DUST AND SAND STORM ELECTRIFICATION

By C. D. STOW University of’ Manchester Institute of Science and Technology

TMOSPHERIC electrification may be divided conveniently into two cate- A gories associated respectively with fair and disturbed weather. Study of the former is almost always concerned with dissipation of electrical charges during what may be called passive meteorological conhtions, whilst the latter deals with the generation or separation of electrical charge in the lower atmosphere associated with vigorous vertical mixing of the air. In the latter case charged particles or aerosols will be separated by virtue of their different fall speeds through the air, and vertical electric fields will develop wherever their charge is governed in magnitude or in sign by their physical size or density.

During fair weather the vertical electric field is of magnitude 100 volts per metre at ground level, there being a net negative charge on the surface of the earth. It is generally believed that this is an equilibrium state maintained principally by thunderstorm activity over the globe, the negative charge on the earth being delivered by cloud-to-ground lightning strokes, and by point discharge a t the earth’s surface beneath storms. Electrification resulting from other disturbed-weather phenomena may frequently act in opposition to the contribution from thunderstorms. For example, it has been generally observed that the fair-weather potential-gradient is enhanced considerably in the locality of a snowstorm or blizzard, but is strongly reversed in the presence of a sand storm.

The electrical activity of snowstorms has been studied in recent years and it is now believed that charge is separated by temperature-gradients, created by momentary contact of ice crystals, or by their asymmetric rubbing together. However, whilst sand storms are in some ways similar to snowstorms, the mechanism responsible for charge separation cannot be similar to the proton migration down temperature-gradients in ice. The material which composes sand or dust storms is electrically insulating and it appears that either a volume impurity or a surface contaminant must be responsible for the observed charging. The little work which has been done relating to sand and dust storm electrifi- cation is described below. Some experiments that are being conducted or planned by the author are mentioned briefly.

SAND STORMS

Storms of sand or dust fall into distinct categories resulting from different meteorological conditions. There are those storms in which the air motion is mainly rotational and disturbances very localised. Translational velocities are low, the essence of such storms being a core of rapidly-rotating air usually not more than 10 metres in diameter extending vertically IOO metres or so. Intense heating of the earth’s surface, usually over desert areas, often creates instability so great that the lighting of a match or the movement of a small animal might

trigger intense vertical convection and subsequent rotation of the rising column of hot air. Dust devils, as they are called, are most frequently seen when there is a horizontal wind of about I metre per second; with stronger winds turbu- lence generally prevents the unstable layer from forming, and above about 4 metres per second they very rarely occur. Gravitational separation of particles occurs within the column of the dust devil. Large dust devils have been known to raise animals and shrubs and cause damage to buildings, though such ex- amples are rare. A large dust devil is illustrated on the cover of this issue.

The other types of storm involve strong horizontal winds with considerable vertical mixing and are less localised than dust devils. In some areas (near Khartoum, for example) shower activity is accompanied by strong down- draughts of cold air which are deflected over the earth's surface and become heated. Air movement is often sufficiently strong to create a local sand storm (Haboob) lasting for about an hour or so. A haboob is illustrated on p. 141. A more widespread disturbance is often created by a large horizontal pressure gradient, the winds lasting for several days, though the intensity of such a storm may be quite low. Gravitational separation of the airborne sand occurs with such storms also.

FIELD MEASUREMENTS

There is little documented evidence of the electrification which accompanies natural sand storms. Hatakeyama and Kubo (1947) have described the results of observations made at Maebashi Meteorological Observatory during 1943 and 1944. They suggested that the sign of the atmospheric potential-gradient created by the storms was subject to a seasonal variation and that the space- charge density within the storms did not exceed I e.s.u. per cubic metre. However, their results were erratic and their conclusions are therefore somewhat tentative. Freier (1960) recorded the electric field due to a large dust devil over the Sahara at Kidal, Republic of Mali. The dust devil was estimated to be between IOO and 200 metres high and about 8 metres in diameter. The trans- lational speed across the desert was about 4 metres per second. The electric- field record is reproduced in Fig. I (solid line) and approximates to an electric dipole whose negative charge is situated above the positive charge. The electric

+ f\ 1

J I Fig. I . The electric field due to a large dust devil. From Freier 1960

field for a dipole of strength M, at a height h, and a distance R between the observation point and centre of the dipole is given approximately by

This equation is shown in Fig. I (dashed line) fitted as closely as possible to Freier's actual field record and it led Freier to suggest that the dipole strength of this particular dust devil was of the order 1.7 x 109 e.s.u.cm. Aircraft observations of Bradley and Semonin (1963), and surface observations of Crozier (1964), concur in many respects with those of Freier. The electric-field records of both Freier and Crozier suggest that a pocket of negative space charge of density about IOO elementary charges per cm8 was maintained a b v e ground level by the vortex. The non-existence of a dipole structure in Crozier's observations suggests that charge separation occurred mainly at the earth's surface, whereas Freier's dust devil separated charge within the column.

