Africa 2003
Sandstorms and Duststorms in UAR
This document formed part of the Africa briefing that used to be given by the UK AIS
The errors are from the original version
The UAR was Egypt 1958-71 and Syria 1958-61
Perhaps in aviation meteorology the duststorms phenomenon is considered to be the most striking weather feature in the different areas of UAR.
It is probably true to say that most accidents in modern aviation can often be attributed to a combination of circumstances, duststorm weather conditions in some form or other figure only too frequently as one of these contributory factors. Since, in aviation, safety is the primary concern of every one connected with aircraft operations, it is not curious that this weather phenomenon has grown to be of special interest to aviators, pilots and airmen as well as meteorologists and aviation weather forecasts.
Definitions
Duststorms and Sandstorms. Dust or sand is raised from the ground by the wind and is carried upwards to a height depending on the meteorological conditions and state of soil including the size of the particles.
According to international convention the phenomenon can be defined as a duststorm or a sandstorm where surface horizontal visibility is reduced to less than one kilometer. A distinction must however be made between a sandstorm and a duststorm; the main difference being the size of particles the air could support in each case. The diameter of the particles in a duststorm is not supposed to exceed 100 microns. (one micron equals 10 nm).
We can define a duststorm as a rapidly moving mass of air which contain large amounts of dry opaque particles reducing the horizontal surface visibility to less than one kilometer.
Dust Haze
The fine dust carried upwards in duststorms of various sorts is gradually dissipated through the atmosphere and gives a general haziness to all air of the desert origin. Air so polluted will retain its haziness for long periods, since particles are too small to fall be gravity at any measurable speed. Such haze may continue for days at a time and commonly extends up to some 3000 metres. Horizontal visibility is seldom reduced to less than 100 metres but the air to ground visibility may be worse than that along the ground.
Haze Layers
Sharply defined layers of haze occur at any level in the troposphere and give horizon when viewed from above. The most common occurrence is when the surface air polluted with smoke or dust is carried upwards by turbulence and convection until it meets a reduced lapse rate or inversion which the up -currents cannot penetrate. Largely on account of the diffuse reflection of light from the upper surface, the layer may completely obscure the ground in any direction far from the vertical, on some occasions the ground may be entirely invisible although the aircraft may be seen clearly by an observer on the ground.
Conditions Favourable for Duststorms
Synoptically the majority of wide spread duststorms which occur in UAR during the different seasons of the year are of the pressure gradient mainly for either southerly or northerly winds.
The development of any pressure gradient duststorm depends on many factors some of these are:
State of soil
To cause a sand or duststorm to develop we should have favourable meteorological conditions to increase both soil granulation and dryness to assist the effluence of sand and dust particles from the earth's surface. Thus the soil if compacted or caked due to any reason, eg. precipitation or deposition of dew, it should be dried and granulated so as to reduce the cohesion forces among the particles through the upper most layer of the soil Long periods of strong soil heating and long periods of advection of warm and dry air produce soil dryness as well as increasing porosity. Moreover alternate heating and cooling of the soil, same as that produced by strong diurnal variation of soil temperature make the soil granulates to its original constituents and the particles become loose.
Extent of the desert surface over which the winds blow & Speed of the layer of air in contact with the surface of sand or dust:
The greater the speed of the moving air, the layer will be the transfer of momentum from it to the surface sand or dust and consequently the raising of sand or' dust from the ground will be more marked.
Degree of surface turbulence:
Turbulence is present to some extent in all air movement near the surface, and it is well known that in ordinary winds, it increases with the wind velocity. It must also increase with the temperature difference between the hot surface of the ground and the air above.
Descending motion associated with turbulence and consequent raising of sand or dust will be more marked, if the overlying air is appreciably cooler. Also it is well known that surface turbulence increases with the roughness of the surface and generally more marked over land than over sea (To the occupants of an aircraft, turbulence is recognised as bumpiness and the difference between flying at low levels over land and over sea in this respect - is well known).
