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Aviation Meteorology

Microscale meteorology and atmospheric hazards

Rev. 37b — page content was last changed 2 July 2011
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Microscale meteorology pertains to atmospheric phenomena that last only minutes or hours rather than days, and cover distances of metres and kilometres, rather than hundreds of kilometres. These phenomena will not appear on synoptic charts and may not be mentioned in weather forecasts, but they are very significant to glider pilots as a source of beneficial atmospheric lift. On the other hand they may be a source of wind shear and major turbulence to all aircraft operating in the boundary layer. Some of these phenomena may also be classified as mesoscale systems.

Minor atmospheric turbulence continually disturbs aircraft flight, but a three-axis (rather than weight-shift controlled) aeroplane's stability system normally copes with such events without pilot intervention. However, there are some atmospheric phenomena that produce moderately to severely hazardous wind shear and turbulence events, depending — to some extent — on the height at which they are encountered and how they are encountered. Such events may result in temporary loss of control or even structural damage, particularly in very light aircraft. Paragliders and powered parachutes require fairly smooth air for normal operations.

For an explanation of the effects of these phenomena on aircraft handling read the 'Wind shear and turbulence' module of the 'Decreasing your exposure to risk' guide.

9.1 Boundary layer turbulence

In meteorology, the term boundary layer is used to describe the lowest layer of the atmosphere in which the influence of surface friction and surface temperature on air motion is important. It is also referred to as the friction layer, planetary boundary layer or the mixed layer and is perhaps 1000 to 5000 feet thick by day and thinning by night. (Under high surface temperature conditions the depth of the layer affected by thermals can be much more extensive; see 'Dry thermals in the superadiabatic layer'.) The term 'surface boundary layer' or surface layer is applied to the thin layer (roughly 50 feet deep) immediately adjacent to the surface (and part of the boundary layer) within which the friction effects are more or less constant throughout, and the effects of daytime heating and night-time cooling are at a maximum.

Air flow becomes turbulent when its natural viscosity cannot dampen out pressure forces arising when air flows past obstacles, through temperature gradients or over/around curved boundaries. In the wake of a topographic or constructed obstacle, the average wind speed is reduced but mechanical turbulence is increased. Some of the velocity energy is converted to turbulence energy; thus intense, intermittent gusts and matching lulls can be experienced on the lee side of sentinel hills, ridge lines and mountain ranges. Turbulence may take any form — eddies, vortices, upflow or downflow — and be aligned in any plane. Turbulence increases with the square of the wind speed. Doubling of wind speed will increase pressure forces, and thus turbulence, by a factor of four. Such mechanical turbulence will affect the aoa of an aircraft flying into it, even exceeding the critical aoa.

The downward vertical component of eddies and gusts can cause an aircraft to sink rapidly. Such turbulence that occurs when an aircraft is flying near the surface, particularly in take-off and landing, may place the aircraft in a dangerous, possibly irrecoverable, situation.

Extract from an RA-Aus accident report: "The pilot took off ... towards a saddle in a range of hills which rise 400–600 feet above the airstrip. While attempting to turn 180 degrees in the lee of the saddle he experienced strong turbulence and sink and was unable to complete the turn before the aircraft collided with the ground."

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9.2 Low-level wind shear

Generally, below 2000 feet agl and over flat terrain, the amount of horizontal and vertical shear, in both direction and speed, is largely dependent on temperature lapse rate conditions:
Greater lapse rate » greater instability » greater vertical mixing » more uniformity of flow through layer and less shear. An exception is in extremely turbulent conditions below a cumulonimbus. But if the environment lapse rate exceeds about 3 C per 1000 feet then convective thermal turbulence will be severe.

Convective turbulence is minimised in stable conditions, so vertical shear in the boundary layer is enhanced, with highest values in the lower 300 feet. That will affect aircraft taking off and landing.

High vertical wind shear values are often attained at the upper boundary of an inversion. An aircraft climbing through the inversion layer, in the same direction as the overlaying wind, would experience a momentary loss of air speed — and lift — through the effect of inertia. Also, the difference in wind velocity between the layers, with shearing instability at the interface, causes the formation of short-lived waves across the interface; much the same way as ocean waves — which grow in amplitude until they curl up and break. The waves produce an extensive but shallow area of moderate to severe clear air turbulence.

However, severe low-level wind shear can also be associated with other phenomena; for example, lee eddies, lee waves and solitary waves.

9.3 Convection currents

When air flows over a surface heated by solar radiation, the surface contact layer is heated by conduction. If the incoming energy is sufficient, the temperature in the lower layer increases and thermals (upward convection currents) rise from the heated contact layer — perhaps initially as bubbles of buoyant air and then developing into downwind slanted, vertical currents of 50–300 metres diameter. The strength of the thermal depends on the heating and thus on the time of day, being weak in the early morning and strongest in mid to late afternoon. But if the wind builds up, turbulent mixing will disorganise the thermals. Areas of sinking air accompany the thermals, surrounding the weaker thermals and, as the day progresses, extending to fill in the inter-thermal gaps.

The thermal cools at about 3 C/1000 feet and if it reaches dewpoint — the convection condensation level — cumulus will form. The release of the latent heat of condensation of the included water vapour warms the air in the thermal, and the rising cumulus convection current increases its buoyancy. If developed enough, it can draw in surrounding moist air and maintain itself as a single, steady, organised updraft or 'pulse', perhaps even forming a towering cumulus or a cumulonimbus. As the thermals grow higher, the spacing between them generally becomes wider, although adjoining thermals may merge at height.

