Weather and Climate (1988) 8: 23-32

CAN WE REDUCE THE HAIL PROBLEM?J. T. Steiner

New Zealand Meteorological Service, P.O. Box 722, Wellington

ABSTRACT

The magnitude of the hail problem in New Zealand is considered. Although there is lesshail damage than in some other countries, the cost is likely to increase as the area used forhorticulture increases. Methods of abating hail damage by the use of protective devices arediscussed. Applications of an improved hail climatology are considered.

Two methods of suppressing hail that have been widely used overseas are reviewed.One method involves the seeding of cloud with silver iodide, or other substances, that are

intended to promote freezing of supercooled water at temperatures higher than those at whichcloud water freezes naturally. It is shown that carefully designed scientific experiments havenot demonstrated any reduction of hail resulting from cloud seeding. However there isevidence that many hail suppression operations in various parts of the world appear to havereduced hail damage.

Hail cannon, which are now being introduced to New Zealand, belong to a class ofattempted hail suppression methods based on explosives. Possible mechanisms for theirclaimed success are reviewed. The lack of firm scientific evidence for their efficacy isdemonstrated.

INTRODUCTION

Hail is defined as precipitation in the form ofsmall balls or pieces of ice with a diameter of atleast 5 mm falling either separately or agglomer-ated into irregular lumps (World MeteorologicalOrganization, 1956).

Hail is formed in tall convective clouds, usuallycumulonimbus. Ice growth takes place on initialice particles by the accretion o f supercooledliquid water droplets which freeze on impact.The resulting hailstone embryo is of low densityas it includes air-filled gaps. Should such objectsfall to the surface they readily crush. Underappropriate conditions o f temperature and o faccretion rate in cloud, the gaps in these objectsare filled, giving rise to the formation of ice pelletsa few millimetres across. Ice pellets may also formby the freezing of raindrops or snowflakes thathave largely melted. Additional accretion on icepellets results in an increase of volume to form ahail stone. The physics o f hail formation isdiscussed further in Mason (1971), English(1973) and Rogers (1979). The distinction be-tween true hail and other forms o f frozen or

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partially frozen precipitation is not always strictlyfollowed and can lead to difficulties in interpret-ing hail statistics.

Hailstones grow while they are maintained incloud having a temperature of 0' C or less by anear balance between their fallspeed (whichincreases with hailstone size) and the cloudupdraught. Eventually, hailstones may grow to asize such that they fall relative to the updraught,or they may be displaced laterally from theupdraught, or the updraught may weaken. Thehailstones can then descend to levels with tem-peratures above 0' C where melting takes place.The larger hailstones fall to the ground withoutmelting completely.

HAIL DAMAGE

The high energy of large hailstones when theyimpact crops or other sensitive surfaces is aprincipal cause of hail damage. Additional dam-age to buildings can arise from the combinedeffects of hail impact, water damage and wind,from hail weight on structures or through hailblocking drainage. The annual cost of damage by

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hail has been estimated as amounting to billionsof dollars.'

It is difficult to compare agricultural hail lossesbetween countries and periods because of varyingcrop values and exchange rates. However, someestimates from countries where horticulture is amajor industry are noteworthy. In Italy, annualcrop losses t o 1973 amounted to $NZ120mdollars or $NZ388 per square kilometre (Mor-gan, 1973). The annual hail loss in an 80 000square kilometre area in the southwest of Francewas reported by Dessens (1986a) as $NZ82 000000 or $NZ1025 per square kilometre.2

Urban hail damage losses can also be very high.A devastating hailstorm in Munich i n 1984caused an insured loss of over $US500m, with thetotal loss possibly twice as much (Munich Re-insurance Company, 1984). Vehicle damage wasa substantial part of the insured loss. A hailstormin Denver caused losses of $US350m in June1984 (Blanchard and Howard, 1986). Hailstormsin Australian cities have caused large losses. Thegreatest losses occured from a storm in Sydney in1976 with insurance claims exceeding $ALIOm(Morgan, 1979).