Lapse rate of temperature in the lower layers
The greater the lapse rate, the larger will be degree of instability, and the greater will be the transport dust or sand, in the vertical.
Classification of Duststorms in UAR
The duststorms which occur in UAR could be classified as follows:
Khamsin type of duststorms
Caused by the steepening of the pressure gradient for southerly and south easterly winds blowing in front of deep depressions when passing, forming or developing near the northern coast of UAR over the desert or over the southern Mediterranean. This type of duststorm occurs during the transitional seasons mainly in spring and affect all UAR.
Perhaps the most important synoptic feature in spring all over the great Africa desert is the high tendency of desert depressions on either synoptic or sub-synoptic scale. Such depressions generally take a preferred eastward track near the North African coast over the desert or over the Southern Mediterranean. In these regions specially in spring the pronounced contract in surface temperature between the water and the desert air acts as a further resource for the genesis and development of such desert depressions.
These depressions are associated in most cases with an active system of strong southerly and southeasterly, hot, dry and dust laden surface winds blowing in front of them. These winds cause frequently intense temperatures and humidity anomalies as well as rising dust and severe duststorms all the north African countries. These winds are given local names in North Africa Sirocco is the fundamental name used for these winds of the continental tropical origin when blowing over the Mediterranean sea and its coasts. They are known by Chilli in Morocco, Algiers and Tunisia, Gibli in Libya and Khamsin in Egypt. If these desert depressions move further to the east, similar violent winds blow over Arabic countries east of the Mediterranean where they are called Simoon.
Winter Type of Duststorm
Caused by the steepening of the pressure gradient for strong southerly winds blowing in front of deep extra-tropical cyclones when passing forming or developing over the east Mediterranean or even over southern Europe. This type occur mostly during later winter and early spring and affect mainly lower Egypt including Cairo area and in some cases middle Egypt.
Cold Front Type of Duststorms
Caused by steepening of pressure gradient for northerly winds in the cold air in the rear cold fronts associated with deep extra-tropical or desert depression when leaving the east Mediterranean and moving eastward or north-eastward. These duststorms occur both in hot and cold seasons mostly during winter and spring. The affect all areas of UAR mainly upper Egypt. Duststorms of local character
- Instability type of duststorms: Caused by the downdrafts along the edges of thunderstorms clouds creating turbulent equally winds which when descending steeply on ground raises dust and occur mainly during the transitional season affecting lower Egypt. This type of duststorm is more common in autumn than in spring.
- Dust Devils: Caused by intense surface heating of dry regions on hot calm afternoon with clear skies, thus causing a very steep lapse of temperature.
Khamsin Type of Duststorms
Khamsin weather conditions in UAR mostly take place in associate with the passage or desert depressions.
The main weather features which are sufficient and necessary to specify what is called Khamsin weather are, the excessively high surface temperatures and the excessively low dew points (or humilities) associated with the invasion of S and SE winds of Continental tropical origin. However, there are other additional conditions which are not sufficient to specify Khamsin weather but may be associated with it in some occasions. These are the strengthening of the S and SE winds which give rise to rising dust and duststorms and the occurrence of active cloud formation; rainfall and thunderstorms.
In spring continental tropical air over its source region in Africa is very hot and dry. The surface air temperature may be as high as 450C and the relative humidity as low as 3 per cent. The Air mass has a very high lapse rate usually dry adiabatic up to 6000 metres and it convectively. Always khamsin cresses or long fetch over the desert of North Africa. In Spring the temperature of continental tropical air stream continues to rise as long as it remains over the desert and reaches its maximum rather near the Mediterranean coasts. Perhaps in the most extreme cases the air may have been previously warmed by forn effect on crossing the mountains between 170N and 230N (situated almost in the middle of the great desert).