Thermals are a principal source of good atmospheric lift for soaring paragliders, hang gliders and sailplanes, and particularly so in the summer.
Dry thermals in the superadiabatic layer
In the arid inland areas of Australia, the very dry continental air produces generally cloudless skies with little or none of the sun's energy being absorbed as latent heat. Most of that insolation is available to heat the surface, making it far warmer than the adjacent air; ground temperatures of 80 C plus have been recorded. (Conversely, at night both the surface and the adjacent air cool rapidly, by long-wave radiation into space, dropping surface temperatures to near zero.) The daytime heating of air in contact with that heated ground produces a superadiabatic layer where the temperature lapse rate exceeds 3 C per 1000 feet. The layer is particularly unstable, with vigorous, accelerating dry thermals, and associated downflow, which may extend to 15 000 feet or more, above the terrain. Such dry thermal convection is much more powerful than that experienced in Europe where the operating limits for recreational aircraft designed for those environments is established. Powered aeroplanes flying in likely conditions should expect vertical gust shear, often with velocities greater than 20 feet per second — occasionally very much greater — and reduce cruising speed accordingly.
A surface eddy flowing into the bottom of a thermal tends to circulate around the central core, which may develop into a vortex stretching up as a spinning column usually for hundreds, but possibly thousands, of feet. A dust devil, dust whirl or willy-willy, 30–50 feet in diameter, is sometimes visible near the surface. Rotation increases as the column elongates. Because of the added vorticity, such thermals are very dangerous to light aircraft taking off, landing or flying at low altitude. The disturbance may not be visible unless it is picking up dust, dry grass or other debris. If you sight dust whirls or disturbed vegetation in the airfield area be prepared for very turbulent conditions. Taxying, parked, even tied-down aircraft, are at risk of considerable damage.

In coastal areas, cooler maritime air moving over heated, arid ground also provides conditions for propagation of willy-willies. The worst dust or sand whirls — extending to perhaps 3000 feet or more — occur in the dry, sandy interior, and can cause engine and visibility problems.

Encounters of willy-willies in flight usually involve a major upset in attitude and height loss, which should generally be countered using the upset recovery technique outlined in the 'Wind shear and turbulence' module of the 'Decreasing your exposure to risk' guide.

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9.4 Shear and turbulence near thunderstorms

Thunderstorms may be classified in four generalised types — single-cell, isolated multicell cluster, multicell squall line and supercell; although supercells may also be multicellular. Their associated surface winds — originating from the downdraughts of cold, dense air — may be both high velocity and extremely turbulent.

Single-cell storms are usually isolated storms moving with the mid-level wind. They are common in summer and occur in conditions where the wind velocity, relative to the cell motion, does not change markedly with height. (CB development has to be strong to overcome the detrimental effects of vertical wind shear). A single-cell storm may last less than 30 minutes, its life being limited to the growth and collapse of a single, large updraught pulse. The diameter of the storm may be less than one nautical mile and it will not move very far during its lifetime — less than 3 nm in light winds. Such storms do not usually produce violent wind shear near the surface, although microbursts may descend from even a mild-looking CB prior to its collapse. Single-cell storms tend to form in the afternoon when convection is stronger. The strong updraughts are very dangerous for hang-glider pilots.

Isolated, single-cell storms, embedded in low-level cloud layers, commonly form in cold winter air streams entering the south-west of Western Australia, southern South Australia and Victoria. They are generally frequent but short-lived, with soft hail and shallow wind gusts, and are caused by destabilisation of the cold air mass. They can be accentuated by orographic effects. The passage of vigorous winter-time cold fronts, preceding Antarctic polar maritime air moving into the same areas, are likely to produce the more severe multicell storms. In summer 'cool changes' of unstable maritime air moving into South Australia and Victoria from the west/south-west sometimes produce severe storms.

Multicell cluster storms (the most common thunderstorm) consist of a series of organised updraft pulses that may be separated by time and/or distance, and be closely or widely spaced. They move as a single unit and perhaps cycle through strong and weak phases. Frontal, pre-frontal, heat-trough and convergence zone systems may produce very vigorous storms several miles wide. By continually propagating new cells, these last an hour or more before the cold downdraft and outflow finally undercuts and chokes off, or smothers, the warm inflow that produces the updraft, and the system then collapses. Each new cell is usually formed in the 'zone of maximum convergence' where the gust front directly opposes the low-level wind.

Weaker multicell storms advance with, or to the left of, the prevailing mid-level wind at an average rate of 10 knots or so; but the strongest storms may turn almost at right angles to the wind. The storm turns towards the flank where the new updrafts are building — the flanking line, which is a line of CU or TCU stepped up to the most active CB. If the new cells are forming on the upwind side, usually to the west or north-west (a back-building storm), it may appear to move slowly, possibly staying in one place for considerable time.

Strong updraught/weak downdraught storms often form in conditions where there is moist air at most levels. Such storms produce heavy rain and may produce severe hail but, because of the lack of dry air inflow, severe low-level shear is unlikely.

In severe storms, with strong updraughts and downdraughts, updraught velocities increase with height, typically 1500 feet per minute at 5000 feet and 3000 feet per minute at 20 000 feet. Updraughts of 5000 feet per minute in the upper part of a storm are not unusual. Downdraught velocities tend to be slightly less at corresponding altitudes. Vertical acceleration loads of 2–3g may be experienced in horizontal flight.

The areas that most concern light aircraft are the low-level outflow regions, where downburst gusts of 50 knots or more may be reached in the initial line squall; also, lightning and hail may exist. The spreading, cold, dense current of the outflow — the gust front — may last for 10 to 30 minutes and be 1500 to 6000 feet deep. This forces the warm, moist, low-level air up and so continuously regenerates the updraught. Thus, an area up to 15–25 nm from a large storm, and 10–20 nm for a medium storm, should be regarded as a 'no-go' area for very light aircraft. An intense, narrow, initial microburst may sometimes be produced, bringing short-lived but potentially disastrous wind gusts of possibly 80 knots.

