.

An inversion layer can trap moisture, dust and so on below it, while above it the conditions remain perfect.
If VFR is conducted at low levels there are dangers, but at higher levels inversions can be a distinct advantage.

Below the inversion layer, flight can be bumpy with some form of turbulence. Flight above the inversion layer is generally smooth.

Pilots must be aware of the possibility of fog forming with reduced visibility below inversions.

Also the possibility of carburettor icing below an inversion especially where moisture is available ie tracking along a coastline.

Inversions being associated with Wind Shear which can affect the climbing and descending profiles.

Subsidence inversion can be associated with extensive stratiform cloud together with a large anticyclone; which can cause “anticyclonic gloom” which can affect VFR flying.

Explain the factors involved in a:
a)  radiation inversion; 
b)  turbulence inversion; 
c)  subsidence inversion; 
d)  frontal inversion. 

a) When the ground cools after sunset, the air touching the surface cools through conduction, and depending on the degree of cooling and mixing, a layer of air of a given thickness forms within which the temperature is coldest at the surface but less cold with height.

b) Turb inversion – discussion 

c) Subsidence inversion may be found in anticyclones due to the subsiding / descending motion of air. The rate at which air subsides in an anticyclone is invariably greater at altitude than lower down; and this acts like a piston descending in a cylinder.
At some height – 6-8,000 ft the piston has warmed a thin layer of air beneath it to temperatures warmer than near the surface – thus an inversion forms at this height.

d) A front is a slanting “dividing line” between two streams of air having different temperatures. Since cold air is always more dense than warm air, it follows that all fronts have the cold air below the warm air

Explain the effect of inversions on:
a)  the formation and development of cloud; 
b)  visibility;
c)  turbulence; 
d)  the relative humidity and Dew Point; 
e)  the increased risk of carburettor icing; 
f)  the presence of wind shear.
g) Aircraft performance 

a) Cloud Formation; 
As a consequence of an inversion – at some height, usually 6-8,000 ft a thin inversion layer forms. Cooling of the rising air involves condensation and this is demonstrated by a layer of stratiform cloud at the inversion level.

b) Visibility; 

Around mid morning the base of this cloud will rise to the stage where its base will reach cloud top and then the cloud is said to have burned off. Above the inversion, we get clear skies with great visibility, where as below the inversion visibility is generally not as good. 

(c) Turbulence; 

Beneath an inversion, light mechanical turbulence is common. Just below the top of the inversion the turbulence may become moderate as the fast-moving laminar air above the inversion interacts with the slower moving air beneath, creating wind shear. Above the inversion, the turbulence will cease, even though the wind speeds will almost certainly be higher than at lower levels. 

(d) Dew point; 

Inversions cap most of the vertical transport of moisture from the earth’s surface. This means that, assuming no
other factors are at play, the water vapour content of this air will slowly increase over time, and so will the dew point
temperature. The relative humidity may or may not increase depending on the temperature of the air beneath the
inversion

(e) The increased risk of carburettor icing; 

Not only does the risk of carburetor icing increase in an inversion, but the severity is likely to increase slightly
as well. This is simply because the air beneath the inversion traps increasing amounts of water vapour. As the
pressure in the carburektr venturi decreases and the air rapidly cools, the air containing a higher water vapour
content will more readily form ice if the temperature falls below 0 degC within the barrel. 

(f) The presence of wind shear.
I

nversions enhance the effect of wind shear by decoupling (or separa=ng) the faster moving air above the
inversion from the slower moving air beneath. Thus, inversions are almost always associated with turbulence
due to wind shear. When inversions occur close to the ground, the risk posed by this wind shear increases .

 (g) Aircraft performance. 

Apart from the turbulence and wind-shear already covered above, most aircraft will experience some degradation in
performance when passing through an inversion. Some light aircraft, particularly some 4 or 6 seaters, are quite
under-powered for the tasks they are often called upon to perform. An aircraft that is loaded full of people and fuel
(but still within the performance envelope at take-of) may well struggle to climb through an inversion. This is
because the air within the inversion is considerably less dense then the air was at take-of due to the combined
effects of increasing temperature and lowering pressure.