Very large changes in surface air temperatures occur when khamsin winds are usually followed by the cold front. Falls of the order of 120 frequently occur, and if 160C has been recorded.
Because of lack of enough water vapour in khamsin dry air, the daily range of surface air temperature is usually very high due to strong nocturnal radiation specially with rather weak winds and clear skies. This gives rise to strong inversions during night. In khamsin, daily range of surface air temperature of the order of 280C occur.
The most marked changes of humidity of the air at the surface occur with the passage of cloud fronts in khamsin conditions. Vapour pressure may rise by about 10 mbs and the dew point by about 100C with the arrival of colder air, from north.
In Khamsin, duststorms raised by the winds in desert depressions give visibility less than 50 metres at times and the wind carrying the dust may cause reduction of visibilities in regions far distant from the originating duststorm. These duststorms reach their worst ahead of the cold front of the desert depression, Occasionally strong to gale gusty winds ahead of the warm front of a khamsin depression may also raise much dust.
With exceptionally deep and extensive desert depression the belt of duststorms may be more than 200 kilometers in width and thick dust may be carried up to 6000 metres. Such dust may travel great distances northwards even over the sea. The extent of visibilities under 200 metres is usually limited to a narrowband and across the width of the air stream but visibilities of about 100 metres may be quite general on great areas near the approach cold fronts. A duststorm at any one place seldom lasts for more than 12 hours, although the depression may cause duststorms for some days as it moves along its track. Generally the dust clears after the passage of the cold front.
The frequency of duststorm in khamsin season (late winter and spring) may be gauged from the fact that they occur chiefly with the khamsin depressions, of which, apart from secondary thermal ones, the are about 8 in the season. They are more common in May. The amount of dust raised by the wind and the height to which it is carried both increases dung the day normally reaching their maximum in early afternoon when convection is most vigorous. Relative improvement with respect to wind and visibility is usually experienced during night.
Khamsin winds in most cases blow from SE. As the cold front approaches the wind becomes southerly and increases with marked gusts, bringing violent duststorms. With the passage of the cold front the wind veers abruptly to NW and the dust almost clears rapidly though the wind may continue strong. Under normal conditions when the wind decreases behind the cold front and dust is no longer raised in the locality, the atmosphere may remain hazy for many hours and sometimes for a day or two due to persistence of suspended fine dust.
Sometimes, the southerly khamsin sets in abruptly replacing a light east to southeast wind causing a sharp rise in temperature, fall of dew point and arrival of a wall of dust: this warm front which is normally cloudless herald the arrival of hot desert air from far to the south. Visibility always remains bad until the cold front passage.
Usually in khamsin conditions with advance of night and the formation of ground inversions, which is well marked in desert dry air, the surface wind drops and there may be no raising of dust.
It is worthy to mention that especially when the cold front passes through during the evening, radiation clouds (low stratus) always from the early morning. This can be explained by the formation of a marked inversion at night due to strong nocturnal radiation and the invasion of the humid cold air in the rear of the forward portion of the cold front in the lower layers.
Generally these clouds in few hours time after sunrise. Usually in Khamsin, continental tropical air has almost dry adiabatic lapse rate and is convectively unstable. Due to lifting by heating from below mainly during afternoon and by strong convergence in the vicinity of the desert depression and its cold front, strong convection to great heights is present but the relative humidity is so low at the surface that cumuliform cloud bases are usually above 3000 metres. Cumulonimbus clouds of high base occur in some occasions but precipitation falling from this high based thunderstorm cloud frequently evaporates completely before reaching the ground. These dry thunderstorms however produce squall winds and may cause severe local duststorms. Severe turbulence in the lower layers and aloft, restricted ceiling and poor visibility accompanied by gusts and squalls near the ground, presents hazards to aircraft.
