State the significance of forecast or observed low level moisture to flight

It is fitting that this should be the last objective in this manual, as it is, in my opinion, one of the most important
objectives in the Civil Aviation Authority Advisory Circular AC61-3 meteorology syllabus. 

Most weather-related accidents and incidents in the GA community are related to one of two things. The first, and
most important is: 

Low-level visible moisture. 

And the second is: 

Wind and wind related occurrences. 

Any observed or forecast low-level moisture should raise a warning flag for a pilot; that is not to say that you should
immediately abandon your current or proposed flight, but rather you should give serious weight to its significance. 

 Increasing low-level moisture will inevitably lead to the development of low cloud and poor visibility in our
mountainous terrain. The combinations and rising dew points and rising ground, with an onshore flow must result in
cloud formation if sufficient lifting occurs. And if that increasing moisture is due to an approaching frontal system,
the cloud bases may start out quite high, however they will lower, and sometimes very quickly, as the dew point
temperature increases. 

And don’t be fooled into thinking you need a mountain or a large hill for this to occur either. The height that the cloud
forms is not related to how high the hills are, but rather how close the temperature and dew point temperatures are.
On several occasions, I have observed broken stratus cloud at around 50 -100ft above Ohakea in moist westerly
flows. Out to sea, the air is clear of all cloud, but as this very humid air comes ashore, two things happen. Firstly, the
air is forced to rise over the sand dunes which are no more than 50 -100 ft high, so this results in lifting and therefore,
cooling. Secondly, as the air encounters the dunes, it slows down slightly because of increased friction. The air coming in from the sea is not being slowed, so it runs into this slower
moving air resulting in convergence, which adds to the lifting – sufficient for low-level cloud to form.

Another smart thing to do is monitor the dew point temperature over time. If the dew point temperature is rising,
more low-level moisture is entering the area. This will not necessarily result in cloud immediately, but if the trend
continues, at some point, low cloud becomes inevitable. 

Often, a front will sweep through an area and dump quite a lot of rain. All through the rain period and often after the
rain has stopped, this added water will evaporate into the air causing the dew point to rise. If this time of rising dew
point temperatures corresponds to a rapidly falling temperature in the clear air of evening, radiation fog becomes a
distinct possibility. 

Too many pilots have lost their lives by continuing to fly into deteriorating weather, which, whether forecast or not,
was either clearly visible before departure, or became visible ahead of them during the flight. In either case, such
weather is always avoidable if good decision-making processes are followed. 

Flying can be heaps of fun, however when things go wrong – and they will from time-to-time – it can rapidly become
a very stressful pursuit. Some things, like a sudden engine failure for example, are drilled into you through practice,
practice and more practice. You hope the engine will never fail, but you instinctively know how to deal with it when it
does happen. 

Being caught out by bad weather though, is simply something that should never happen. ALL flights at PPL level into
poor weather must have resulted from a poor decision by the pilot. Either you took off in already existing poor
conditions or you decided to continue the flight into poor weather ahead – forecast or not. 

A Decision-making Flow chart:
Figure 102 presents a GOOD decision-making flow chart for any flight with regard to poor weather. The aim of every
flight should ultimately be to reach the yellow highlighted box. You don’t have to have a beer and tell war stories, but
the fact that you could if you chose to, means you have reached the ultimate goal – you’re still alive and, hopefully,
the aircraft is still in one piece. 

Everything that has gone before in this book, has been written to educate you, and prepare you, for some of the
weather experiences you will come up against during your flying career as a recreational pilot. 

A bad forecast, a good forecast, or even the lack of a forecast, never killed anyone. Forecasts are simply planning
data written on a piece of paper or on a computer display. They are certainly designed to help you make good
decisions, but they don’t make the decision for you – you make those decisions yourselves

As pilots, you alone hold the key to a long and enjoyable participation in aviation. Train yourself to set your own
weather limits and don’t ever push those limits. The weather can be very unforgiving to aviators. Don’t let that one
bad decision become the last decision you ever make.

Insert Flowchart of good decision making during a Cross Country Flight

Describe the causes, factors involved and techniques commonly used to avoid or minimise the following;

Motions within the atmosphere can be broken down into waves, from the smallest waves: gusts with a wavelength of
just metres – to the largest: planetary waves with wavelengths of 10,000 km or so. All turbulence as experienced by
an aircraft is a function of the interaction of the plane in flight with one or more of these waves at an appropriate
wavelength.
When thinking about how an aircraft will be affected by wave motions, we need to consider the size of the aircraft
and its speed of travel. This is because severe turbulence is experienced when the wavelength and amplitude align
with the aircraft’s movement through the air. 

