Science, and transportation united states senate



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numerical models that accurately predict cloud development and the effects of

seeding should minimize the risk of unexpected events. 22

MODIFICATION OF SEVERE STORMS

Severe storms have a greater immediate impact on human life and

property than most other weather phenomena. A major portion of

losses due to natural disasters results from two of the most destructive

kinds of severe storms — hurricanes and tornadoes. During an average

year the U.S. mainland is threatened by 8 tropical slorms and experi-

ences over 600 tornadoes. 23 Among the results of the annual devastation

from these storms are the loss of hundreds of lives and the accumula-

tion of hundreds of millions of dollars in property damage.

Perhaps the most important problems to be attacked in weather

modification are associated with the abatement of severe storms. While

rainfall augmentation promises borderline economic value at best, al-

ternatives which can contribute more significantly to severe water

shortages may prove more suitable. On the other hand, the annual

threat of tolls in damages and fatalities from hurricanes and tornadoes

will persist year after year, and research directed toward modification

of these severe phenomena requires continued support. There have been

dramatic attempts, with some successes, in demonstrating the potential

reduction of the hazards of hurricanes ; however, almost no research

has been directed toward tornado suppression.

Hurricanes

A hurricane is an intense cyclone which forms over tropical seas,

smaller in size than middle-latitude cyclones, but much larger than a

tornado or a thunderstorm. With an average size of 500 miles (800

kilometers) in diameter, the hurricane consists of a doughnut-shaped

ring of strong winds in excess of 64 knots which surrounds an area of

extremely low pressure and calm at the storm's center, called the eye. 2 *

The generic name for all vortical circulations originating over tropi-

cal waters is "tropical cyclone." When fully developed with sufficiently

strong winds, such storms are called hurricanes in the Atlantic and the

eastern Pacific Oceans, typhoons in the northwest Pacific, baguios in

the Philippines, Bengal cyclones in the Indian Ocean, and willy-willies

near Australia. For a tropic cyclone whose winds are in the range of

33 to 64 knots, the official name' in the United States is a tropical storm.

The hurricane season is that portion of the year having a relatively

21 Fuquay, "Lightning Damage and Lightning Modification Caused by Cloud Seeding,"

1974. p. 612.

22 Ibid., p. 606.

23 Feieral Coordinator for Meteorological Services and Supporting Research. "Federal

Plan for Meteorological Services and Supporting Resenrch : Fiscal Year 1973." U.S. Depart-

ment of Commerce, National Oceanic and Atmospheric Administration, Washington, D.C.,

January 1972. p. 1.

24 Anthes, Richard A.. Hans A. Panofskv. -Tohn J. Cahir. and Albert Rango. "The Atmos-

phere." Columbus, Ohio, Charles E. Merrill. 1975. p. 150.

102

high incidence of hurricanes and usually is regarded as the period



between June and November in the Northern Hemisphere. 25

Owing to their duration, which exceeds that of earthquakes, and to

their violence, which approaches that of tornadoes, hurricanes are the

most destructive natural phenomena. Prior to Hurricane Agnes in

1972, whose total damage exceeded $3 billion, the annual hurricane

property losses in the United States amounted to about $450 million,

although two hurricanes in the 1960's, Betsy (1965) and Camille

(1969), each caused damage exceeding $1.4 billion. 26 Improved tech-

niques in hurricane detection and warning have dramatically reduced

the number of deaths caused by hurricanes ; however, property losses

have continued to grow, as a result of increased population and activi-

ties in vulnerable coastal areas, with the attendant concentration of

new houses, buildings, and other facilities of higher replacement value.

Figure 8 shows the simultaneous increase in property losses and de-

crease in deaths due to hurricanes in the United States in the 20th

century through 1969.

Devastation and fatalities occur essentially from three phenomena

associated with hurricanes : the force of the winds in the storm itself,

the storm surge on coastal areas, and flooding which can result from

excessive and widespread rainfall as the storm moves inland. Since

wind force varies with the square of the wind speed, a 50-mile-per-hour

wind exerts four times as much force as a 25-mile-per-hour wind. Ac-

cordingly, a 10-percent reduction in maximum windspeed yields a de-

crease in wind force of about 20 percent. 27 Attempts to modify hurri-

cane winds can thus be expected to reduce storm damage caused by

winds in approximate proportion to the corresponding reduction in

wind force.

25 Federal Coordinator for Meteorological Services and Supporting Research, U.S. Depart-

ment of Commerce, National Oceanic and Atmospheric Administration, "National Hurricane

Operations Plan," FCM 77- 2. Washington, D.C., May 1977, pp. 6-7.