Khamsin type of duststorm due to desert depressions
Desert depression form usually in the lee of Atlas mountains either when a cold north westerly or north easterly outbreak of air moves towards the Atlas Range. The depressions give rise to Sirocco or Khamsin winds and are therefore call sirocco or khamsin depressions. There is at all times of the year a tendency for all depressions to form such of the Atlas mountains but they are activated most frequently in spring when the 'Mediterranean Frontal Zone' has moved well south into the western desert. During the transitional period (March-May) the sea temperature in comparison with that of the rapidly heated land are still rather low and the Mediterranean Front is often very sharply defined.
During this season and to a lesser extent in autumn we have the greatest activity of desert depressions and they are at that time perhaps the most important type of Mediterranean depressions.
Desert depressions develop most readily when a polar or artic stream from the NW or NE flows in their rear when this happens there is a corresponding induced flow of continental tropical air northwards, often bringing dust with it to give duststorms which are most common over north Africa in spring.
A desert depression generally begins to move eastwards along the Mediterranean front. If there is pronounced northward steering the depression moves north-eastwards to Europe before reaching the east Mediterranean. If there is a high pressure established over the Balkans and the Black Sea, the desert depression to the African coast either over the sea or overland affecting the UAR. This always is maintained if there is a continue flow of polar wind around the depression.
There may be frontal rain ahead and to the north of the depression and showers in the polar air. This is most likely to occur with depression of late winter and early spring. In April and even May the air ahead of the depression is very dry and duststorms are likely to occur in the strong to gale force winds that flow southeast ahead of the warm front at times and in the warm sector, and from southwest just ahead of the cold front. Visibility may be reduced to well below 200 metres in the duststorms, dust may be carried northwards over the sea. Generally the duststorms cease when the wind veers to NW and decreases in speed.
Although depressions moving inland on tracks South of the North Africa are not frequent, they are the most troublesome to the weather forecaster owing to the scarcity of observations in the neighbourhood of the track and because they may bring gales and duststorms to Lower and Upper Egypt for many hours or more. These depressions are likely to give thunderstorm at least at medium and high levels mainly near the tip of the warm front. The first sign that development of such a disturbance is taking place is a steady fall of pressure over a wide area of Libya and Egypt although only small waves may be apparent on the Mediterranean Front.
The development and steering of these depressions as most of the depressions in the Mediterranean are associated with the general thermal pattern and winds in the upper troposphere as the case of other depressions in middle and high latitudes.
Generally the motion of such depressions is closely related to that of the associated upper trough. They may develop and move quickly eastwards if a good supply of polar or arctic cold air is maintained rather far ahead. Otherwise they may remain almost stationary for days but on the whole slow movement is more common than rapid movement.
Khamsin type of duststorms due to thermal secondary desert depressions
The above mentioned desert depressions as most of the depressions in the Mediterranean are generally small be comparison with those of higher latitudes. However, they are large enough to be considered of the synoptic scale.
Another type of desert depressions of subsynoptic scale frequently occur in spring and affects the areas of the UAR. These depressions form and develop on the desert near the North African Coast in the warm sector of the controlling desert depression east of the main cold front. They form when the pressure gradient of the holt continental tropical air in the warm sector is weak enough to permit for the genesis of a shallow surface of separation between the heated dry continental tropical air stagnating south of this surface and the modified continental tropical air originating from the Mediterranean and prevailing north of the surface.
Such secondary desert disturbance usually develops rapidly and splits out of the controlling system shooting rapidly eastwards or north-eastwards steered by the strong thermal winds in the lower layers, while the main desert depressions stays related with the upper cold trough.
These secondaries are mainly thermal ones. They are so shallow that they hardly appear at the 850 mb level. Duststorm of rather local character develop in their vicinity. After passage of the depressions may follow. Usually more than one pass and the Khamsin conditions last for a long time (7-10 days) till the main cold front passes and destroys the original system.
The sources of energy for the development of these depressions are mainly the heat given by insolation to the surface layers and perhaps the heat energy that can be supplied to the surface air layers by baked blown dust. Consequently development and movement of such depressions are subject to pronounced diurnal variations. They get deeper and faster during day while at night they have a tendency to fill and stagnate.