There is an area of extreme, low-level shear at the leading edge of the storm, between the nose of any identifiable shelf cloud and the position the gust front has reached; possibly 1–3 nm ahead of any rain curtain.

Vertical wind shear is usually detrimental to early development of CB cells. However, if there is:
  • strong vertical wind shear, backing and strengthening with height,
  • associated with a deep surface layer of warm moist air,
  • below a mid-level layer of dry air,
  • with an inversion separating the layers,
  • and a rapid decrease in temperature with height above the inversion,
  • then the ideal conditions are created for a severe multicell storm; or a supercell storm if the surface wind is greater than 20 knots and the vertical wind shear exceeds about five knots for each 3000 feet.
The capping inversion keeps a lid on development until the lifting force builds up sufficiently to burst through the inversion and great buoyancy develops in the colder, upper layer. Upper-level divergence and a jetstream will also enhance the vertical motion.

Strong wind shear both tilts the updraught and provides the means to rotate it (storm updraughts usually do not rotate) leading to the development of a supercell storm.

A supercell is a severe storm with a strong, continuing, organised main updraft and co-existing strong downdrafts, controlling and directing the inflow (which may have a velocity of 30–50 knots) into the cell from the surrounding atmosphere. It will usually diverge to the left of the prevailing mid-level wind. There may be broad, anti-clockwise rotation — as viewed from below — of the cloud base beneath the main updraught. Humid, rain cooled air from the downdraught may also be pulled into the normal inflow (which is often visible as scud beneath the CB). This causes part of the cloud base to lower, forming a circular wall cloud at the updraught base. If vorticity increases within the cloud, a tornadic funnel may form. A gustnado may form on the leading edge of a gust front under a shelf cloud or similar cloud bank, lasting up to several minutes. The gustnado is a brief, intense downburst vortex indicated by rotating scud.

Broad-scale rotation of a storm cell forms a mesocyclone, 1–10 nm in diameter, with a surface pressure drop of a few hPa at the centre; although a 30 hPa drop has been recorded. Supercells may last for several hours as organised systems and commonly form in warm, moist, north/north-east flow into a surface trough, and along the Great Dividing Range during summer.

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9.5 Convective downbursts

The CB downdraft can become concentrated into a downburst — a fast-moving plunge of cold, dense air. Peak wind gusts in the squall* usually last less than ten minutes, often 3 to 5 minutes, but extremely hazardous vertical gust and horizontal shear results, with extreme turbulence at the leading edge or 'gust front'. The downburst may be 'dry' or associated with precipitation ranging from virga* showers to heavy rain showers — 'wet'. The cold outflow wedges under warmer, moister air and pushes it up. A curling outflow foot of dust, tree movement or precipitation from the surface touchdown point may be visible on or near the surface. A shelf cloud often forms above the leading edge as the warmer, moister air condenses.

(*In meteorological terms a squall is a wind that rises suddenly, exceeds a velocity of 22 knots and is sustained for a least a minute then dies quickly. Gusts are shorter lived. Virga is precipitation that evaporates before reaching the surface.)

Initial downburst and outflow

Microbursts are a more concentrated downburst form, often associated with warm to hot and relatively dry conditions at low levels, and convectively unstable moist air aloft with high (5000 to 10 000 feet) based CU or TCU. If the cloud is forming when the surface temperature/dewpoint spread is 15 C to 25 C then the microburst potential is high. The high spread means the atmosphere can retain much more water vapour. Rain falling in, and from, the cloud is evaporating (virga), thus cooling the entrained air, resulting in downward acceleration of the denser air. Consequently, flight through, under or near precipitation from a large CU involves considerable risk. Significant hail is unlikely. The most dangerous area is the horizontal density current vortex ring close to the touchdown point. The ring moves outward from the contact point at high speed until it disintegrates into several horizontal roll vortices spread around the periphery. The vortices may continue to provide extreme turbulence for several minutes; inflight breakup of aircraft is possible. The maximum horizontal winds occur about 100–200 feet above ground level. Flying directly through the outflow ring would see a 180° reversal in gust direction, and extreme shear.

Roll vortices stage

In bushfire conditions the firestorms associated with dry microbursts are particularly dangerous to firefighters.

Microbursts occur under only 5–10% of CB but a less concentrated, longer-lasting gust front macroburst is normally associated with the entire cold air outflow of the larger storm cells. The severe gust fronts from a microburst extend for less than 2 nm, while those from a macroburst extend much further. The vertical gusts within the downburst, perhaps with a velocity twice the mean, may produce a microburst within the macroburst. (Unfortunately as a consequence of some high-profile airliner disasters in the USA, probably due to storm downbursts, the 'microburst' term now seems to be applied to all downburst events.)

The following is extracted from a report by an RA-Aus pilot who apparently encountered a springtime cluster storm on the southern edges of the Great Dividing Range, north-east of Melbourne, only 13 nm from home, but — fortunately — in a very tough recreational aeroplane.

"I had encountered a few small rain showers that lasted 15-20 seconds when all of a sudden I noticed the altimeter going nuts ... the next thing to happen was the Cobra Arrow was lifted and it felt like it was just thrown over end first, I pulled the power and then the fun really started; I was now heading to the ground 2000 feet below at over 160 knots ... inverted and going down quick. I can recall just yelling. I pushed down elevator and commenced a bunt — or the upward half of an inverted loop — then a half roll. That's got it up the right way then I was thrown to the right at the same time dislocating my left shoulder, inverted again and rolled back to upright then to the left and bang in went the shoulder; all the time just flying and waiting for something to give! I managed with good luck and a lot of skill to get out of this situation ... I have done a fair amount of aerobatics and I think it more than saved my life this day. I started to ease the power and flew clear of the main front, leaving the mountains two minutes later in blue skies and sunshine and almost nil wind.