20 Gentry, K. Cecil, "Hurricane Modification." In Wilmot N. Hess (ed.). "Weather and

Climate Modification," New York, John Wiley & Sons, 1974, p. 497.

27 Ibid., p. 498.

103


Figure 8. — Losses in the United States from hurricanes, 1915 through 1969, in

5-year periods (from National Oceanic and Atmospheric Administration).

_ As a hurricane moves across the coast from the sea. the strong winds

pile up water to extreme heights, causing storm surges. The resulting

onrushing water wreaks damage to shoreline and coastal structures.

The severity of the storm surge is increased by the hurricane-generated

wind waves which are superimposed on the surge. From Hurricane

Camille, the storm surge at Pass Christian, Miss., was 24.6 feet, higher

than any previous recorded tide. As a result, 135 people were killed,

63,000 families suffered personal losses, and Mississippi alone sustained

$1 billion in damage. 28 The height of the storm surge depends both on

Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," 1975, p. 159.

104

the windspeed and the shape and slope of the sea bottom offshore. If



there is a sharp dropoff in depth not far off the beach, the rise of the

sea level will be small, for example. Nearshore attempts to modify a

hurricane could lead to uncertain results, depending upon local condi-

tions. If the windspeed is reduced without moving the position of

maximum winds along the coast, the overall effect would likely be a

reduction in storm surge. However, should the modification activity

result in developing a new windspeed maximum at a different location,

the surge might increase or decrease, depending on bathymetry and

bottom topography. 29 Solutions are not yet clear, and the storm surge

prediction problem is being studied intensely with the use of numerical

models.

Major hurricane damage can often be attributed to heavy rains and

the massive and sudden flooding which can result as the storm move's

inland. In mountainous regions especially, the floods from such rain-

fall can be devastating in losses to both life and property. Such flood-

ing was a major contributor to the 118 deaths and $3.5 billion in prop-

erty destruction 30 which resulted in June 1972 from Hurricane Agnes,

which set the record of achieving the greatest damage toll of all U.S.

hurricanes. Ironically, Agnes caused almost no major damage as it

went ashore. Hurricane modification activities which have been at-

tempted or are contemplated are unfortunately not designed to reduce

the rains significantly, but are intended rather to reduce the maxi-

mum winds. 31

Generation and characteristics of hurricanes

A hurricane can be thought of as a simple heat engine driven by

temperature differences between the center of the storm and its mar-

gins. At each level the central column must be warmer than the

surrounding area to insure maintenance of the strong convection on

which the storm depends. 32 While the energy which forms extratropical

cyclones is provided by temperature differences between different air

masses, the energy which generates and maintains hurricanes and

other tropical cyclones is derived from a single air mass through

condensation of water vapor, and there are seldom present any of

the frontal activities which are characteristic of storms originating

in temperate latitudes. The moisture-laden winds continuously supply

water vapor to the tropical storm, and the condensation of each gram

of the vapor releases about 580 calories of latent heat. Within this

thermally driven heat engine tremendous quantities of energy are

converted from heat to mechanical motion in a short time, a fact

readily apparent from the fury of the winds. The daily power of the

energy liberated within a hurricane has been estimated to be about

ten thousand times the daily power consumption in the United States. 33

The importance of tin 1 ocean in providing moisture to a hurricane

is seen in the weakening and dissipation of the storms after they have

crossed coastlines and travel over land.

20 Gentrv. "Hurricane Modification," 1974. p. 499.

30 National Advisory Committee on Oceans and Atmosphere. "The Agnes Floods.: a Cost-

Audit of the Effectiveness of t^c Storm and Flood Warning System of the National Oceanic

and Atmosnheric Administration," a report for the Administrator of NOAA. Washington,

D.C., Nov. 22. 1972. p. 1.

:;1 Gentrv. "Hurricane-Modification." H>74. n. 490.

^Donn. William L. "Meteorology." 4th edition. New York. McGraw-Hill, 1975, p. 336.

"Ibid., p. 338.

105


Exactly how hurricanes form is not yet fully understood. They

are all generated in the doldrums (a region of equatorial calms),

though rarely if ever within latitudes closer than 5 degrees from the

Equator, over water whose temperature is at least 27° C. The relatively

high surface temperature is necessary for initiation of the convection.

Hurricanes are relatively rare features even of the tropics, and the

exact triggering mechanism is not yet known. 34 Their origin is usually

traced to a low pressure disturbance which originates on the equatorial

side of the trough of an easterly wave.

Such a tropical disturbance moves slowly westward and slightly

poleward under the direction of the tropical east winds. If conditions

are right, this cluster of thunderstorms intensifies as it reaches the

region near the boundary between the tropical easterlies and the

middle-latitude westerlies, at about 25° latitude. It may then follow

a path which reverses toward the east as it leaves the tropics. The

tracks of 13 major hurricanes in the Northwest Atlantic Ocean are

shown in figure 9.