Some horticultural damage estimates are avail-able for recent New Zealand hailstorms. A stormin the Bay of Plenty in January 1987 was reported("Evening Post", 2 February 1987) as havingcaused a loss of $6 000 000 to kiwifruit growers.The combined cost of hail and wind damage,mainly horticultural, from the November 1984storm in west Auckland was $4 000 000 (McGill,1987). The October 1986 hailstorm in HawkesBay is estimated to have caused 30 percentdamage to 959 hectares of horticultural crops at acost of about $4 000 000. When the losses fromlesser or less publicised storms are added, thetotal may already amount to more than tenmillion dollars per annum. Between 1981 and1986 the area used for horticulture increased by55 percent. I f this expansion of the area used forproducing valuable but vulnerable crops con-tinues, horticultural losses caused by hail can beexpected to increase.

It is therefore timely to examine the hailproblem in New Zealand. This paper examinesthe possibility of reducing hail damage. A secondpaper (Steiner, 1988) examines the climatology

Hail Reduction?

and characteristics of New Zealand hailstorms.The principal results o f that study are sum-marized below.

Analysis o f reports o f hail at climatologicalstations reveals that the incidence of hail gen-erally increases from north to south and from eastto west. The analysis suggests that the hailfrequency is highest in winter or in spring in mostparts of New Zealand. Analysis of the reportingof hail in hourly weather observations indicatesno preferred time of day for hail occurrence.

A data set of reports of severe hail in news-papers (Neale, 1977) was extended and re-analysed. A very different seasonal and diurnaldistribution is found for damaging hail. In east-ern areas and in Southland more than 60 percentof all severe hail occurs in spring and summerafternoons. More than 50 percent occurs at thesetimes in Marlborough, Nelson and the Bay ofPlenty-Taupo area. In other (western and north-ern) areas, the seasonal and diurnal variability isless pronounced.

The different distributions above probablyresult from the inclusion of many small frozenparticles in the climatological and hourly hailreports.

Fourteen hailstorms that had caused severelosses to the apple or wheat crops were studied.All of the hailstorms occured in the rear of themain cold front o f a depression. Most wereassociated with a cold and unstable tropospherebut without the temperature or instability beingnecessarily extreme. A mesoscale or large scalecirculation pattern conducive to upward motionis favourable to the occurence of damaging hail.

HAIL ABATEMENT

Methods for the reduction o f hail damagebased on hail climatology are considered below.The international effort in hail suppression byintervention in cloud processes is reviewed sub-sequently. The prospects fo r mitigating haildamage in New Zealand are considered in thefinal section.

I f a detailed climatology of the inccurence,quantity and size of hail is available it can be usedfor planning purposes. From such data the hailrisk can be determined Vento and Malossini

Henderson (1987), from correspondence with Lloyds of London, quotes a 1986 estimate of at least 4 billion USdollars, or at the 1986 exchange rate, at least 8 billion New Zealand dollars.The Italian and French hail losses have been converted to New Zealand currency at the exchange rate for theappropriate period.

Hail Reduction?

Fig. 1. Distribution of damage (as measured bythe percentage of the yield lost) in a soybean fieldin Illinois, USA in 1975. The area was generallyflat and open but there was a clump of trees(shown) in the northwest corner. (From Towery etal, 1976). 13-17 m

trees

(1982) suggest that when this information iscombined with a knowledge of the damagingpotential of hail to crops at various phenologicalstages, cost/benefit analyses can be established.These can determine the viability of particularcrops in given locations.