Explain the factors in he development of the following;

1. a) Radiation inversion
2. b) Turbulence inversion
3. c) Subsidence inversion
4. d) Frontal inversion

a) Radiation Inversion:

•  – occurs overnight in clear sky conditions
•  – earth cooling down due to long wave radiation emitting to space
•  – temperature above the inversion is unaffected by surface cooling
•  – radiation fog and mist can only form in the presence of a radiation inversion

b) Turbulence Inversion:

Turbulence inversions (fig 60) are created at the top of the friction layer. They usually start with a uniform
environmental temperature lapse rate of around 1 to 2 degrees Celsius per 1000ft.
Turbulence is induced within the friction layer by a wind of at least 10 knots blowing over surface obstacles like small
hills, trees and buildings. The depth of the turbulence and therefore the friction layer is determined by the roughness
of the surface and the speed of the wind. Turbulence inversions can occur at heights between about 1000 and 5000
feet, however they are most common at around 2000 to 3000 feet.

So, the air beneath the friction layer becomes turbulent (but mostly only light from an aviation perspective), and this
tumbling motion means that some of the air is rising and some is falling. The air that is rising is subjected to less
pressure and so it expands and cools adiabatically. The descending air is compressed and warmed adiabatically. The
rising and falling air mixes with the environmental air. The cooling in the top half of the layer is of-set by the
warming in the bottom half of the layer. A new ‘steeper’ lapse rate – one approximating the Dry Adiabatic Lapse Rate
of 3oC per 1000 feet – is created. The air above the friction layer is unafected by the turbulence below, and so it
remains warmer than the induced cooling beneath the newly created inversion.
If suficient moisture is present for the cooling to reach saturation point, Stratocumulus cloud is produced with the
tops capped by the top of the inversion. Sc is a cloud induced by turbulence in this case.

 c) Subsidence Inversion;

Subsidence inversions have their roots in the upper troposphere during the initial formation process of a developing
surface high pressure area. High-level air just beneath the tropopause converges and begins to sink and, as more air
converges into the same area, the surface pressure starts to rise. The sinking air is subjected to increasing pressure
and is therefore compressed, and so it warms adiabatically.
A subsidence inversion is first created in the mid-troposphere. As the air continues to descend, the inversion
develops and becomes stronger. The descent usually stops somewhere between 3,000 and 8,000ft above MSL,
where it meets weak convective currents rising of the surface (see fig 61).
The sinking air above a subsidence inversion originates in the upper troposphere where the air is very dry and
typically cloud-free. If you fly up through one of these inversions, there will be a very marked decrease in relative
humidity and dew point as you do so, and consequently, there is likely to be very little cloud, if any, above a
subsidence inversion.

d) Frontal Inversion

 A frontal inversion occurs at any frontal surface when warm air is forced to rise over the top of a layer of colder air.
The height at which the inversion is intercepted depends on your position relative to the surface position of the front.
Closer to this position and the inversion will be lower; further away and the inversion will be higher (see fig 62).
In radiation, turbulence and subsidence inversions, the atmosphere tends to ‘dry out’ above the inversion. Frontal
inversions are diferent in that the air temperature and the dew point temperature will remain close together through
the inversion because the cloud layer will often be continuous through this area, with heavy rain falling through it.

Define:
a)  inversion; 
b)  isothermal layer. 

a) In certain circumstances the temperature in a layer of air increases with height rather than decreases.
This is known as an inversion

b) An isothermal layer is similar to an inversion, but instead of experiencing increasing temperature with height, the temperature remains constant with height

Describe the effects of vertical and horizontal wind shear on aircraft operations.

Wind shear can be a major cause of aircraft accidents when present at low levels.

Vertical wind shear 
-When an aircraft descends into wind through a shear zone below which the wind is lighter, the aircraft’s indicated airspeed will reduce, its angle of descent will steepen and rate of descent will increase.

Horizontal wind shear

Horizontal wind shear is similar to vertical wind shear; except that the winds are almost direct opposite winds. This creates an area where the winds meet, where turbulence, speed changes and the controllability of the aircraft all change. 

Define wind shear.

The speed of the wind and its direction is not normally constant with height or even within the horizontal

When there is a difference in either speed or direction within a shallow layer or within a narrow horizontal zone, wind shear is present

 

Explain how applying Buys Ballot's Law can:
a)  determine the location of high and low pressure areas; 
b)  establish possible errors in altimeter reading. 

a) By briefly putting the aircraft’s tail to the wind in one’s mind and remembering that the area of low pressure is to the right.

Example pictured that of northern hemisphere. 

b) Flying into a low pressure area will alert the pilot to altimeter inaccuracy