Bicknell (1967, private communication), using a rotating-vane field-mill, made measurements on behalf of the author whilst travelling through the north-westem regions of the Sahara. Thirteen separate observations of the vertical electric field at ground level and one metre above ground level were taken, each observation being at a different locality. All but one of the observ- ations concerned the electric field created by widespread storms and covered a wide range of wind velocities. With one exception the fields observed were opposite to the average fair-weather field, corresponding to either negative charge above the field mill or positive charge below it. For a moderate storm the field at one metre above the ground was about 5 kV m-l, rising to typically 50 kV m-l a t ground level. During more vigorous storms readings greater than 20 kV m-l were observed at one metre above the ground, and greater than 200 kV m-l a t ground level. These observations suggest that the field was due mainly to positively-charged sand particles blowing along the ground rather than more dispersed clouds of negatively-charged sand at altitudes greater than one metre. Unusually gusty conditions occasionally caused field-reversals a t one metre above the ground but not a t ground level, a pheno- menon which is explicable if at these times the heavier, positively-charged particles are momentarily blown to heights in excess of one metre. Bicknell made one observation of a dust devil of diameter 6 metres whose translational velocity was about 2 metres per second. Negative fields of about IOO kV m-l were recorded at its centre, with an occasional positive excursion. The fair-weather field was typically positive and of magnitude 500 V m-l.

There is an increasing number of references to electrical activity caused by dust storms several hundred kilometres from the point of observation. Indirect evidence of negative charge being transported to great heights has been given by Harris (1967, 1968) 'who was not primarily examining the phenomenon of sand storm electrification. Violent reversals in the fair-weather electric field were observed when high-altitude dust haze (Harmattan) occurred in West Africa, giving negative values as high as 4000 V m-l. The effect was subject

to diurnal and seasonal variations and on occasion appeared to be suppressed for reasons which are not yet clear. The author believes that the electric fields measured in these cases were due to charges, of predominantly one sign, on fine dust which had separated gravitationally and been borne aloft and carried great distances.

LABORATORY EXPERIMENTS

One of the earliest documented experiments on the electrification of blowing dust is that of Rudge (1914). He disrupted sand and other materials such as red-lead, flour, iron filings and chalk by means of an air jet, and measured the sign of the charge carried on the larger blown particles, and also on the small particles suspended in the air, using gold-leaf electroscopes. The electroscopes always showed opposite signs of charge and in the case of sand under normal conditions of temperature and humidity the larger particles were positively charged and the smaller particles negatively charged. Experiments in which pairs of different sizes of single crystals of quartz were rubbed together showed that the larger crystal always became positively charged and the smaller crystals negatively charged, regardless of which face of the crystals was used. None of the electrical measurements was quantitative.

Shaw (1927) studied the manner in which electrical charge was separated when two rods of similar material are rubbed ‘asymmetrically,’ that is, the areas of contact on each rod differ. Peculiarly, he found that there was no charging for either glass or vitreous silica. Impact of one rod against another yielded a net negative charge on the rods for almost all his materials, including glass and vitreous silica. Shaw suggested that the electrification of sand storms was explicable in terms of this process. Again the results were only qualitative. In later experiments by Shaw (1929)~ the charge was measured on various powders which had been blown through a tube whose inner wall could be made from any chosen material. One arrangement was that in which sand was blown through the tube lined with sandpaper. He found that the sandpaper became positively charged and the blown sand negatively charged. The charge separated could be increased either by increasing the air-jet velocity or by increasing the air-jet temperature. Shaw was not able to find a satisfactory explanation for these results.

Debeau (1944) measured the electrical charges separated when silica was poured through a system of nickel funnels. He observed that the silica became negatively charged and that the nickel became positively charged. Values of the charge separation were obtained using atmospheres of oxygen, nitrogen and hydrogen under various pressures. His work produced useful results, especially concerning the preparation of silica to obtain reproducible results. Gill (1948) reinterpreted some of the work performed by Debeau, especially that concerned with the relationship between charging and environmental pressure. Debeau thought that the effect of gas pressure was to alter the absorption rate of gas into the silica, but Gill showed that the effect of pressure was to alter the threshold of corona discharge from the particles.