Winter Type Duststorms
Depressions entering the Mediterranean area are generally formed as secondaries to more northerly depressions of extratropical origin moving east or northeast.
Lee depressions occasionally form in the Ionian Sea, the Aegean Sea and in the region of Cyprus, but the formation of new depressions in these areas is rare and normally confined to the winter months (December to February). What more usually happens is that depressions already in the area become rejuvenated and develop rapidly as a north easterly continental polar or arctic air stream moves towards the central or Eastern Mediterranean. They frequently become independent and controlling when reaching the East Mediterranean.
The isobars of the southern part of a winter depression run more or less from west to east and extend over a large area. Accordingly strong SW winds may blow in front of the depression over north UAR giving rise to rising dust and duststorms over lower Egypt and at times over Middle Egypt, mainly ahead of the cold front.
Although these SW winds are of continental tropical origin, they are usually in this season originated over the northern parts of the desert just South of the coast of Africa where the mean surface temperature are rather low in winter. It may be mentioned in this connection that this type of duststorms have not to be confused with the khamsin type since conditions of high temperatures and very low dew point characterizing the khamsin type are not fulfilled in the winter type.
There is a great variety in the extent and severity of duststorms of this type, depending on the strength of the wind and the state of the ground. They reach their worst in the strong winds ahead of the cold front generally in a belt of 50 to 100 km wide just ahead of the front.
With the passage of the cold front the dust may blow again if the place is not too near the coast specially when there has been no rain with the front and when the cold air is unstable as is often the case early in spring. In such cases a duststorm may continue for some hours after the passage of the cold front unless favourable conditions the storm degenerates into occasional dust.
Although visibilities in duststorms of this type may be reduced to very low values (50 metres or less) dust does not usually rise to very great heights as the case of khamsin type duststorm perhaps due to lack of heating from below and rather unfavourable state of soil.
Cold Front Type of Duststorms
In winter and early spring there is a marked tendency for Mediterranean depressions to remain stationary near Cyprus for some time. Under favourable synoptic conditions, and extremely cold stream of polar continental air from Russia and central Europe flow southward around the depression causing its regeneration into a depression which becomes more and more vigorous, the longer it stays stationary. In such a case the polar air is intensively cold and has a relatively sea track so that the associated cold front is well marked. As the depression moves eastwards, the cold air rapidly spread southwards across Egypt and Sudan as it meets no physical obstacle in Nile valley. This air on meeting and under running the warm desert air to the south mainly in Upper Egypt and Sudan, quickly develops a cold front which if sufficiently active, causes duststorms over these regions, otherwise it may cause only rising dust.
As the cold front moves south and southeastwards across the desert the following factors operate in such a manner that great quantities of sand are carried aloft within the polar air mass.
- Scooping action of the down slope of the cold front.
- Strong turbulent flow behind the front.
- Instability caused by heating from below.
The movement of cold front on surface is appreciably retarded and less marked during the hotter part of the day due to surface heating of the cold air and loss of temperature contrast in the forward portion. With the decreased of insolation, th the nose of the cold front becomes active again. However, with advance of night and the formation of ground inversion, which is well marked in the desert area, the surface wind drops considerably and there may be no raising of dust therefore it may be difficult to recognise the front on the surface chart.
The existence of strong northerly winds between 600 and 1500 metres may mark the position of the front during night. After sunrise when the inversion layer is destroyed, the strong Northerly flow above the frictional layer comes down to the surface and suddenly a very wide area is affected by blowing duststorms. The worst conditions perhaps are usually experienced between 0600 and 0800 UT in Upper Egypt.
Generally visibilities improve gradually when the surface wind drops but air polluted with fine dust particles may retain its haziness for long periods and the air to ground visibility may be in some cases mainly in Upper Egypt worse than the horizontal visibility which also frequently drop to low values.