An attempt to obtain a detailed climatology ofhail in Canterbury was made by Miller (perscomm) using cards mailed to volunteer sup-plementary observers and requests through aradio station for additional information afterknown hail falls. To develop and maintain asystematic record of hail and hail damage is amajor undertaking, requiring the co-operation ofall land users and insurers, and the commitmentof appropriate people to archive and analyse thedata. I n addition to its uses for planning, theavailability of the data would serve both insurersin setting appropriate rates and growers in decid-ing if any offered insurance was worthwhile. I twould also provide some guidance for evaluatingany hail supression effort. Comprehensive highresolution hail archives have been developed inAlberta (Wojtiw, 1975) and France (Dessens,I986a).

A further possibility for reducing hail damageis to use protective screens above the crop. \lentoand Malossini (1982) suggest these may be ap-propriate for particularly valuable crops. The hailclimatology, microclimatic effects of the coverand its lasting properties would all need to beconsidered in evaluating the merit of such screen-ing.

Towery et al (1976) demonstrate that trees canmarkedly reduce hail damage in their immediatevicinity when hail is associated with strong winds.The trees have three effects. Some hail is inter-cepted directly by the trees protecting crops

178 m

Wind direction

v w i t h hail

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immediately downwind. The trees also create achange in the air flow so that the area in the lee ofthem is partially sheltered with hail deflectedlaterally. Wind speeds will also be less in the lee ofthe shelter so the total hail kinetic energy, whichresults both from the vertical fall speed of the hailand the wind speed, will be less. The impact of aclump of trees on the hail damage to an otherwiseopen field of soybeans is shown in Fig. 1. I f thereis a preferred wind direction with hail thenshelter belts can be planted perpendicular to this

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direction to reduce crop damage (Vent() andMalossini, 1982). The wind direction and estimat-ed speed during hailstorms are additional itemsthat should be included in developing local hailclimatologies.

HAIL SUPPRESSIONMost forms of weather modification do not

cause changes that are so direct and immediatethat the change can irrefutably be ascribed to thetechnique employed. Thus, the evaluation of aweather modification experiment requires that ithave both physical and statistical plausibility(World Meteorological Organization, 1986). Thefirst criterion requires that there is a demon-strable linkage — by established physical prin-ciples, by calculation, or simulation, or preferablyby measurements — between the modificationaction taken and the sequence of ensuing events.The second criterion implies that a trial of thetechnique is undertaken involving some random-ization. A series of periods are divided on arandom basis into those in which the technique isemployed and those when it is not and the resultsof the two sets are compared.

Two techniques for hail suppression have beenwidely employed: seeding of clouds with silveriodide or other materials, which induce freezingto occur at warmer temperatures than otherwise,'and the use of cannons or explosive devices.

(a) CLOUD SEEDING

There are two ways in which seeding is post-ulated to reduce hail severity. The most common-ly pursued is the beneficial competition approach(Iribarne and Pena, 1962). Seeding is intended tocause a vast increase in hail embryos, none ofwhich grows to large hail because of competitionfor the available liquid water. Another approach,premature rainout, involves seeding o f cloudelements at an early stage, so that particles whichmight otherwise become hail embryos fall out ofthe cloud as rain from lower levels rather thanascend to the higher levels where hail formationtakes place (Ludlam, 1959).

The most systematic test of hail suppression bycloud seeding was undertaken in Switzerlandfrom 1977 to 1981 (Federer et al., 1986). Abeneficial competition methodology that wasdeveloped in the Soviet Union was employed; itinvolves the discharge of large amounts of silveriodide into a particular zone of the cloud usingrockets (e.g. Sulakvelidze et al., 1974). In thisexperiment, the hail kinetic energy, as measuredfrom a network of hailpads, was compared withthe energy that could have been expected fromthe atmospheric measurements prior to the hailevent. This statistic was compared for seeded andunseeded storms, the decision to seed or not seedhaving been made randomly. N o significantreduction of hail kinetic energy was found. Anearlier experiment in Colorado, USA also failedto demonstrate the effectiveness of the Soviettechnique (Crow et al., 1979). A randomisedexperiment in Argentina, using ground basedgenerators of silver iodide, was also inconclusivebut there was some evidence that there was adecrease of damage to crops in frontal situations,but an increase in other situations (Iribarne andGrandoso, 1965).

