GEJALA KILAT

8/7/2019 GEJALA KILAT

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GEJALA KILAT

1. General karakteristik

The physical manifestations of lightning have been with us from the remotest times, but only

comparat ively recently have the phenomena become even part,ly understood. Franklin in hiselectrical experimentsbetween 1740 and 1750 succeeded in identifying lightningas the staticelectricity of his time. Beyond this fact little was learned until within the past 35 years. The realincentive to obtain additional knowledge lay in the necessityof the electrical industry to protect againstits effects. As longer transmission lines were built the need for reduction in outages due to lightningbecame more acute. This placed more stringent requirements upon lightning arresters and otherprotective devices. Largely through the co-operation of the utilities and manufacturers and throughthe use of special instruments such as the klydonograph, cathode-ray oscillograph, surge-crestammeter, Boys camera, and fulchronograph, information of a very valuable character has beenobtained regarding stroke mechanism and the voltages and currents associated with lightning.

1. Charge FormationIn spite of the great interest in the manner in which charges arise in thunderclouds, the question

is still controversial. Some half-dozen theories have been advanced, but those of C. T. R. Wilson and ofG. C. Simpson or modifications of them have received most consideration.Both theories postulate ascending currents of air and relative motion of rain drops of different

sizes. Wilsons theory depends for its explanations upon the presence of large numbers of ions in theatmosphere. Many of these ions, both positive and negative, attach themselves to minute particles of dustand extremely small drops of water, called Aitken nuclei, to form large ions as contrastedwith unattached or small ions.

Over land the number of small ions of each sign ranges from about 300 to 1000 per cubiccentimeter, and the large ions from 1000 to 80 000 per cubic centimeter. The small ions do not play animportant part in Wilsons theory. The mobility of an ion is the steady velocity that can be attained under avoltage gradient of one volt per centimeter. The large ions have very low mobility ranging from 0.0003 to0.0005 centimeter per second. Under a gradient of 10 000 volts per centimeter this would correspond to avelocity of only 3 centimeters per second.

Macky, in a study of the behavior of water drops when exposed to electric fields, found that a droplet ofradius p centimeters becomes elongated until at a critical field determined by the relation F1 = 3875 itbecomes unstable, A luminous glow is formed at each end and the energy absorbed thereby results inevaporation of a portion of the water forming the droplet. This sets a limit to the size of drops in athunderstorm.Thus, no drops greater than 0.15 centimeter in radius can persist in fields of 10 000 volts per centimeter.Air pressure has no influence upon the field at which this occurs. Macky suggests that in general thefields within thunderstorms will rise to a value of the order of 10 000 volts per centimeter before dischargeoccurs,

Wilsons theory premises the existence of the normal field which occurs during fair weather. Thisis generally directed downward, the direction which convention has adopted as positive. In magnitude it is

of the order of one volt per centimeter at the surface of the earth and gradually decreases with altitudeuntil at 30 000 feet it is only about 0.02 volt per centimeter. A relatively large drop of water (of say onemillimeter radius) in such a field will become polarized by induction, the upper side acquiring a negativecharge and the lower side a positive charge (see Fig. 1).

The velocity of fall under the influence of gravity of such a charge will be 590 centimeters persecond, which is large with respect to the velocity of the slowly moving ions even under the maximumfield strength of 10 000 volts per centimeter. At the under surface of the drop a selective action withregard to the slowly moving ions occurs. The negative ions tend to be attracted and the positive ionsrepelled. No such selection occurs at the upper surface. As a result of this action, the drop accumulates


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negative charge. With the loss of the negatively charged ions the remaining large ions arepredominantly positive.

The smaller drops descend with a lower velocity and thus their velocity becomes more nearlyequal to that of the velocity of the large ions under the influence of the electric field. It becomes possiblethen for the small drops of water to pick up positive charge by impact with the positive ions.