The most worrying thing about the whole ordeal was that I had seen a small front about 3 miles to the west. It had actually run past me. I was looking towards home and feeling pretty good but in the mountains anything can happen. The microburst came back up a valley and changed direction almost 180 degrees. I can remember the trees just getting smashed about. I got a real close-up view of them as the back blast of the burst was shoving me upwards. I was only about 200 feet above them.

After landing at Coldstream we were able to watch the cell's continuing progress from the ground. It moved around the hills over Healesville then south towards Silvan before coming back around and passing directly over the airfield."

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9.6 Squall lines

The usual precipitation downdraft associated with an individual CB cell tends to be concentrated towards the leading edge of the storm where the cold, heavy outflow spreads out at ground level, forming a small, high-pressure cell 10–15 nm across. The dense air lifts the warmer, moist air in its path and may initiate an extremely dangerous, self-amplifying, convective complex.Within this, neighbouring storm cells consolidate into a towering squall line of large thunderstorm cells ranged across the prevailing wind direction. At locations in the path of the squall line, the resultant line squall occurs as a sharp backing in wind direction, severe gusts, temperature drop, hail or heavy rain and possibly tornadoes. If the squall line is formed in an environment of strong mid-level winds the surface gusts may exceed 50 knots.

Australian Bureau of Meteorology radar image of squall line Squall lines vary in length; some of the longest are those that develop in a pre-frontal trough 50–100 nm ahead of a cold front. These squall lines may be several hundred nautical miles in length and 10–25 nm wide moving at typically 25 knots; their very high altitude anvils extend considerably further. The squall line shown in the adjacent BoM weather radar plot is about 250 nm long. The squall lines form ahead of the front as upper air flow develops waves ahead of the front; downward wave flow inhibits and upward wave flow favours uplift.

Squall lines are a common northern Australian feature. They develop along active areas of the Inter Tropical Convergence Zone, within the feeder bands of tropical storms, along sea breeze fronts or other convergence zones, and in the summer heat trough. In south-east Australia they may also be associated with fast-moving winter cold fronts, producing severe winds and heavy rainfall.

During daylight hours the squall line may appear as a wall of advancing cloud with spreading cirrus plumes; the most severe effects will be close to each of the numerous CB cells. The convective complex releases a tremendous amount of latent heat and moisture, which may be sufficient to generate a warm core mesoscale cyclone, and consequent poor flying weather, lasting several days.

9.7 Storm avoidance

It can be seen that any downburst encounter — whether the vertical gust or the turbulent horizontal outflow — will be deadly to any light aircraft; any thunderstorm activity or potential activity should be given a very wide berth. Stay well away from any storm sighted — perhaps 10 nm for single cells to 25 nm for the largest storms — and never attempt to fly between storm cells. Be prepared to reverse course if it looks doubtful. Never fly under a CB base, and expect that storm cells may be embedded within an otherwise innocuous cloud layer. It is known for hail to fall from an apparently clear sky; this, in fact, originates from the high anvil of a CB many miles away and, of course, a lightning strike will certainly ruin your day. An encounter with heavy rain may produce total loss of visibility combined with a loss in both airspeed and lift.

Before any flight, check the online BOM weather watch radar and the area forecasts for storm activity or developing winds. Don't place total faith in the written forecast — check the latest surface chart for the position of pre-frontal zones, convergence zones, developing inland lows, surface troughs, dips in the isobars or other conditions that might indicate possible storm development or increasing winds. Remember that the latter also brings increasing gusts and thus low-level shear and turbulence; 15 minutes spent checking might save 15 weeks repair — for you and/or your aircraft. Check the sky all round at a reasonable height after take-off; if you have any doubts about what you see, scrub the flight!

Light aircraft should not be operating in the vicinity of thunderstorms. The following is an extract from an RA-Aus fatal accident investigation report.

"The pilot departed Holbrook airfield in a Sapphire aircraft for his private strip about 30 minutes away ... a line of large thunderstorms were active in the area and a witness reported that one of the nearby cells not only had virga visible below the cloud but also exiting horizontally ... the pilot was aware of the approaching weather and, indeed, was trying to beat it home ... the aircraft impacted the ground in a near vertical attitude ... about 100 metres short of the threshold of his strip ... the owner of the adjacent farm on which the aircraft crashed stated that there were thunderstorms within five kilometres and that a wind squall had passed through the area at the precise time the sound of impact was heard."

Michael Thompson's storm chasing diary at ozthunder.com/chase/chase.html provides some excellent reports and photographs of storm encounters in eastern Australia.

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9.8 Tornadoes, landspouts and waterspouts

A tornado is a rapidly rotating, narrow air column extending from the updraught base of a CB to the ground. Intense tornadoes usually develop from areas of rotation inside supercells. One theory is that the horizontal vortices produced by the low-level shear are tilted upward by the updraught inflow initiating the rotation within the cell, which develops into a mesocyclone. The vortex — deriving its energy from the latent heat of condensation released from the warm, moist inflow — spins at perhaps 30 knots, accelerating if the column contracts. Another theory is that the tornado forms when a smaller, more rapidly rotating updraught causes part of the storm base to lower — thus forming a rotating wall cloud from which a condensation funnel cloud appears, which may reach the ground. The funnel is usually located on the edge of the storm?s main updraught, close to the downdraught.

The tornado diameter at the tip can vary from a few metres to a few hundred metres. Winds at the outer edge may reach 100 knots and there may be a substantial pressure drop within the core, with the magnitude being about 30 hPa per 1000 feet of funnel length.