Gill and Alfrey (1949) allowed sand to slide down an inclined metal surface situated in an electric field. The sand was caught in a Faraday cage and its charge measured. In the absence of an applied field the charge acquired by the sand was negative. They found that the magnitude and sign of the separated charge varied with the applied field in a simple fashion and could be interpreted as induction charging. Gill and Alfrey suggested that perhaps all static charging was a consequence of induction charging which was initiated by surface fields produced at interfaces. Their work was extended by Peterson (1949) who measured the induction charging of borosilicate glass spheres rolling along an inclined nickel plane as functions of the humidity of the environment, the s p e d of rolling, the path length and the gas pressure. Even though great precautiqns were taken when preparing the spheres it appears that surface impurities were present which permitted charge transfer to take place across the nickel-glass interface.

Considerably more detailed studies, directly related to the electrification of dust storms, were performed by Kunkel (1950) in which the charges on dust particles dispersed into the air were measured as a function of particle size. He showed that where the dust particles were dispersed from a container made of the same materials, e.g. quartz from a quartz container, the charging for a given particle size was symmetrical. When the dust was dispersed from a con- tainer made of a different material charging was asymmetrical, there being a predominance of one sign of charge over the whole particle-size spectrum.

Henry (1952) demonstrated, by means of asymmetric rubbing, with special reference to insulators, that for any substance whose concentration of free- charge carriers is a function of temperature, charge transfer should occur if a temperature-gradient is applied to a specimen or if two pieces of the same substance, of different temperature, are brought into contact. He also pointed out that such temperature-differences can result from asymmetric rubbing, a process which certainly exists in a sand storm. No conclusive experiments have been performed with quartz.

Unlike Rudge, Harper (I955), who performed experiments to determine the charge transfer between quartz crystals, observed that charge transfer was notably anisotropic. Wagner (1956) extended the work of Peterson using modern outgassing processes in order to clean the surfaces of his working materials. Amongst other materials, he studied charging using fused quartz, synthetic crystalline and natural crystalline samples. No anisotropic charging was observed of the type described by Harper, presumably because of the improved cleaning techniques, though doubt does exist on this point.

Barcilon (1967) has modelled the dust devil both theoretically and expefi- mentally. He was concerned solely with the flow field and not with the sepwa- tion of electrical charge, though it should be possible to determine the degree of gravitational separation of dust particles from his results, thereby enabling the effectiveness of the dust devil as an electrostatic generator to be assess+.

Attention is drawn to four books which contain much material pertinent to the investigation of sand and dust storm behaviour. Two, Loeb (1958) and

Harper (1967), are principally concerned with static electrification in some detail and involve material directly related to the problem of charge separation. The remainder are related to the mechanical aspects, Bowden and Tabor (1950,1964) being solely concerned with the nature of physical contact between solids, and Bagnold (1941) dealing with the physical transport of sand either through the air or over various types of desert surface.

CONCLUSIONS

I t is evident that there are deficiencies in both the field observations and laboratory experiments. To date, no field investigations have been made other than the most rudimentary measurements of variation of the earth’s vertical potential-gradient near the ground. To understand the behaviour of the electrification of natural sand storms it is necessary to correlate the space- charge at various heights and on various sizes of dust particle, the ion concen- tration in the air a t various heights, the individual charges on airborne particles, and the surface-charge density a t the ground, with wind speed, ambient temperature and dust type. To the knowledge of the author no such investi- gations have yet been performed. Because natural storms are infrequent long periods of time may elapse before similar conditions recur. I t is therefore necessary to compare results obtained from natural storms with those from simulated storms in the laboratory. Within the confines of a laboratory it is possible to vary at will parameters such as wind speed, wind temperature and sand or dust type, thereby isolating more easily the factors which most affect electrical charging. An investigation similar to this was made by Latham and Stow (1965) to study the electrification of blowing snow, with considerable success. However, laboratory simulation is somewhat limited by the physical boundaries of the environmental chamber used, and occasionally it is limited by the energy input available.

Whilst field measurements and laboratory simulations enable the electrical behaviour of a sand or dust storm to be evaluated, often giving useful information about the charge-generating mechanisms, the specific charge-transfer processes involved may only be investigated by more detailed experiments. The experi- ments described in the previous section provide only scanty evidence of the charging mechanisms involved. The results of Rudge and Shaw aTe only quali- tative and were obtained under conditions which were not clearly defined. Even the quantitative results of Debeau, Gill and Alfrey. and of Peterson, were subject to the influences of unwanted surface-contaminants. I t is un- fortunate that the results of Wagner concerned metal-sand contact-electrification since his experiments are probably the most reproducible. Only Kunkel’s observations are both quantitative and reliable; they concerned sand-sand electrification, but the experiments were not specifically designed to investigate sand storm electrification.