Figure 81 demonstrates how a light aircraft and a heavy aircraft may
experience completely opposite extremes of turbulence at different wavelengths.

Incidentally, in the severe turbulence generated downstream from a mountain range, these two wave lengths and
many others besides will be mixed together, meaning that regardless of size and/or speed, all aircraft are likely to
experience severe turbulence in these conditions. 

 (a) Convective (thermal) turbulence; 

Mature Cb’s contain very strong updrafts and downdrafts in juxtaposition to each other. These vertical winds can
reach speeds more than 5000 ft per minute in New Zealand, thus generating a very violent overturning motion,
sufficient at times to tear the wings of an aircraft. Since no VFR private pilot should be caught flying inside a Cb
cloud, the internal turbulence should not affect them.
Another source of turbulence associated with Cb’s is caused by Microbursts – columns of rapidly descending air
beneath the Cb cloud base. Despite common misconceptions, microbursts can and do develop in New Zealand and
extreme care should be exercised when considering taking of or landing with a Cb just of the end of the runway.
Almost all New Zealand’s microbursts will be ‘wet’ microbursts i.e. accompanied by rain, and therefore clearly visible.
If encountered, microbursts can force an aircraft down to the ground (figure 82).

Another source of convective turbulence is a by-product of the microburst. When a microburst hits the ground, it
spreads out horizontally, creating a phenomenon known as a First Gust or Gust Front (see figure
83). A First Gust or Gust Front is the boundary between the cold outflow air resulting from the microburst and the
warm inflow air feeding a Cb. The warmer air, being less dense, rises over the cold air, frequently creating what is
known as a roll cloud on the leading edge of an advancing Cb cell. In New Zealand, this gust front may precede the
Cb cell by up to 5km and the roll cloud may not be visible if there is insuficient moisture in the air.
It is not uncommon for the surface wind to change direction by 180o instantly with the passage of the gust front, and
for the wind to change from 10 knots or so ahead of it to gusts of 40 to 50 knots behind the gust front – a wind shear
of perhaps 50 or 60 knots.
Attempting to cut in front of an approaching Cb and gust front to land into a 10-knot head wind is fraught with
danger. If the gust front catches you before touchdown, the best you can hope for is a very hard landing. It just gets
worse after that.

(b) Mechanical turbulence – small scale and large scale; 

For the most part, large scale turbulence has been covered in chapter 8.32, Mountain Weather. However, there are a
few additional aspects of large scale turbulence that should be considered.
As most mountains have rugged terrain, and waves are associated with strong winds, the friction or boundary layer
will be deep and turbulent immediately above and in the lee of the ranges. The turbulent zone generated beneath
these waves may extend hundreds of kilometres out to sea (see figure 84).

In addition, strong updrafts and, more importantly, strong downdrafts are likely to exist. These vertical winds will
often exceed the performance of your aircraft, meaning that if caught in an updraft, the aircraft will soar like a glider
and gain height despite the pilot’s best efforts to lose height.
If the aircraft gets caught in a downdraft, the experience can be much more alarming, as even at full power and with
the aircraft set up for maximum rate of climb, it may well be descending at several thousand feet per minute. This
coupled with rising ground…well, you get the picture, and it’s not nice.
Low ground speed is another hazard, although it can be advantageous too. A low ground speed means you will be in
the danger zone for longer. Countering this is the fact that a low ground speed will give you a little more time to
make decisions about escaping or turning away from the ridge line.
A local low-level obstruction to the strong surface flow will create tumbling and turbulence downstream from the
object. Immediately downwind the air will be dumping toward the ground, creating a localised down-draught.
Helicopter pilots operating into pads immediately down-wind of a building or a row of trees should be very cautious
in strong wind scenarios.

(c) Wake turbulence;

Wake turbulence forms of aircraft wing-tips because of the high pressure under the wing being forced around the
end of the wing towards lower pressure above. 

Fig. 85 Wake Turbulence generated from Aircraft Wing-tips. 