Some 15 to 20 tornadoes are reported annually in south-east and south-west Australia. Their intensity and size is predominantly classified as ?weak and short-lived? (1–3 minutes). They usually move from the north-west at 30 knots or so and damage a strip perhaps 50 metres wide by 2 kilometres long. (In April 1960, though, a tornado in jarrah forest near Collie, Western Australia cut a swathe 240 metres wide and 30 kilometres long, destroying tens of thousands of trees.) Although tornadic storms can occur in any season, day or night, they are often associated with dewpoint temperatures exceeding 10 C and an inversion at 6000 feet or so. Bushfires may trigger their development. Areas of high incidence are west of the Dividing Range from southern NSW to central Queensland, western Victoria and the south-west corner of Western Australia. A tornado that struck Brisbane in November 1973 produced winds estimated at 135 knots. Also a wind velocity of 90 knots was reported in the fatal tornado at Sandon, Victoria in 1976.

Fujita damage scale number for tornadic winds:
  • F0 35–62 knots: light damage (covers Beaufort scale 8 to 11)
  • F1 63–95 knots: moderate damage — caravans overturned, cars pushed off roads. (Beaufort scale 12 starts at 63 knots)
  • F2 96–135 knots: considerable damage — roofs off, large trees uprooted, light missiles
  • F3 136–180 knots: severe damage — house walls off, heavy cars lifted and thrown
  • F4 181–225 knots: devastating damage — well constructed houses levelled, structures blown some distance, large missiles generated
  • F5 226–275 knots: incredible damage — strong timber houses lifted and carried considerable distance to disintegrate, car sized missiles fly in excess of 100 m.

Landspouts and fair weather waterspouts develop, from the surface up, in a superadiabatic or similar layer within an environment with little vertical shear. The landspouts and waterspouts tend to develop from low-lying eddies along wind shifts which, in the unstable atmosphere, roll up into vertical vortices about 0.5 nm in diameter. If a vortex happens to get caught in the updraught under a TCU or developing CB then the updraught stretches (and contracts) the vortex, and the tornado-like landspout or waterspout may form. The funnel is usually indicated by dust in a landspout, but the moist sea air will provide a visible condensation funnel, plus a sheath of spray, around a 'fair weather' waterspout. In Australia most waterspouts occur in northern waters. But the world record height of a waterspout, off the New South Wales south coast in 1898, was measured from land by theodolite at 5014 feet, but this was most likely a tornadic waterspout; i.e. a tornado moving out over coastal waters. Multiple or cluster spouts may form in the one location.

   Photographs and descriptions of tornadoes, gustnadoes and waterspouts observed in south-east Queensland can be viewed in the Brisbane Storm Chasers Web site.

9.9 Other pre-frontal turbulence

Cold fronts generally travel south of 25° S latitude and west to east. Their passage produces pre-frontal/frontal wind shear, the severity of which increases with the speed of frontal movement and the temperature differential across the front. For example, a front moving at 10 knots with 5° C differential would probably produce only light/moderate shear, while one moving at 30 knots with 10° C differential is likely to produce very severe shear.
New South Wales Southerly Buster
The NSW Southerly Buster is an intense, pre-frontal squall leading a cold front moving up from the Southern Ocean. It occurs maybe 30 times per year, with about 10 major events usually in spring and summer. The phenomenon is a shallow density current, 20–50 nm wide, centred on the coast and surging northward at 15 knots with 30–60 knot gusts. The temperature may fall 10–15 C over a few minutes and there may be extreme low-level turbulence. A spectacular roll cloud may form above the nose of any frontal cloud, but usually there is little cloud and consequently little warning.

A prime cause of the Southerly Buster is the interaction of a shallow cold front with the blocking mountain range that parallels the coast; frictional differences over land and sea uncouple the flow. Other phenomena lead to intensification of the temperature gradient between the warm air mass and the cold density current; for example, a hot north-westerly or a warm dry foehn wind preceding the squall. Severe thunderstorm activity may result from the forced lifting of warm, humid air.
Sea breeze fronts
In coastal areas, differential diurnal heating promotes development of on-shore breezes which, during the day, grow in strength to 'moderate breeze' and, due to Coriolis effect, begin to back. The surface wind is a resultant of the sea breeze vector and the gradient wind vector. In hot land conditions, the sea breeze front (a density current) can travel 100–200 nm inland by midnight, if not blocked or diverted by terrain.

The cool air lifts the warmer inland air (providing a lift source for gliders) and, if conditions are suitable for deep convection, a squall line may develop and propagate along the convergence line of the surface flow. Opposing sea breeze fronts, such as occur in Cape York, may cause strong convergence disturbances when they meet. Along the eastern Queensland coast, typically between September and March, storm lines of CB up to 100 nm in length form inland in mid- to late-afternoon then move towards the coast, and are out to sea by mid-evening. Such squall lines may be difficult to avoid if encountered unexpectedly.

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9.10 Low-level jets

Low-level jets may form by interaction between anticyclones and mountain barriers — particularly in the area west of the Dividing Range in northern NSW and southern Queensland. This produces a zone in the friction layer, which may extend 50 nm plus, where wind velocity is highly geostrophic and concentrated both vertically and horizontally, so that large, low-level shears are produced.

Core speeds of 25–30 knots,and up to 50 knots, occur in an otherwise light surface wind area, particularly early to mid-morning in winter, with the anticyclone centred over the interior. The overnight cooling of the western slopes produces a horizontal temperature gradient. A low-level jet in a circuit area is very dangerous to light aircraft.

9.11 Lee wind downflow, eddies, rotors and vortices

Pilots of aircraft flying on the lee side of higher topographic features — particularly if taking off or landing, or flying parallel to a ridge — should be aware that the downflow (sinking air) encountered can exceed a powered aircraft's climb capability; there is usually no indication of the downflow other than that sinking feeling!