In addition to such scientific experiments,seeding is or has been used as an operational toolin many countries. In such operational seeding norandomization is employed; all events expectedto produce hail are seeded. Some evaluation canstill be made by comparing hail measurementsfrom radar, hail pads or crop losses. The com-parisons may be between the target area and somesupposedly unaffected nearby area — the control— or between the period of seeding and someother period, usually the hail seasons prior tocommencement of the seeding operation. Evalu-ations may also employ a combination of thesemethods by comparing the ratio of a hail para-meter in the target area to that in the control areain the seeded period and an unseeded period.Difficulties in interpreting the comparisons in-clude the possible impacts of changes in generalweather patterns, in crop management practiceand in loss reporting.4

3 Pure water droplets do not freeze until the temperature reaches about -40' C. Freezing is induced at highertemperatures because of the presence in the atmosphere of impurities which act as nuclei for ice formation.However significant numbers of ice crystals are found only at temperatures below -15"C or -20' C.

4 As an illustration of the difficulties in evaluating operational weather modification consider a hypothetical hailsuppression operation at Nelson over three summers. The probability of no hail (or smaller frozen particles)occuring in summer at Nelson is 79% (Steiner, 1988). Assuming that there is no relationship between hailfall inone summer and the succeeding one the probability of three summers in a row without hail is 49%. Thus if no hailat all occurred during the operational period no claim could be made for the success of the experiment since thereis an almost even chance of this happening without intervention.

•

Hai l Reduction?

Hail Reduction?

TotalProgramme* S e a s o n s

ReleasetMethod

Type of#Evaluation

DataSource

Change in Change inDHailfall R a i n f a l l

SignificanceLevel(hail)

AFRICA1 Kenya 8 A,G S/N-S C,R -28% +12% 0.05(1968-1975)2 South Africa 4 Aj S/N-S C -35% <0.01(1973-1977)AMERICA3 Hudson Valley 2 G T-C C,R A weak suggestion of lessUSA (1956-1957) hail with no effect on rainfall4 NE Colorado 1 A,G T-C HP,R Decrease Increase Not sig.USA (1959)5 Texas 6 A T-C I,R 48% +5% 0.05(1970-1975)6 SW of North 1 5 A T-C I -31% n.a. not testedDakota USA (1961-1975)7 W of North 4 A S/N-S HP,I,R -30% +23% variousDakota USA (1969-1972)8 North 1 0 A X I 43% n.a. .002Dakota, USA (1976-1985)9 South Dakota 4 A T-C I -20% +7% not sig.USA (1972-1975)10 Utah USA 5 A,G T-C I-R -69% +11% .10(1976-1981)AUSTRALIA11 Tasmania 6(1961-1966)

R X D,I no sig.change

not sig.

EASTERN EUROPE12 Bulgaria 8 R S/N-S C -50 to n.a. n.a.(1972-1978) -60%13 Hungary 7 R X I -50 to n.a. 0.05(1976-1982) -55%14 USSR 1 7 R S/N-S I -80% n.a. n.a.(1968-1984) Combined results all areasWESTERN EUROPE15 France 1 8 G X 41% n.a. .01(1965-1982)16 Greece 2 A T-C C,R -52% +6% .10(1981-1985) 2 S/N-S HP,R -75% +9% 0.0317 Spain 5 A S/N-S -20% n.a.1979-1983)

A summary o f seeding operations w i th theobjective of suppressing hail of which evaluationshave been reported, is shown as Table

TABLE 1: RESULTS OF SOME HAIL SUPPRESSION OPERATIONS.