Moreover it may be mentioned in connection with night inversions expressed previously that haze layers may occur in associate with these inversions, the surface air polluted with fine dust particles is carried upward by turbulence until it meets the inversion which the up current cannot penetrate. This phenomenon happens mostly when the cold front passes the place late in the afternoon or in the evening so that the inversion formed at night is quite strong due to both radiation and the existence of the polar cold air in the lower, just in the rear of the front.
For an aircraft when landing in such conditions its landing lights if put on cause diffuse reflection from the upper surface of the haze layer. This diffuse reflection may obscure the ground completely in any direction far from the vertical.
Duststorms of Local Character:
Instability Type of Duststorms
Duststorms of this type are associated with the development of thundery activity and occur mainly during the transitional periods affecting Lower Egypt. They are more common in autumn than in spring.
Down drafts along the edges of the thunderstorm cloud, create turbulent squally winds which on descending steeply to ground raise dust causing duststorms of local character. The dust raised by the squally winds is carried away from the storm edges resulting duststorms even far from and mainly ahead of the thunderstorm cell. Severe turbulence poor visibilities and restricted ceiling always present hazards to aircraft.
Dust Devils
The raising of dust in the form of dust devils is the second localized phenomenon which may result in a deterioration in visibility. They occur with calm light winds over intensively heated loose soil and are familiar features in the hot months in all coastal regions is the S & E Mediterranean specially the later. In UAR they may only grow to 100 feet or so in height before dissipating. They are of minor importance in UAR but in Palestine they may reach heights of 600 to 700 ft. They drift in the prevailing wind and may rotate in either direction.
Effects of Duststorms on the Safety of Aviation
Hazards of aircraft operations
No doubt dust and sandstorms present serious hazards to aircraft operations during in flight, take off, or landing stages. They may also seriously affect parked aircraft engines.
In the respect the following aspects have to be taken thoroughly into consideration by pilots, aviators and airmen operating in UAR during duststorms weather conditions previously discussed.
Deterioration of visibility
One of the main hazardous elements in duststorms is the deterioration of horizontal, vertical and slant range of visibilities. In such conditions aircraft accidents due to impact during flight, flying into high lands or any other terrain obstacles, and failure in safe landings or take-off s are rather probable. The navigational facilities available in the aircraft as well as in the aerodrome defines the capability of aircraft to fly, take-off, or land in such weather conditions.
Fuel reserves would be such that adverse wide-spread duststorm (or rising dust) weather conditions would be less of a problem since there would be enough reserve fuel to permit a very wide choice of alternate aerodromes even far outside the affected areas.
In connection with surface haze layers previously mentioned associated with strong night inversions, care must be taken when aircraft is landing in such conditions. Diffuse reflections from the upper surface of the haze layer, or sunlight during the day or of the landing lights if put on during night; may completely obscure the ground in any direction far from the vertical although perhaps, the surface horizontal and vertical visibilities may be rather good.
Air to ground slant range visibility is poor, however even from above the haze layer. Visibility within the haze varies greatly depending largely on whether the pilot is facing into or away from the sun. When haze is present it is often different to land an aircraft into the sun and perhaps when landing lights are on during night.
It is known that many of aircraft accidents that occurred in UAR were due to bad duststorm weather conditions mostly during khamsin season and mainly during the landing stage.
Damage of Aircraft engines and propellers
In severe dust or sandstorms, the blowing dust or sand may cause extensive damager to aircraft engines and propellers by penetrating sand or dust particles either during flight or to in adequately protected parked aircraft on ground out of suitable shelters.
Static Electricity
The contact between the blown dust or sand and the aircraft skin can increase considerably the electric potential difference between the aircraft and the ambient air. When the potential difference reaches a certain value spark discharge occurs. The phenomenon is usually noticed first as radio noise particularly on high and medium frequencies, VHF reception is affected to a much lesser extent. In extreme cases a visible discharge is observed around some parts of the aircraft. Although generally static electricity is not dangerous there have been rare occasions where a discharge has caused breakage of windows screens and plastic panels.