(Of course glider pilots will find atmospheric upflow on the windward side of the ridge providing the opportunity for 'ridge soaring'.)

Strong sink conditions may occur on the lee side of mountains, ridges, valley walls, hills and islands, and even extend above the height of the barrier. The severe sink associated with this lee side downflow is a function of wind speed and slope angle. For example, 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 per minute — greater than the maximum climb rate of many ultralights. This downflow airstream may be non-turbulent, particularly when associated with standing wave conditions, so a pilot may not have an early indication of the danger. Turbulent eddies/curl-overs within the downflow may add to the ambient sink rate.

sink vector diagram

The following is an extract from an RA-Aus fatal accident investigation. Note: the Capella aircraft was last sighted in flight over a lightly forested area not far above tree-top height and thought to be intending to land in the grounds of a winery familiar to the pilot. The aircraft impacted the ground almost on the apex of a small rise and about halfway down the slope in a lawn area. Weather was fine with good visibility, and wind was 10 to 15 knot northerly with strong gusts.

"Indentations in the ground and damage to the aircraft indicate that the aircraft had initially contacted the ground travelling in a north-westerly direction at a relatively low forward speed but with high downward force. The wind direction and strength combined with the topography at the accident site (a long east-west ridge to the north) would have combined to produce a small standing wave with significant downflow. An aircraft approaching at minimum speed and tree top height could expect significant sink in that area. This could translate to loss of airspeed if the pilot was concentrating solely on touching down on a given spot."

Injuries suffered when an aircraft sinks with high vertical decelerations are usually very much more severe than those suffered in horizontal decelerations of similar magnitude.

Some pilots have expressed the opinion that a light aircraft cannot get into real trouble in a lee sink situation because the airstream must level out before reaching the surface and so will take the aircraft with it. This is not so; inertia is related to mass and the mass of a molecule of metal is far greater than that of an air molecule.

Eddies with large sink rates, possibly greater than 1000 feet per minute — lee wind eddies — may occur, in only moderate wind conditions, on the lee side of mountains, ridges, hills and islands. Sink will be particularly dangerous when accompanied by high temperature (i.e. high density altitude) and high aircraft loading. Airfields along the eastern Australian coastal strip will be influenced by lee downflow and eddies when the westerlies are blowing during August to October.

lee downflow and eddies

Vortex-like turbulence tends to develop when slope gradients exceed one in three [18] and it appears at a lower level than the long horizontal vortices associated with lee waves. As the vortices 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 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.

Horizontal lee eddies can also develop from friction with the mountain side; this normally requires an inversion at or below the mountain top with a strong, sustained wind exceeding 20 knots. The eddies may be visible if cloud forms under the inversion.

Wake vortices, similar to those produced from aircraft wingtips, can develop in the lee of lone hills and peaks in strong, sustained wind conditions. The strong — often twin — spiral turbulence can be felt at a distance ten times hill height and at altitudes considerably above and below hill height. In 1966 a BOAC Boeing 707 suffered in-flight breakup in such conditions, while giving passengers a view of Mt Fuji on a cloudless day. A search and rescue aircraft recorded airframe loads of +9g /−4g when flying through the same vortices. Ravine winds can also develop wake vortices.

Ravine or gap winds occur in narrow gaps which that part a mountain range. The pressure difference between the two sides of the barrier when moderate to strong wind flows across the range creates a pressure gradient — with consequent strong, turbulent winds in the ravine and flowing from the exit. This also applies to gullies, to some extent.
Effect of windbreak eddies
Turbulent windbreak eddies will form in the lee of obstacles such as trees adjoining an airstrip. The distance they spread from the windbreak is dependent on the density and height of the trees. Generally, the windbreak affects airflow for a horizontal distance equal to ten times the height of the tree line, if the flow is perpendicular to the windbreak; the more turbulent flow is closest to the trees. There will also be a significant lee-side downflow extending over the windbreak shadow, its vertical component being dependent on the ambient windspeed. Such downflow conditions require that take-off and approach speeds are higher than normal, and that ample clearance is provided — not a place to be low and slow! In addition, in conditions of high solar radiation, the differential heating of airstrip surfaces caused by partial shading can promote turbulent vertical eddies over the take-off area.

The following is an extract from an ATSB fatal accident investigation.
"The pilot and his passenger were conducting a private flight in the pilot's Jabiru aircraft in the Southport area. Several other pilots heard the pilot advise over the radio that he was conducting a simulated engine failure and glide approach. The aircraft subsequently impacted a steep embankment short of runway 19 at Southport aerodrome and on the extended runway centreline. The embankment was approximately 2 m high, about 210 m from the displaced approach threshold and 30 m short of the sealed runway surface.

An examination of the wreckage indicated that the aircraft had impacted the embankment in a moderately nose-high, left wing-low attitude. Damage to the propeller indicated that the engine was delivering significant power at the time of impact.

Local procedures required that pilots conduct right circuits when operating on runway 19. Tall trees adjacent to the aerodrome induced localised mechanical turbulence, windshear and downdrafts when the wind was from the southeast. At the time of the accident, the wind was recorded on the Gold Coast Seaway as 150 degrees at 15 knots, gusting to 18 knots.

It is likely that the aircraft entered an area of turbulence and high sink rate generated by the prevailing wind over the adjacent trees. Given the evidence of significant power at the time of impact, it is possible that the pilot had initiated a go-around at a stage in the approach from which it was not possible to establish a positive rate of climb."

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9.12 Mountain waves

Mountain waves or lee waves are a manifestation of an internal gravity wave. Such waves occur fairly frequently over, and in the lee of, the mountainous areas of south-eastern Australia, and in the lee of the mountains along the east coast in strong westerly wind flow conditions.