The most widespread operational seeding iscarried out in the USSR where, by 1984, the 10areas of operations extended over 80 000 square

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*Data sources:-1,3,4,5,10,16 Henderson (1987), 2 Mather (1977), 6,7,9 Changnon (1977) 8 Smith et al. (1987), 11 McCarthy (1984), 12Stantchev and Simeonov (1985), 13 Marko et al. (1985), 14 Burtsev (1985), 15 Dessens (1986b), 17 Sanchez et al. (1985).

-j- Release method: A aircraft; A j jet aircraft; G ground generators; R rocket.# Type of evaluation: T-C target-control area comparison; S/N-S seeded/non-seeded period comparison; X more complex

comparison.t Measurements used: C crop damage; R radar data; I insurance data; HP hailpad data; D days of hail.0 Changes in rainfall and significance level (hail): ma. not available, not. sig. not significant.

There have been some attempts to augment rainfall in New Zealand by cloud seeding but seeding for the purposeof reducing hail damage has not been tried.

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kilometres (Burtsev, 1985) — about 1/3 o f thearea of New Zealand. From Burtsev's data, theaverage reduction in hail losses over the entireareas of operations in the years 1968 to 1984 is 80percent. Using the same type o f rockets, re-ductions o f hail losses by more than hal f havebeen obtained in some other eastern Europeancountries.

Many of the other operations listed appear tohave achieved significant reductions in hail. O fparticular note is the long term French operationreported by Dessens (1986b). He compared hailinsurance data f rom an area o f 55 000 squarekilometres in the southwest of France, in whichsilver iodide is released from 455 ground gener-ators, w i t h data f o r the rest o f the country,without generators. A significant difference be-came apparent from 1965, when the silver iodiderelease technique was perfected. The ratio of thehail damage in the two areas was reduced by 41percent. Unless this change can be shown asbeing related to changes in large scale weatherpatterns, or some other phenomenon, i t seemsreasonable to assume it is a genuine result of theseeding. The benefit to cost ratio for this oper-ation is stated to be about 24.

The differences in results between experimentsand some operations are diff icult to reconcile.Difficulties in interpreting the operations havealready been noted. I t must also be rememberedthat hailstorms can be highly variable i n hailkinetic energy and in their dynamics. N o t al lhailstorms in a given area are similar, and i t ispossible that in some areas the overall statisticsinclude cases where some hailstorms cause re-duced damage through seeding, while others areunaffected or possibly cause more damage. I n -deed both Burtsev (1985) and Stantchev andSimeonov (1985) comment on the occasionalparticularly severe storm, which is not ameliorat-ed by seeding. Possibly in the areas where seedingappears to have been effective the storms aremore uniform and amenable to control by theparticular seeding methods that have been em-ployed. The inability to accurately forecast hailand thus to initiate suppression may be a problemin some regions. A further possibility is that thereis a delayed effect o f silver iodide seeding, assuggested by Bigg and Turton (1985). I f this wasthe case, then there could be large numbers o fparticles that act as nuclei for freezing on bothseeded days and some unseeded days i n theexperiments.

Hail Reduction?

The region of the cloud where the beneficialcompetition technique is considered to be effec-tive is in the upper parts o f the interior o f thecloud, where there is strong upward motion.However, penetrations o f this cloud region o fSwiss hailstorms by an armoured and instrument-ed aircraft (Waldvogel et al, 1987) has demon-strated that the region already contains much iceand little supercooled water. This demonstratesthat, at least in these clouds, attempts at hai lsuppression by beneficial competion is unlikelyto be of value.

There is some experimental and modellingevidence for the success of the premature rainoutapproach to hailstorms in Alberta, Canada. It hasbeen demonstrated there that ha i l is usuallyinitiated in a series of "feeder" clouds which formupwind of the main cloud mass, and then mergewith i t . A comparison o f ice particle countsobtained by aircraft f lying through seeded andunseeded feeder clouds indicated that seedingcould increase the ice content of feeder clouds bya factor o f ten (Humphries e t al, 1987). T h eaircraft was unable to penetrate the main cloudmass. However, numerical modelling of the pro-cesses i n these storms (Farley, 1987) demon-strated that seeding leads to some increase o frainfall, with a decrease in hail.