Turbulence
Mechanical and Thermal Turbulence
One of the main aircraft hazards arise from severe turbulence associated with duststorms weather conditions. Mainly in khamsin type duststorms, conditions are very favourable for both mechanical (frictional) and thermal turbulence to be raised. This is due to both the existence of strong winds blowing over the rough surface and the vigorous convective currents set up by heating from below due to strong insolation and by convergence of the convective unstable airmass in the vicinity of the developing disturbance.
Surface heating results also from the passage of the cold polar air over the warm land surface as it happens with the cold front type duststorms previously
Either mechanical or thermal turbulence comprises both vertical and horizontal fluctuations of wind, these are practically inseparable and develop more easily as the lapse rate becomes steeper. The eddies of thermal origin are often of larger dimensions and causing stronger gusts than those produced frictionally. Therefore thermal currents are more noticeable as a rule to occupants of an aircraft then are the frictional eddies. In Khamsin weather conditions instability and consequently thermal eddies extends to great heights.
Near the surface extreme gusts occur when some of the faster moving air above the frictional layer is brought down to the surface by the thermal eddies. This happens some times in association with the destruction of marked surface inversions usually at about 10 am setting up to the duststorm accompanied by strong turbulence as it happens with the cold front type of duststorms mainly in Upper Egypt.
In certain conditions thermal currents develop into large systems extending 2 kilometres or more horizontally and several thousand metres vertically taking probably some minutes to pass over the station. In these circumstances the variations of wind take the form of squalls rather than gusts. The strongest gusts recorded are usually associated with such squalls. Both direction and speed at the time of squall may differ widely from the prevailing geostrophic values. The strongest gusts are in many respects more important than the mean wind for they may be cause of severe structural damage.
The frequency of bumpiness increases with the speed of flight through a series of convective currents. A high speeds more up and down currents are crossed during a given time interval. Thus the increased speed causes the turbulence to be more severe due to the more rapid changes in the vertical motion of the aircraft. The lighter and the slower the aircraft the more susceptible it is to displacement from its flight path. Thus aircraft size, its speed and weather conditions are interrelated in determining the turbulence a particular pilot will experience.
When flying into turbulent conditions aircraft may be placed in an unusual attitude or may have its velocity increased either by the turbulence itself or by the pilot in his attempts to retain control and retain altitude. The effectiveness of gusts in causing structural failure will be considerably accentuated under such circumstances. In this respect the opinion is that attempts to control the aircraft should be kept to a minimum since the gust load may easily be increased by cover control. It is recommended also to fly in a straight and level altitude, keeping it as constant as possible avoiding unnecessary manoeuvring. This lessens the stresses imposed upon the aircraft.
Turbulence associated with surface inversions
In connection with night inversions previously mentioned and associated with the passage of cold fronts in both Khamsin and cold front types of duststorms it is worthy to mention that a narrow zone of wind shear with its accompanying turbulence is often encountered when an aircraft climbs or descends through the inversion. Wind speed and or direction sometimes changes very abruptly with altitude in this zone. A potential hazard to aircraft immediately after take-off or on the final approach for landing is an extreme form of wind shear associated with such strong temperature inversions near the ground. In a typical case intense night-time radiational cooling forms a surface layer a few hundred feet thick of relatively cold air underneath moving layer of warmer air above the inversion.
Because of the difference in speed between the warm air and the trapped cold air a narrow zone of strong wind shear forms along their boundaries especially when the wind in the warm air is strong enough. An aircraft climbing or descending through this zone may encounter a large loss of air of climbing aircraft, the abrupt change of wind speed can cause a substantial loss of altitude or airspeed. This situation can be dangerous since the zone is only a few hundred feet above the ground and the aircraft is at altitudes too low to recover.