The development of the intense storm which might result from the

conditions noted above is described in the following way by Anthes

et al. :

The increased inflow toward the center of falling pressure produces increased

lifting of air, so that the thunderstorms become more numerous and intense. The

feedback cycle is now established. The inflowing air fuels more intense thunder-

storm convection, which gradually warms and moistens the environment. The

warmer air in the disturbance weighs less, and so the surface pressure continues

to fall. The farther the pressure falls, the greater the inflow and the stronger

the convection. The limit to this process would occur when the environment is

completely saturated by cumulonimbus clouds. Further condensation heating

would not result in additional warming, because the heat released would exactly

compensate for the cooling due to the upward expansion of the rising air. 35

34 Ibid.

35 Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," 1975, p. 154.

106

Figure 9. — Tracks of thirteen major hurricanes in the Xorth Atlantic from 1879



through 1955 (from U.S. Naval Oceanographic Office, Publication No. 21,

Sailing Directions for the West Indies, 1958).

As the storm forms, the winds begin to strengthen about the center,

increasing especially to the right of the direction in which the center

is moving, normally on the poleward side. The clouds organize them-

selves into a system and dense cirrus move forward in the direction

of the movement of the center. Suddenly, the pressure falls over a

small area and hurricane force winds form a tight band of 20 to 40

107

miles radius around the center. The well-organized clouds show a



spiraling structure, and the storm acquires an eye, a small nearly

circular area, coinciding with the region of lowest pressure. The winds

in the eye are light and variable and the clouds are scattered or

entirely absent. 36 As the storm matures, the pressure ceases to fall

and the maximum winds do not increase further. Now the storm ex-

pands horizontally and large amounts of air are drawn in. As the

storm expands to a radius of about 200 miles or more it becomes less

symmetrical. Figure 10 is a vertical cross-section of the structure of

a typical mature hurricane, showing the direction of flow and cloud

distribution. 37

In spite of the great damage and fatalities caused by hurricanes,

their effects are not completely destructive. In many areas of South-

east Asia and the west coast of Mexico, tropical storms are depended

upon for a large part of the water supply. Throughout the Southern

United States, hurricanes have also provided valuable drought relief. 38

- Hurricane and other tropical cyclones are always characterized by

high wind velocities and by torrential rains. Wind velocities of 60 to

70 knots and more are normal for such storms. The air rotates rapidly,

moving spirally toward the center. Maximum gusts exceed 100 knots

and may reach 200 knots, although such high speeds are unrecorded

since instruments are blown away or made inoperable at these wind

speeds. 39

Figure 10. — Vertical cross section through a hurricane, showing typical cloud

distribution and direction of flow, as functions of height and distance from

the eye. (From Anthes, Panofsky, Cahir, and Rango, 1975.)

Compared with extratropical storms, hurricanes are generally small,

circularly shaped zones of intense low pressure, with very steep pres-

sure gradients between the center and the periphery. The pressure

drop between the eye and the periphery is quite large, 20 to 70 milli-

bars being typical. The winds are in a constant circular cyclonic

motion (counterclockwise in the Northern Hemisphere and clockwise

in the Southern Hemisphere) ; however, the center of the storm is a

36 p P tterssen. Sverre. "Introduction to Meteorology," second edition, New York, McGraw-

Hill. 1958, pp. 242-243.

37 Anthes. Panofsky. Cahir. and Rango. "The Atmosphere," 1975. p. 157.

ssReihl, Herbert, "Introduction to the Atmosphere," New York, McGraw-Hill, 1965, pp.

178-179.

39 Gentilli. J.. "Tropical Cyclones." In Rhodes W. Fairbridge fed.). "The Encyclopedia

of Atmospheric Sciences and Astrogeology." Reinhold, New York, 1967, p. 1028.

* Widely scattered

_ — — shallow cumulus

1000


Distance from hurricane center (km)

108


calm region of low pressure, called the eye. which is about 10 miles

across on the average. The warm dry character of this region is due

to subsiding air, which is necessary for existence of the storm. Around

the eye is the wall, consisting of cumulonimbus clouds and the at-

tendant extreme instability and rising motion; in the wall area adja-

cent to the eye, heavy rains fall. Out from the central zone altostratus

and nimbostratus clouds mix to form a layer with a radius as great

as 200 miles. At higher altitudes and reaching to the outer regions

of the storm is a mixture of cirrus and cirrostratus clouds. 40

In a mature hurricane a state of relative equilibrium is reached

eventually, with a particular distribution of wind, temperature, and

pressure. Such distributions for a typical hurricane are shown sche-

matically in figure 11. Note that the greatest pressure change and the

maximum windspeeds are in the region of the wall clouds, near the

center of the storm. 41

Figtjbe 11.— Radial profiles of temperature, pressure, and windspeed for a mature

hurricane. The temperature profile applies to levels of 3 to 14 kilometers;

pressure and windspeed profiles apply to levels near the surface. (From

Gentry, 1974. )