Thus i t is apparent, that at least in certainareas, some methods o f seeding seem to havebeneficial effects, as determined from reducedcrop losses. There is also some limited physicaland modelling evidence for the success o f thepremature rainout technique. Nevertheless, thestatement of the World Meteorological Organiz-ation (1986) that " the requisite combination o fphysical and statistical evidence o f hail sup-pression is lacking" remains valid.(b) OTHER M O D I F I C AT I O N M E T H O D S

Another form of hail suppression is the use ofvarious types o f cannon, o r other methods o fcreating loud noise or explosions. These methodshave a long and colourful history (Oddie, 1965;Morgan, 1973; Changnon and Ivens, 1981;Chassany, 1982). Cannons especially made forhail suppression came to be used extensively inAustria in 1897, and their use quickly spread toneighbouring countries. The cannons appearedinitially t o be successful, b u t b y about 1902doubts emerged. I t was recognized that hail hasnatural inter-annual variabil i ty, and that theearlier apparent success could be fortuitous. Norwas there an acceptable theory for the cannon

Hail Reduction?

effects. By about 1905 interest had waned. InFrance in the 1950's, rockets were used to createexplosions in cloud to reduce hail damage (Ruby,1953); success was claimed but the method wasnot subject to a systematic statistical evaluation.An experiment using rockets for hail suppressionwas undertaken in the tea estates of Kenya from1963 to 1967; there was an apparent reduction incrop damage (Sansom, 1968) but this method ofhail suppression seems to have been abandonedthere.

In recent years the use of a vertically pointingcannon which explodes acetylene at intervals of afew seconds has again found favour amongfruitgrowers. A high success rate is sometimesclaimed. One make of these devices is marketed inmany countries and such cannon are being intro-duced in New Zealand.

There are a number of theories as to how suchdevices could modify natural cloud processes soas to reduce hail. For some of these theories aneffect has been identified by laboratory experi-ment (Goyer and Plooster, 1968; Plooster, 1972;Favreau and Goyer, 1967) o r by numericalsimulation (Foster and Pflaum (1985), but onlywhen the pressure perturbation in the cloud is atleast 100hPa.

The pressure perturbation at the ground nearone type o f cannon (emitting vertically) wasmeasured by Mezeix et al. (1974). It amounted toonly about 3hPa at a distance of 40 metres. Asound intensity o f 96dBa was measured at ahorizontal distance of 60 metres from a super-ficially similar cannon in New Zealand (L WGardner, pers. comm ) This corresponds to apressure perturbation o f only 1.3 hPa. Theenergy level directly over the cannon may bemuch higher. I f an additonal 50 dBa is added tothe New Zealand measurement to allow for thedirectional effect o f the cannon configuration(suggested by S. Singal, pers. comm.) the mag-nitude of the pressure perturbation just above thecannon mouth amounts to — 400hPa. Howeverthe intensity must become attenuated as thesound wave propagates away from the source.Assuming this pressure perturbation at a heightof 1 metre above the cannon mouth, then at 1000metres, which would typically be near cloud basein New Zealand, the pressure perturbation wouldonly be 0.4 hPa i f the energy dispersion isisotropic. Even i f the dispersion is not initiallyisotropic because of the effect o f the cannonshape, the pressure perturbation near cloud base

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will still only be at most a few hectopascals, and inthe higher regions of the cloud where hail isformed, i t wi l l be much less. Thus pressureperturbations in cloud are at least two orders ofmagnitude too small to have effects on cloudphysical properties. The brevity of the periods ofmaximum energy of the sound pulses furtherreduces the possibility of any effect.