The intensity of such turbulence varies mostly with speed of the warm air since the cold air is always calm.
Thunderstorms
Perhaps the most serious hazard to aircraft operations in UAR occur when thunderstorms previously mentioned occur in association with khamsin widespread type of duststorm.
Besides hazards associated with wide spread duststorms previously mentioned, other serious hazards to aircraft operations are added due to flying through thunderstorms in such weather conditions even if the thunderstorms are dry thunderstorms. Some of these hazards are, severe turbulence due to strong up and down draughts inside the thunderstorm cell, danger from hailstone, ice accretion, lightning strikes, static electricity and the instrumental errors due to flying inside the storm etc etc.
Mainly in khamsin duststorm weather conditions, due to the prevailing hot, dry and dust laden wind, conditions on the surface even indoors usually are nearly intolerable and it does not help doing even light physical efforts.
In addition, flying including landing and take off in such weather conditions makes the pilots feel uncomfortable. These considerations as well as unexpected diversions and delays in departures and arrivals of flights, affect undoubtedly the behaviour of both pilots and ground operations personnel, and may strain their nerves. This will be no means be reflected to flight operations and may affect their needed efficiency.
Advice to pilots and operator's local representatives
The following aspects should always be taken into consideration by pilots and operators local representatives in carrying out their duties.
Air reports and debriefing
Meteorological air-reports and debriefing must be highly stressed to the fullest possible extent particurlarly in unstable weather conditions. Air reports provide the meteorologist with more information to give more accurate forecasts to meet well the operational requirements for the benefit of both pilots in command and operators themselves. A brief verbal debriefing can often convey to the forecaster considerably more information than a few sot reports and there is the added advantage that the forecaster can also ask questions about any particular features.
The operator's local representative
Being appropriately qualified in meteorological training in order to carry out the duties assigned to him, must acquaint himself with existing meteorological situations and the probable and possible developments and consult with the meteorological office as necessary.
From the time of issue of the flight and aerodrome forecasts up to the time of departure any amendment to forecast should be notified directly or by the operator's local representative should check the continuing validity of the forecast particularly in the case of delayed departures.
One of his major duties is maintaining a flight watch on all flights for which he is responsible and recommending appropriate action to meet changing circumstances. He may also be responsible for ensuring that pertinent meteorological information is brought to the attention of the pilot-on-command before departure and during flight.
Briefing
In all cases whether the pilot-in-command has assistance from an operator's local representative or not, the need for the extent of meteorological briefing may vary considerably according to circumstances. However in the Khamsin season even for short routes since weather conditions are relatively unstable, there is likely to be a more frequent requirement for a comprehensive meteorological briefing.
Alternate course of action during flight
Due to lack of enough observations and forecast limitations the pilot in applying the weather reports and forecast as his flight progresses, should have always in alternate course of action in mind in case the weather goes 'Sour'. He should know alternative weather possibilities. For example if the direction of movement of a wide spread duststorm is known the pilot knows his best alternate action in case they move faster than anticipated. Only then can important deviations from the expected weather be recognized in time of action.
In order to make more effective use of the weather service a pilot or operator's local representative has:
- not to make his own forecast, and to leave that job to the authorized meteorological personnel.
- not to try to get the forecaster to make his decision of whether or not he will fly, only the pilot can make the decision.
- to fix limits of weather conditions beyond which he will not fly and stick to them.
- not to expect the weather features on the chart to look typical as illustrated in a book. No two weather situations are exactly alike.
- when in flight to monitor volmet broadcast and apply and make use of efficiently 'the meteorological procedures during flight including the basic (or are) meteorological watch system'.
- to beware of situations which are or threaten to be border line between VFR and IFR. These are the situations but cause most weather connected aircraft accidents.
- to remember the difference between a weather report and a weather forecast.
- to use the weather and not to fight it.