The rotation generated slowly sinks and expands outward behind the generating aircraft (see Figure 85). It can be
disastrous if encountered at low-level, particularly during the take-off and landing phases of flight, when the induced
roll and yaw occurs with little height for recovery.
Wake turbulence only forms when the wings are loaded, so its generation ceases on touchdown and doesn’t develop
until the generating aircraft rotates on take-off. The turbulence generated will be worse if the generating aircraft is
heavy, slow and clean i.e. no flap and landing gear up. In calm conditions, the vortices generated of the wing-tips
during a landing or take-off will sink to the ground, and then spread out horizontally – away from the runway in opposite
directions. If, however, there is a slight cross-wind, one of the vortices may be pushed slowly toward and then over
the operational runway. Be very aware of this possibility.
There are several different options open to a pilot in terms of avoiding wake turbulence. The first two, dealing with
the landing and take-off phases of flight, are detailed in Figure 86. 

An approach above the approach path, and a
touchdown beyond the touchdown point of the generating aircraft will keep a light aircraft out of the unsafe zone.
Likewise, a rotation prior to the rotation, and a steeper climb rate than the generating aircraft will avoid the problem.
Where these options are not available, the next best option is to wait. There are recommended minimum time delays
which need to be applied between generating aircraft and following aircraft. If at a controlled airfield, ATC will advise
you of the hold time appropriate for the aircraft types involved.
Light aircraft can also generate wake turbulence which can be significant for following aircraft if close behind,
especially if conducting a streamed landing as part of a formation.

In the cruise, a light aircraft` should maintain a separtion of 5 NM behind a medium weight aircraft` and 6 NM
behind a heavy aircraft`.
And a final point of note: helicopters may also generate very dangerous wake turbulence, particularly large
helicopters like the RNZAF NH90’s.

8.32.12 With regard to VFR flight in a light aircraft in mountainous terrain, describe the meteorological factors that should be considered during the flight planning phase and en-route

Every weather situation presents somewhat different conditions in mountainous terrain. On any two occasions when
the weather patterns look the same, the weather experienced along a well flown route may be surprisingly dissimilar.
For this reason, it is not possible to give complete instruction on this objective. There are, however, some points in
relation to each of these elements that may prove useful.
The following points are made on the assumption that conditions are flyable in and around the Southern Alps.
Note: When flying in the mountains it is essential that you prepare well. This includes:

•  – Knowing your own limitations and sticking to them
•  – Knowing the limitations of the aircraft
•  – Being fully prepared for the flight – including;
•  – Having the latest up to date weather forecast
•  – Studying the route thoroughly, including spot heights for saddles etc
•  – Considering weather a valley is wide enough to make a safe U-turn in
•  – Knowing the position of wires, and clearly marking them on the map
•  – Planning and conducting a passenger brief
•  – Ensuring everyone on boards is wearing suitable clothing and footwear
•  – Thoroughly pre-flighting the aircraft – make sure navigation lights are working
•  – Make sure everything is secured
•  – Planning and allowing for escape routes
•  – Having an adequate flight following and alerting service in place

So, this objective asks you to consider six specific points in relation to the meteorological factors and track selection
when flying in the mountains. They are:

(a) Cloud base;
As a rule, the cloud base will be much lower on the windward side of the ranges – often unflyable. On the lee side of
the divide, cloud bases will be high – often well above the highest ridges. Some lower level cloud may exist in the
form of rotor clouds. These should be avoided at all costs.
Cloudscapes in mountainous terrain can change very rapidly. A slight increase in moisture advected into an area of
rising air can result in the sudden formation of cloud where none existed previously. For this reason, always keep
checking behind you to ensure your escape route remains open. 

Cloud that is forming rapidly above or about far of ridges is a good indication that dew point temperatures are rising.
Rising dew points generally mean more cloud development and lowering bases, and such cloud development can
occur rapidly in mountainous terrain. 

(b) Turbulence;
To windward, light to moderate turbulence is common. Severe turbulence, though possible, is rare except in
thunderstorms.
In the lee of the ranges, severe turbulence is relatively common, especially in rotor zones, but it’s also possible
anywhere within the friction layer. In addition, hydraulic jumps as described in 8.32.6 above can create downdrafts
that may force a light aircraft into the ground. 