Mezeix et al. (1974) also operated a hail cannontilted t o emit i n a horizontal direction, andexamined its effect on ice particles suspended infront of it, confirming laboratory results of List(1963). Only within 25 metres of the cannon wasany motion of the particles or their supportingwires detected, but even in this range, there wasno sign of any fragmentation of the ice particles.The cannon was also repeatedly fired (in theregular vertical position) at cloud on 4 occasions,with at least 150 explosions in each case. No effectwas detected in the visual cloud characteristics.However it must be pointed out that the cloudswere not cumulonimbus.

It is surprising that hail cannon are generallyclaimed to affect the area immediately surround-ing them. No statistics o f storm motion areavailable in New Zealand but overseas studies ofhailstorms suggest they can move rapidly, e.g.Dessens (1986a) and Carte (1963) indicate aver-age speeds of 15 and 13 m/s. Thus any effect froma vertically pointing cannon might be expected tobe somewhere downwind.

Another possibility of hail suppression couldbe some modification of the electrical propertiesof the cumulonimbus cloud with which hail isusually associated. The makers of one type of hailcannon claim that it works through some elec-trolysis process. No detailed explanation is givenand there is no reference to any accepted physicaltheory. The minimal requirement for cannon tocause hail suppression through some electricalprocess, would be the detection of changes inatmospheric electrical properties above the can-non. The manufacturers of hail cannon couldcontribute to the plausibility of these devices i fthey could either further explain the thory, ordemonstrate an electrical effect.

Huang et al. (1981) consider that firing gunsinto clouds has an effect on cloud dynamics. Theinherent difficulty of such a technique was point-ed out a long time ago in New Zealand (Bates,1907): the total energy of a storm is several ordersof magnitude greater than that of any explosion orgun-shot that could be contemplated. I f a con-

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vective circulation speed of 10 m/s then thekinetic energy amounts to 0.511. Any gun firemust have several orders o f magnitude lessenergy. Huang et al. present some evidence ofchanges in the radar echo following the repeatedshelling of hailstorms with anti-aircraft guns. Inmost cases pronounced changes were reported asfollowing the shelling, often with the appearanceof holes in the echo pattern. Because changes inthe radar echo can occur naturally, a carefulcomparison of the evolution of shelled and otherevents would be required, as well as an acceptabletheory, before the value o f shelling could beaccepted.

Thus, on the basis o f field, laboratory ormodelling tests, a plausible theory has not beenestablished for cannon and other hail suppressionsystems based on explosives or gunfire. Further-more, there are inadequate statistics to confirmthe value of such methods. The same difficultiesapply to evaluation of operations, as were in-dicated for hail suppression by seeding: it is notenough to compare hail damage over a few yearswhen a cannon is in use, with an earlier periodbefore its installation.

DISCUSSION AND SUMMARY:A HAIL PROGRAMME FOR NEW ZEALAND

Strategies for reduction of hail damage havebeen reviewed. Application of such strategiesrequires a more detailed hail climatology. I twould be desirable to establish a computer ar-chive o f New Zealand hail data in as high aresolution as possible. It should not only includeareas currently used for horticulture, but alsoother potential horticultural areas. I t is con-sidered that organizations concerned with hort-iculture, the insurance industry and the Meteor-ological Service could all be involved in thisdevelopment. The cooperation of horticultural-ists and other land users in providing data wouldbe essential.

Data from meteorological radars would also bevaluable for developing the hail climatology.Moreover such data would be a high priority forthe guidance of any attempts at weather modific-ation.

Methods of hail suppression attempted over-seas have been reviewed. In the case of cloudseeding there are scientific theories to explain the6 There is not unanimity in France on the value of this method. See Mezeix (1987).

Hail Reduction?

effect, and statistics showing a benefit in manycloud seeding operations. However, randomizedexperiments have not demonstrated a significantseeding effect.

There is no acceptable theory for the use of hailcannon or other explosive devices. Many usersare convinced of the value of these devices, butthis does not provide proof of their efficacy.

