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Submitted by Charlie Galea
Micro Meteorology
� Micro scale meteorology pertains to atmospheric phenomena which last only minutes or hours rather than days, and cover distances of metres and kilometres, rather than hundreds of kilometres. Thus these phenomena will not appear on synoptic charts and may not be mentioned in weather forecasts, but they are very significant to light aircraft operating VFR within the boundary layer.
Minor atmospheric turbulence continually disturbs aircraft flight but a 3-axis (rather than 'weight-shift' controlled) aircraft's stability system normally copes with such events without pilot intervention. However there are some atmospheric phenomena which result in moderate to severe turbulence events. Such events may result in temporary loss of control or even structural damage, particularly in light aircraft. Weight-shift controlled aircraft - trikes and powered 'chutes - require much more pilot input in turbulent conditions than the normal 3-axis controlled aeroplane.
Boundary layer turbulence
The term boundary layer is used to describe the lowest layer of the atmosphere, roughly 1000 to 5000 feet thick, in which the influence of surface friction on air motion is important. It is also referred to as the friction layer or the mixed layer. The term surface boundary layer or surface layer is applied to the thin layer immediately adjacent to the surface, and part of the boundary layer, within which the friction effects are more or less constant throughout, rather than decreasing with height, and the effects of daytime heating and night time cooling are at a maximum. The layer is roughly 50 feet deep, varying with conditions.
Air flow becomes turbulent when inherent viscosity cannot dampen out pressure forces arising when air flows past surface obstacles, through temperature gradients or over/around curved boundaries. Doubling of wind speed increases the pressure force by a factor of four.
(Viscosity: the property of a fluid whereby it tends to resist relative motion within itself. If different layers of a fluid are moving with different velocities viscous forces come into play tending to slow the faster moving layers and to accelerate the slower moving. Air has a low viscosity so that friction or drag between air layers moving at different velocities is generally weak but in some conditions severe turbulence may occur.)
In the wake of an obstacle the average wind speed is reduced but turbulence is increased, some of the velocity energy being converted to turbulence energy, thus intense intermittent gusts and matching lulls can be experienced on the lee side. Turbulent eddies may form in any plane - horizontal, vertical or anywhere between, and these eddies will have an impact on aoa of an aircraft flying in them. The vertical component of eddies and updraughts / downdraughts can impose a high structural load on an aircraft.
The ICAO turbulence definition, including the supplementary g loading � positive or negative, relative to the normal 1g load � follows. It can be seen that aircraft handling can be affected with just a 0.5g change in load factor:-
� Very low - below 0.05 g - Light oscillations
� Low - 0.05 to 0.2 g - Choppy - slight, rapid, rhythmic bumps or 'cobblestoning'
� Moderate - 0.2 to 0.5 g - Strong intermittent jolts.
� Severe - 0.5 to 1.5 g - Aircraft handling affected
� Very severe - above 1.5 g - Increasing handling difficulty, structural damage possible
The velocity of near surface winds is constantly changing, fluctuations in direction of 20 degrees or so and in speed maybe 25% either side of the mean, occur every minute. In an unstable friction layer the rising air in thermals is complemented by down-currents from the top of the layer, where the wind velocity approximates the gradient wind - i.e. the direction is backed by 20 - 30 degrees from the wind at the surface, and the speed is greater. The descending air retains most of these characteristics when it arrives at the surface thus the gust will back and increase in speed. In strong, habitually turbulent wind conditions, gust ratios (maximum gust to mean wind speed) are typically 1.3:1 over open sea, 1.6:1 over open country and 2:1 plus over rough terrain.
Slope effect
In a slope effect, air currents encountering mountain-tops slip and rise along them. If there is enough wind, the air currents may go above the summits to a height of one-third of the difference in level. The updraft will be characterised by the mountainous profile. A shallow slope gives a weak updraft, while a steeper slope logically gives a more marked updraft.