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 20 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 vortex or rotor flow. These vortices differ from the streaming rotors formed in lee wind eddies. They are closed with a long horizontal axis; form in the lee of, and parallel to, a well-defined escarpment, and remain fixed in position. Curl-overs may also be produced by friction slowing the near-surface downflow. Usually cloud will not form in the vortex but should it do so, it may range from scraps of scud to a long, solid roll cloud.

mountain wave turbulence

Turbulence in and under the rotor area, i.e. from the mountain height down, will be severe to very severe. Some evidence of the rotor may be seen on the surface — rising dust, sudden and erratic wind changes, etc. Readers interested in the techniques recommended for flight in such conditions should check www.mountainflying.com

If conditions are suitable, lenticular cloud that appears along the crests may reveal the waves; the stationary clouds continuously form and dissipate in the vertical air motion.

Vertical movement of 2000 feet per minute is common in lee waves and could be much greater; the vertical component being dependent on wave length and amplitude. Lee wave downflow can easily exceed the climb capability of any powered light aircraft. In suspected lee wave, or potential vortex, 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; never attempt to cross a ridge at 90° when flying into wind in potential lee wave conditions.

Wave length tends to increase with stronger wind aloft, and is also affected by temperature and stability conditions. The shorter the wave length, the steeper the ascents and descents.

Amplitude depends on airstream plus the shape and size of the ridge. It will be at a maximum within the stable layer, particularly if the layer is shallow with great stability. The larger the amplitude, the further the air moves up and down. Over a plain, the wave effect can continue for 100 nm. The disturbance may extend to the stratosphere. Depending on length and amplitude, mountain waves may produce considerable areas of smooth, laminar uplift and sink — much sought-after by experienced sailplane pilots. Mountain waves are unlikely to break unless the amplitude is high, but if they do break then moderate to severe clear air turbulence will result.

A resonating mountain or orographic wave will produce strong, adiabatically warming downslope winds — called foehn in Europe, chinook in the Rocky Mountains area and Canterbury north-wester in New Zealand. In January 1943, a temperature rise of 27 C (− 20 C to 7 C ) was recorded in the space of two minutes in Spearfish, South Dakota. The resonating waves may reach extreme heights and may produce downslope windstorms exceeding 100 knots in the lee of high, extensive mountain barriers. Updrafts and downdrafts in excess of 3000 feet per minute are common; 7000 feet per minute has been reported in the USA.

Weak foehn winds occur regularly in the south-east Australian coastal strip under the influence of westerly or north-westerly flows; they can bring unseasonal warming to areas around Lakes Entrance, Victoria, for example.

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9.13 Valley winds

Valleys and gullies tend to develop their own rather turbulent air circulation, somewhat independently of the ambient wind overflow. They have a tendency to flow up or down the valley/gully regardless of the general wind direction. However, if the overflowing wind exceeds 20 knots or so then significant downflow and turbulent eddies may form over the windward slopes of larger valleys, whilst rising air may be experienced over the leeward slopes. Thus aircraft contemplating a 180-degree turn within such a valley should first move over to the leeward side before commencing the turn; if available an appropriate flap setting should be used to allow a slower speed, smaller radius turn. This minimises the risk of encountering turbulent downflow on the windward side.

Circulation within valleys may also be modified by solar heating of the valley slopes. Anabatic winds form during the day when hillside slopes are heated more than the valley floor. The differential heating of contact air causes air to flow upslope. Wind speeds of 10 knots or more may be achieved.

9.14 Solitary waves

Solitary waves — external gravity waves or undular bores — are common in the dry interior of northern Australia, particularly in spring prior to the wet season. They occur as severe, low-level clear air disturbances (a horizontal vortex) accompanied by a transient surface wind squall. When sufficient moisture is present a long, continuously forming roll cloud may appear with base at 500–1000 feet agl and top at 3000–5000 feet agl. Long distance soaring capability is provided by the uplift at the front of solitary waves.

The roll cloud (and thus the vortex) may extend for several hundred nautical miles. Because it forms along the wave leading edge updraft and evaporates in the trailing edge downdraft, it appears to roll backwards. The wave may manifest itself as one large amplitude wave closely followed by several smaller diminishing waves.

Solitary wave disturbances seem to be generated on an inversion by a disturbance such as late afternoon thunderstorm activity, the collision of opposing sea breeze fronts or the interaction of the northern end of a cold front with a developing nocturnal inversion.

The waves, usually a 'family', propagate at a speed of 15–30 knots, relative to the ambient air flow, in a low-level stable layer under an inversion at 1500–2000 feet or so with a deep stable layer above. The neutral layer enables the wave to propagate without being damped and to travel long distances; i.e. the layer acts as a wave guide.

The Gulf of Carpentaria Morning Glory is a product of the late-afternoon interaction of the sea breeze fronts on Cape York. The north-easterly sea breeze, aided by the prevailing easterly/south-easterly winds, is more dominant than the westerly sea breeze. The westerly breeze increases the depth of the cooled surface layer and produces a sharp gradient in the low-level wind profile. The surging higher-density air from the north-east collides with the westerly flow. This builds a long ridge of the cooler, denser air protruding into the inversion. The resulting disturbance in the inversion layer, when the convergence collapses at night, produces solitary waves in the boundary layer that propagate to the south-west on the nocturnal inversion. The waves reach the southern Gulf coastline about dawn and provide an amazing soaring ride for sailplane and hang-glider enthusiasts. Similar phenomena occur in other parts of the world but are not as extensive, or as regular, as the Gulf of Carpentaria phenomenon.

   Photographs of magnificent Gulf of Carpentaria roll clouds can be viewed on the Morning Glory web site. Be sure to view the Gulf of Carpentaria satellite image for 8 October 1992 (8:00 am local time) to see the Morning Glory threaded diagonally right across the Gulf.