(c) Adverse and favourable winds; 

The path that the wind takes when flowing around and over mountain ranges is incredibly complex and is influenced by many factors. They include:

• – Wind strength
• – Angle of the flow near the ranges
• – Shape of the mountain ranges
• – Stability of the air
• – Vertical profile of wind speed and direction
• – Orientation of the valley systems
• – Location of passes and higher peaks – Effects of multiple ranges

 In some valleys, the flow will align itself with the valley – either up it or down it. Flying in these valleys is generally not
a problem. In others, however, the wind may be across the valley, and this frequently sets up a rotation in the vertical
which could be clockwise or counter-clockwise depending on wind and terrain considerations (see Figure 78 below).
In this case, it would be advisable to establish which side of the valley is updrafting, and which side is down-drafting.
Avoid the down-drafting side if possible – the combination of rising ground and descending air can be disastrous.
Flying on the updrafting side, however, will help you stay clear of terrain.

d) Visibility; 

Generally, visibility will be poor on the windward side of the range in rain, drizzle or snow. On the lee side visibility is
usually very good; however, curtains of rain or virga may exist some distance downstream from the main divide.
There are, of course, exceptions when pockets of poor visibility may still exist on the lee side, particularly if a front is
passing overhead.

In very strong winds, visibility at low-levels in east coast valleys may be reduced due to blowing dust and sand,
especially after extended dry spells in the braided river systems.
Another problem associated with visibility is one related to visual illusions and loss of horizons. Learn to visualise
where the horizon is and superimpose this line on the mountains in front of you. This will help you maintain both
attitude and altitude. 

(e) Track selection; 

Obviously, selecting a route or track to get from A to B depends on the weather on the day.
If you are lucky and the weather is fine, you can save time by flying high and in a straight line from departure to
destination, or you might use the good flying conditions to get down into the valleys and enjoy the scenery. 

If the weather is a little dodgy, however, often taking the long way around is the best option. For example, if you want
to get from Christchurch to Queenstown, the best option may be to fly down the coast to Invercargill, then fly north
via Lumsden and the valley following SH6 to Queenstown. It all depends on the weather on the day. 

Plan a route through the passes etc to your destination, but always be prepared to change your plans if the passes
are closed due weather. And of course, be wary of a pass closing behind you. 

Regardless of the track chosen, there are some useful tips to flying in the mountains. 

With little or no indication as to which way the wind is rotating in some valleys, choosing the correct side of the valley
to fly in may be problematical. The CAA GAP publication on Mountain Flying recommends that when flying up or
down a valley with a high trafic density (around Mt Cook for example), you should always fly on the right-hand side
so that opposing traffic will always be on the opposite side of the valley. This is certainly worthwhile in the major
valley systems where there are sometimes many aircraft flying. However, in lesser valleys, flying on the RHS may put
you in the down-drafting air – not a good idea when close to rising ground. Better to fly on the up-drafting side of the
valley and to always allow enough room to do a 180 degree turn. 

NEVER enter a valley which is too narrow to do a 180º turn in. 

When crossing a ridge, do so at a 45 deg angle. Thus, if sink is encountered, the turn back towards safety will be at
a shallower angle of bank with less wing loading and less distance to cover. Always be wary of sink approaching
the ridge-line, or during any turn-back.
In stronger winds, expect stronger sink, so approach ridge-lines at a greater height. And remember, the wind blowing
through passes or saddles is often stronger than the wind blowing over the higher ridge-lines. If you get lost flying
in the ranges, follow the biggest valley you can find downstream. Not only will the valley tend to broaden
downstream, but it will eventually lead to bigger rivers, roads, and towns where you can re-orientate yourself.

 (f) The anticipated timing of any expected weather change;
Thorough pre-flight planning, including asking yourself “what if?” will help you to be ready for in-flight conditions
that you were not anticipating. Weather in the mountains can and frequently does change very quickly. Set your
personal minima and stick to them. If conditions are deteriorating and are getting close to your personal pre-set
minima, play your Joker card and turn back immediately. 

Note any scraps of cloud developing where none previously existed. They may indicate that higher moisture content
air is being advected into the area.
During the hours prior to the planned flight, keep an eye on the sky. If upper level lenticular waves are slowly moving
away from the generating range, the upper level winds are increasing, even though the surface winds may not be.

Another rule of thumb:

If the mountain airfield TAFs have three or more QNH forecast lines indicating RAPIDLY falling pressure, consider cancelling your proposed flight. The weather WIILL deteriorate over the next 5-10 hours and once the deterioration starts it will be rapid