Decisions on undertaking weather modific-ation activities are not necessarily made by scien-tists. It is important that scientists take an interestin such activities and offer advice. It would be inthe interests of both those undertaking weathermodification, and the wider community, i f allsuch activities were recorded: summaries of theoperations and of the results should be kept insome standard form.

There is no method capable of totally suppress-ing hail. Of the methods of hail suppression thatappear to have some beneficial effects, none canbe recommended with absolute confidence.

I f it was felt that some form of hail suppressionshould be attempted in New Zealand, despite thislack of confidence, then the French technique ofusing ground based generators of silver iodideand sodium iodide (Dessens and Pham VanDinh, 1968; Dessens, 1986b) should be adopted.6It is preferred because of its apparent successover a long time period, ease of operation andhigh benefit to cost ratio. In the area of Francewhere this technique has been employed, most ofthe damage comes from hailstorms associatedwith cold fronts. A study of severe New Zealandhailstorms (Steiner, 1988) suggests that they arenot usually frontal. Hence the method might notyield the same results here. I t should also benoted that this type of technique is intended tosuppress hail over a large area rather than toprotect individual properties.

There are some additional caveats associatedwith the introduction of any form of hail sup-pression. Effort needs to be put into developinghigh skill in forecasting potential hailstorms. Theavailability of weather radar would be beneficialin this regard. The possibility of a suppressiontechnique leading to a rainfall increase, and theconsequences of such an increase on horticultureand other activities, also needs to be considered.Because of the limited historic data on hail, i tmight be difficult to demonstrate any effect of thehail suppression attempt.

Hail Reduction?

ACKNOWLEDGEMENTS

I wish to thank D r N. D. Gordon, Dr H. R.Larsen and Mr A. A. Neale for reviewing draftsof this material and D r S. Singal for advice onacoustics.

REFERENCES

Bates, D., C., 1907. Rain-making experiments at Oamaru(Report of Meteorological Observer On). Return to anOrder of the House of Representatives dated 13 Novem-ber 1907. H34a. (Also published as: Report upon the dryperiod and rainmaking experiments at Oamaru, NewZealand. Monthly Weather Review 36, 208-213, 1908).

Bigg, E. K., and Turton, E., 1986. Delayed effets of cloudseeding with silver iodide. Journal of Climate and AppliedMeteorology 25, 1382-1386.

Blanchard, D. O. and Howard, K. W., 1986. The Denverhailstorm of 13th June 1984. Bulletin of the AmericanMeteorological Society 67, 1123-1167.

Burtsev, I. 1., 1985. Crop protection against hail damage inthe USSR. Papers presented at 4th WMO Conference onWeather Modification, Honolulu. WMP Report No. 2 ,World Meteorological Organization, Geneva, 613-623.

Carte, A. E., 1963. Some characteristics of Alberta hailstorms.Alberta Hail Studies 1962/63. Scientific Report MW 36,Stormy Weather Research Group, McGill University,1-16.

Changnon, S. A., 1977. On the status of hail suppression.Bulletin of the American Meteorological Society 58, 20-28.

Changnon, S. A., and Ivens, j. L., 1981. History repeated: theforgotten hail cannons of Europe. Bulletin of the AmericanMeteorological Society 62, 368-375.

Chassany, J., 1982. La grele a travers les ages. Rapporttechnique No. 44, Groupement Nationale D'etudes desMeaux Atmospheriques, 27pp.

Crow, E. L., Long, A. B., Dye, J. E., Heymsfield, A. J., andMielke, P. W. Jr., 1979. Results of a randomized hailsuppression experiment in northeast Colorado. Part I I :surface data base and primary statistical analysis. journalof Applied Meteorology 18, 1538-1558.

Dessens, J., 1986a. Hail in southwestern France. H a i l f a l lcharacteristics and hailstorm environment. Journal ofClimate and Applied Meteorology 25, 35-47.

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