I t is clear that the problem of electrification of sand storms will be resolved only by an intensive laboratory investigation involving both the simulation of sand storms and the study of the electrical behaviour of sand and dust particles

under a variety of conditions that will distinguish the effects of, for example, purity, particle size and nature of contact. The author is at present setting up quantitative experiments of the type performed by Rudge and Shaw. Measure- ments will be made of the maximum charge which can be carried by irregular sand or dust particles and the relationships between particle structure and particle charge will be investigated together with the possible existence of charging by asymmetric rubbing.

REFERENCES

BAGNOLD, R. A.

BARCILON. A.

BICKNELL, J. A.

BOWDEN, F. P. and TABOR. D.

BRADLEY, W. E. and SEMONIN, R. G.

CROZIER, W. D.

DEBEAU. D. FREIER, G. D.

GILL, E. W. B. GILL, H. W. B. and

ALFREY, G. F. HARPER, W. R.

HARRIS, D. J

HATAKEYAMA, H. and KUBO, T.

HENRY, P. S. H.

KUNKEL, W. B.

LATHAM, J. and

LOEB, L. B. PETERSON, J. W. RUDGE, W. A. D.

SHAW, P. E. '

STOW. c. D.

WAGNER. P. E

The physics of blown sand and desert dunes. Methuen Press,

A theoretical and experimental model for a dust devil. London

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Press, London The friction and lubrication of solids, 11. Oxford University

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The electric field of a New Mexico dust devil. .I . Geophvs. _ _ 69, PP. 5427-5429

Phys. Rev., 66, p. g The electric field of a large dust devil. J . Geophys. Res.,

Frictional electrification of sand. Nature, 162, p. 568 Frictional electrification. Zbid., 163, p. 172

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Proc. Roy. SOC. A. 231, p. 388 Contact and frictional electrification. Oxford University Press Nature, 214, p. 585 Proc. Conf. Univ. AsO. Atmos. Ele., Tokyo On the ;ariation oi the atmospheric potential gradient

caused by the dust storm. J . Met. SOC. Japan, 25, p. 45 The role of asymmetric rubbing in the generation of static

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a cloud. J . Appl. Phys., 21, p. 820-832 A laboratory investigation of the electrification of blowing

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A. 122. pp. 49-57

DUST STORM IN THE SUDAN

Photograph by J . A k y s A haboob approaching Sodiri, Sudan, g July 1954. The wall of dust lies beneath a layer of cloud that is probably forming in warm air scooped up by the advancing edge of cool air that contains the dust storm. This cool air was produced in the downdraught of a distant shower, the anvil of which may well be the cloud shown in the upper part of the picture

Photograph by E. M . Squires

141

Wave cloud above cumulus: Malham Tarn, Yorkshire

CLOUDS OVER CONICAL MOUNTAINS

Photografih by K. Gordon Mt Demavend, looking north-eastwards from the foothills of the Elburz Mountains, Iran

Photograph by R. B. Gofin

Mt Ngauruhoe, looking north-eastwards from the ski grounds on Mt. Ruapehu, North Island, New Zealand

The shapes of wave clouds to some extent demonstrate the type of airflow over moun- tainous country. Flow over long ridges is largely two-dimensional and can lead to elongated bands such as the one shown on page 141. But when a mountain i s conical the flow becomes three-dimensional and can result in a more or less circular cloud cap such as the one over M t Demavend shown at the top left. On other occasions, possibly when the lower atmosphere is convectively unstable and is therefore more favourable to the develop- ment of an upslope eddy in the lee of the mountain, the cap is displaced downwind from the summit. An example is shown at the bottom left, where the cloud is in the lee of Mt Meru. The photograph was taken towards sunset during the season when north-east winds predominate. In the picture above, a multilayered cap is displaced to the left in a south- easterly airstream.

ANTICYCLONE OVER SCANDINAVIA

Crown Copyvight

This picture was received from Essa 8 at 0943 GMT on 15 March 1969. Clear skies over Scandinavia reveal the fjords and snow capped mountains of Norway. The Gulf of Finland and most of the North Baltic is frozen. Afforested areas appear relatively dark despite their snow cover. The anticyclone responsible for the clear skies brought cold easterly winds to most of Britain.