Thus, the original charges which were distributed at random and produce an essentially neutralspace charge, become separated. The large drops carry the negative chargesto the lower portions of thecloud and the small drops retain the positive charge in the upper portion. hccording to Wilsons theory thelower portion of the cloud is negatively charged and the upper portion, positivclv. This mechanism ofdischarge has been verified ,experimentally in the laboratory by Gott

3who actually obtained charge

separation by this process.

The theory of G. C. Simpson4 also has been substantiated in part by laboratory experiments. Ithas been shown that a water drop of radius greater than 2.5 millimeters becomes flattened or unstablewhen it falls through still or ascending air. A large number of smaller drops are formed. The terminal orsteady-state velocity of drops 0.25 centimeter in diameter is eight meters per second, which thusconstitutes the limiting relative velocity of rain drops. No drops will fall to earth in an ascending current ofair cxcceding eight meters per second. It has also been shown that when water drops break up, theresulting droplets become positively charged and the air negatively.

The meteorological conditions within a cloud according to Simpson are shown in Fig. 2. Theunbroken lines represent lines of flow of the air, their distance apart being inversely proportional to thewind velocity. The air enters the storm from the right and passes under the forward end of the cloudwhere it takes an upward direction. Within the cross-hatched oval marked 8 the vertical component of thewind is more than eight meters per second; and Out side less. For the reason just stated no water can fallthrough this area. The dotted lines show the general path Of the larger drops as they fall to earth. Theballoon-like Surface of which the oval 8 forms the bottom represents a boundary within which the upwardvelocity is still very high. Only the larger drops are able to descend within the volume so formed and noneare able to penetrate the oval 8. The drops that do fall within this volume will be broken and the partsblown upward. The small drops that have been blown upward will recombine and fall back again, and sothe process will be continued.

The distribution of electrical charge that will result from the conditions represented in Fig. 2 isshown diagrammatically in Fig. 3. The mechanism by which charge separation occurs is explained clearlyby Simpson as follows:

In the region where the vertical velocity exceeds eight meters a second there can be noaccumulation of electricity. Above this region where the breaking and recombining of water drops takeplace (the region marked B in Fig. 3) here, every time a drop breaks, the water of which the drop iscomposed receives a positive charge. The corresponding negative charge is given to the air and isabsorbed immediately by the cloud particles, which are carried away with the full velocity of the air current(neglecting the effect of the electrical field in resisting separation). The positively charged water, however,does not so easily pass out of the region B, for the small drops rapidly recombine and fall back again,only to be broken once more and to receive an additional positive charge. In this way the accumulatedwater in B becomes highly charged with positive electricity, and this is indicated by the plus signs in the

diagram. The air with its negative charge passes out of B into the main cloud, so that the latter receives anegative charge. In what follows, the region B will be described as the region of separation, for here thenegative electricity is separated from the positive electricity. The density of the negative charge obviouslywill be greatest just outside the region of separation, and this is indicated in Fig. 3 by the more numerousnegative signs entered in the region around A.

In contrasting the two theories, it may be observed that Wilsons theory leads to the conclusionthat the lower portion of a cloud is negatively charged and the upper portion positively. Simpsons theoryas given above, on the other hand, leads to the converse-that an intense positive charge resides in thehead of the cloud and that negative charge is distributed throughout the rest of the cloud. Wilsons


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experimental observations of field changes next to the ground indicated that a charge of positive electricmoment, that is, a charge distribution equivalent to a positive charge above a negative charge, isdestroyed in the process of most lightning discharges. In addition the results of magnetic-lin investigationson electrical systems, as discussed hereinafter, indicate that approximately 90 percent of all strokes lowernegative charge to the transmission system.

The direct contradiction between these two theories led Simpson and Scrase5 to investigate thecharge distribution in a more direct manner. Free balloons equipped with clock-operated apparatus tomeasure electric gradient, atmospheric pressure, and relative humidity were released during storms. Itwas found that in general the main body of a thundercloud is negatively charged and the upper partpositively charged. A concentration of positive charge appears to esist frequently in the base of the cloud.hccording to Simpson and Scrase t,he cloud structure of the type shown in Fig. 4 offers a satisfactoryexplanation of practically all the soundings obtained in their investigations.