Be warned : there are traps associated with the slope effect; the rising zone on one side corresponds to a falling zone, where the airflow often quickly becomes turbulent and an aircraft caught in this trap will not be able to avoid making a premature end to its flight.
In order to exploit a slope effect, it is advisable to fly in a straight line along the line of the crest (make sure you correct for drift, since otherwise you will find yourself on the wrong side). This must always be done on the windward side, in order to avoid being thrown down the wrong side of the hill.
Lee wind eddies
Turbulent eddies with large sink rates, possibly greater than 1000 feet/min, lee wind eddies may occur, in only moderate wind conditions, on the lee side of mountains, ridges, hills and islands. Strong sink conditions may extend above the height of the barrier. Aircraft flying on the lee side, particularly if flying parallel to a ridge or taking off or landing, should be aware that sink rates encountered can exceed the aircraft�s climb capability.
The severe sink associated with the lee side downflow is a function of wind speed and slope angle. e.g. If the horizontal wind speed is 29 knots and the slope angle is 15 degrees then the ambient downslope velocity is about 30 knots ( 29 / cosine 15� = 29 / 0.97 = 30 ). The sink vector is equivalent to sine 15 degrees ( 15 / 60 ) = 0.25 x 30 = 7.5 knots or about 750 feet / minute � greater than the maximum climb rate of most ultralights. Vertical gusts (eddies) will greatly increase the ambient sink rate.
Rotor type turbulence tends to develop when slope gradients exceed one in three ( 18� ) and it appears at a lower level than the long horizontal rotors associated with lee waves. As the rotors stream downwind severe turbulence may be encountered at and below the hilltop level and for some distance downstream. Pre-conditions for these streaming or trailing rotors or vortices are a stable layer, a wind vector component across the barrier exceeding 20 knots and this component should decrease considerably not far above the barrier.
Conditions favourable for the formation of strong mountain waves, and which would be provided in the outer fringes of a high pressure system, are:
� An isothermal layer or inversion at about ridge height, sandwiched between a low level unstable layer and instability, or low stability, aloft.
� A wind, in excess of 15 knots, crossing a ridge at a high angle and increasing in velocity with height.
A sharp change in wind direction within the stable layer and a large amplitude wave may induce stationary rotor flow. These rotors differ from the streaming rotors formed in lee wind eddies, being closed with a long horizontal axis and forming in the lee of, and parallel to, a well defined escarpment and remaining fixed in position. Usually cloud will not form in the rotor but should it do so it may range from scraps of Cu fractus to a long solid Sc roll cloud.
Turbulence in and under the rotor area, i.e. from the mountain height down, will be severe. Some evidence of the rotor may be seen on the surface - rising dust, sudden and erratic wind changes etc. In potential rotor conditions it is advisable to clear the lee side of a ridge or escarpment at an altitude well above it and to cross the ridge lines at an oblique angle.
MicroMeteorology in Malta
Sometimes when we study meteorology we tend to associate most of the factors to a country abroad, much bigger than ours where there are mountains, lakes, rivers etc. But here in Malta the structure of the land can produce meteorological phenomenas than can be dangerous.
Places like Dingli cliffs and most of the West coast can cause a lot of turbulence on the windward side when we have winds blowing West (Punent) to South West (Lbic) and one has to be very careful when flying in those areas in these conditions. The best place to fly is from the edge inwards or else high enough to clear any turbulence. On the positive side one can take advantage from the updrafts caused on the edge (paragliders do that). But never fly under the cliffs it is definitely very dangerous.
Other areas that can cause a lot of trouble when the wind will be blowing from a certain point are hills like Mtarfa, Laferla Cross hill and areas in Gozo. If you are flying beneath the top of the hill in certain circumstances like those mentioned in MicroMeteorology you can find yourself in the turbulence strong enough that a microlight cannot cope with.
It is very important to know our surroundings and circumstances, much like fishermen do with the sea, in order to enjoy flying in its best form and for a long time.
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