The occurrence of several roll clouds arriving in the Burketown, Queensland area from the north-east, south-east and south, during the same morning has been recorded. When opposing solitary waves meet, they pass through each other and reform their shape and velocity.

If unaccompanied by a roll cloud, solitary waves arrive unannounced, presenting a very severe low-level wind shear and turbulence hazard to aircraft. With suitable surface conditions, aircraft flying at low levels may be warned by a line of raised dust. With the passage of a wave, the closely spaced updrafts and downdrafts may each exceed 2000 feet per minute and the transient wind gusts may vary surface wind by 30 knots or more; not something to fly into head-on, but providing an outstanding ride for the capable pilot.

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9.15 Aircraft wake vortices

Aircraft themselves induce another form of mechanical turbulence. All aeroplanes (and helicopters) develop wake vortices in flight, their size and energy being dependent on both the aircraft's mass and the dimension of the lift coefficient. The latter, of course, is dependent on aoa and wing configuration (i.e. flap and high-lift device settings) so, for any particular aircraft, its wake vortices are greatest at the slowest flight speeds — at rotation for take-off followed by the climb out, plus the approach followed by the flare for landing.

The relatively large surface area and the shape of weight-shift trike wings, at high aoa during take-off and landing, generate significant vortices that may trap any following aircraft with a low wing loading. In light winds, the vortices generated by aircraft the size of twin turboprops tend to persist for at least a couple of minutes as they slowly sink a couple of hundred feet below the flight path and, of course, drift with the wind. Gusty wind conditions or contact with the ground will dissipate vortices more rapidly but will spread additional turbulence while doing so.

It is often thought that an aircraft encountering the wake vortices from an aircraft of similar size would not be unduly upset; however, this is not so and particularly if the vortex is of higher energy such as that generated by a high lift coefficient STOL aeroplane. Such encounters with relatively small vortices can be very dangerous if there is insufficient height to recover from any consequent uncommanded roll and yaw; and, of course, the upset will increase in severity as the relative mass of the vortex-generating aircraft increases. The most likely points of wake encounter are when turning base to final behind an aircraft landing from a straight-in approach and before touchdown or after lift-off if too close to any aeroplane.

Certainly it is wise for light aircraft to anticipate and avoid encounters with the vortices from significantly larger aircraft. The general concept is to follow at least two minutes behind them in take-off or landing, and try to maintain a flight path somewhat above (which may not be possible) and upwind of the preceding aircraft. (In 1994 a Mooney 201 aircraft failed to do that when taking off behind an RAAF Hercules at Wagga Wagga, New South Wales, and ended up as wreckage alongside the runway.)

9.16 Clear air turbulence

Turbulence above the boundary layer and not directly associated with convective cloud is clear air turbulence [CAT]. CAT is usually associated with regions of strong vertical wind shear and temperature inversions; with jet streams, particularly in convergence/divergence areas; or with internal gravity waves, generated in the lee of mountain regions. The waves may break at various altitudes and distances from origin, generating many patches of CAT. Thus CAT is not just a concern for high-altitude aircraft; it can also adversely affect aircraft flying at comparatively low altitudes.

Gravity waves, with consequent turbulence near thunderstorm tops [TNTT], also propagate from the intrusion of strong convective clouds into a stable upper layer. Upper-level turbulent patches vary in length from one to thirty nautical miles and are usually less than 2000 feet deep. Aircraft loads of minus 1g to plus 3g may occur.

Upper-level frontal zones form independently of surface fronts in conjunction with jet stream intensification and with strong temperature gradients. The frontal zones are characterised by subsiding dry air and a downfold in the tropopause. Strong wind shear at the zone produces severe CAT.

9.17 Effect of heavy rain

Flight through rain causes a water film to form over the wings and fuselage; if the rain falls at a rate exceeding perhaps 20 mm per hour, the film over the wings is roughened by the cratering of drop impacts and the formation of waves. The effect, which increases with rainfall rate, is a lowering of the lift coefficient value at all angles of attack, with laminar flow wings being most affected and fabric wings least affected. The stall will occur at a smaller angle of attack; i.e. the stalling speed increases, which is further compounded by the increased weight of the aircraft.

The water film will increase drag, and the encounter with falling rain will apply a downward/backward momentum, which may be significant to a light aircraft. Propeller performance is degraded and water ingestion may affect engine output.

Thus the rain effect can be hazardous when operating in conditions of low excess aircraft energy — typically when taking off, landing or conducting a go-around. Visibility through a windscreen may be zero in such conditions, so a non IFR-equipped aircraft will be in difficulties.

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Further reading
The online version of CASA's magazine Flight Safety Australia contains some articles relating to microscale meteorological events, which are recommended reading. A categorised index of articles of interest to recreational pilots contained in Flight Safety Australia since 1998 is available on this site. Particularly check the articles in the 'Micrometeorology and weather emergencies' category; there are also relevant articles within the other categories.

Next –  aircraft icingAviation Meteorology ground school
The next section of the meteorology ground school covers aircraft icing

Aviation meteorology guide modules

| Meteorology guide contents | The atmosphere and thermodynamics (part 1) | Thermodynamics (2) and dynamics |

| Effects of altitude — contained in the Flight Theory Guide module 2 & module 3 |

| Cloud, fog and precipitation | Planetary-scale tropospheric systems | Synoptic scale systems |

| Southern hemisphere winds | Mesoscale systems | Micrometeorology — atmospheric hazards |

| Airframe and engine icing | Atmospheric electricity | Atmospheric light phenomena |

| Aviation weather reports and forecasts |

Copyright © 2000–2011 John Brandon     [contact information]

Page edited September 2009 by RA-Aus member Dave Gardiner