The positive charge at the top of the cloud gives rise to the positive field encountered at theground as the storm approaches and as it recedes. The negative charge contained in the lower halfproduces a negative field everywhere under the cloud except where the local concentrations of positiveelectricity produce positive fields. Further verification of this fact is offered by data obtained by Simpsonand Scrase by recording ground gradients during the passage of storm clouds. From the records of 20storms it was found that the average length of time for which the potential gradient was appreciably

disturbed from its fine-weather value was i3 minutes. Ry centering each record about the midpoint of thetotal period and dividing the record into five-minute intervals, the curve in Fig. 8 shows the percentage offrequency of positive potential gradient. The parts of the curve above the line corresponding to 50 percentrepresent a preponderance of positive gradient and those below a preponderance of negative gradient. Itshows that the approach and recession of a storm usually are accompanied by positive gradients whilethe center of the cloud produces a negative gradient.

This is what would occur if the lower portion of the cloud carried negative charge and the upper portionpositive charge. As between the Simpson and Wilson theories, the induction theory of Wilson seems tooffer an adequate explanation of negative charge in the lower regions of the cloud and the concentrationof positive electricity higher up in the cloud. It does not explain the positive charge found at t,hc base ofthe clouds. However, quoting from Simpson and serase:

Our observations have shown quite conclusively that the boundary between the positive electricity in theupper part of the cloud and the negative electricity in the lower is in every case in a region of the cloudwhere the tempernture is well below the freezing point and generally below-10 degrees centigrade.In this part of the cloud raindrops cannot esist. The cloud particles may be supercooled water, but oncoalescing they would immetliately freeze. The precipitation in the upper part of a cloud is in the form ofcrystals, either needles or plates, which tend to lie horizontally and to fall slowly in a series of nearlyhorizontal motions, first in one direction and then in another, These crystals cannot play the role of theraindrops in Wilson a theory, for in the first place they are nearly perfect nonconductorsand so do not becomeelectrically polarized, and, even if they do conduct, their shape and orientation is not favorable to theformation of induced charges, and finally their rate of fall relatively to the air is very slow. It is clear, therefore, thatWilsons influence theory cannot explain the separation of the charges found in the upper part of the thunder-clouds.

It is well known that during blizzards in polar regions which are accompanied by large masses of blown snow, very

strong electrical fields are set up near the earths surface. These fields, with very few exceptions, are positive in

direction that is to say, in the same direction as the field in the upper part of a thunder cloud Simpson, in hisdiscussion of the observations made in the Antarctic (Simpson 1919), suggested that the impact of ice crystalsresults in the ice becoming negatively charged and the air positively charged. The general settling of the negativelycharged ice crystals relatively to the positively charged air would then result in a separation of electricity with thepositive chargo above the negative. This explanation, however, has not yet been confirmed by satisfactory laboratoryexperiments. Whatever tho physical explanation may be, there seems little doubt that the upper separation of charge

in a thunderstorm is in some way connected with the presence of ice crystals.


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There appear therefore to be two different physical processes taking place in a thunderstorm to protluce theelectrical effects:

One is confined to the upper parts of the cloud where the temperature is below the freezing point, and the secondoccurs in the lover part of the cloud where the temperature is above the freezing point. There is reason to believethat the former ia associated with the presence of ice crystals and the latter with raindrops, probably in the waydescribed by Simpson in his breaking-drop theory.

Fig. 5 represents Simpsons revised diagram to illustrate the meteorological and electrical conditions in athundercloud. This differs from his early conception illustrated in Fig. 3, in that a positive charge resides inthe upper portion of the cloud above a region of separation from the negative charge, in which the

temperat,ure is between - 10 and -20

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