Modification of hurricanes

Since the damage inflicted by hurricanes is primarily a result of the

high windspeeds, the principal goal of beneficial hurricane modifica-

40 Jerome Williams. John J. Hipsinson. and John D. Rohrhoujjh. "Sea and Air: The

Naval Environment," Annapolis. Md.. U.S. Naval Institute. 1968, pp. 262-263.

41 Gentry. "Hurricane Modification." 1974. pp. 502-503.

109


tion is the reduction of the severity of the storm's maximum winds.

The winds result from the pressure distribution, which, in turn, is

dependent on the temperature distribution. Thus, hurricane winds

might be reduced through reduction of temperature contrasts between

the core of the storm and the region outside.

Gentry notes that there are at least two important fundamentals of

hurricanes which have been established through recent studies, which

suggest possible approaches to modification of the severity of the

storms : 42

The transfer of sensible and latent heat from the sea surface to the

air inside the storm is necessary if the hurricane is to reach or retain

even moderate intensity.

The energy for the entire synoptic-scale hurricane is released by

moist convection in highly organized convective-scale circulations lo-

cated in and around the eye of the storm and in the major rain bands.

The first principle accounts for the fact that hurricanes form only

over warm tropical waters and begin to dissipate after moving over

land or cool water, since neither can provide sufficient energy flow to

the atmosphere to maintain the intensity of the storm. The second

principle explains why such a low percentage of tropical disturbances

grow to hurricane intensity. Possible field experiments for beneficial

modification of hurricanes follow from these principles. On the basis

of the first, techniques for inhibiting evaporation might be employed

to reduce energy flux from the sea surface to the atmosphere. Based

on the second principle, it might be possible to affect the rate of release

of latent heat in that small portion of the total storm which is occupied

by the active convective-scale motions in such a way that the storm is

weakened through redistribution of heating. 43

Gentry discusses a number of possible mechanisms which have been

suggested for bringing about changes to the temperature field in a

hurricane. 44 Since the warm core development is strongly influenced

by the quantity of latent heat available for release in air columns ris-

ing near the center of the storm, the temperature might be decreased

through reducing the water vapor in these columns, the water vapor

originating through evaporation from the sea surface inside the region

of high storm winds. It has been suggested that a film spread over the

ocean would thus reduce such evaporation. No such film is available,

however, which could serve this purpose and withstand rupturing and

disintegration by the winds and waves of the storm. Another sugges-

tion, tiiat the cooling of the sea surface might be achieved through

dropping cold material from ships or aircraft, is impractical, since

such great expenditure of energy is required. It has also been postu-

lated that the radiation mechanisms near the top of the hurricane might

be modified through distribution of materials of various radiation

properties at selected locations in the clouds, thus inducing changes to

the temperatures in the upper part of the storm. This latter suggestion

needs further evaluation both from the standpoint of its practicality

and from the effect such a change, if included, would theoretically have

on storm intensity.

The potential schemes for hurricane modification which seem to be

practical logistically and offer some hope for success involve attempts

42 Ibid., 1974. p. 503.

« Ibid., p. 504.

44 Ibid., p. 505.

34-857 O - 79 - 10

110


to modify the mechanism by which the convective processes in the eye-

wall and the rain bands distribute heat through the storm. Since water

vapor is condensed and latent heat released in the convective clouds, it

should be possible to influence the heat distribution in the storm

through changing the pattern of these clouds. 45 Recent success in

modifying cumulus clouds promises some hope of success in hurricane

modification through cloud seeding. By modifying the clouds in a hur-

ricane, the storm itself may be modified, since the storm's intensity will

be affected through changing the interactions between the convective

(cloud) scale and the synoptic (hurricane) scales. 46 Figure 12 shows

how the properties of a hurricane might be redistributed as a result

of changing the temperature structure through seeding the cumulus

cloud structure outside the wall. The solid curves in the figure repre-

sent distributions of temperature, pressure, and windspeed identical

with those shown in figure 11 without seeding; the dashed curves rep-

resent these properties as modified through seeding. 47

The first attempt at hurricane modification was undertaken by sci-

entists of the General Electric Co., on a hurricane east of Jacksonville,

Fla., on October 13, 1947. Clouds outside of the wall were seeded with

dry ice in order to cause freezing of supercooled water, so that the ac-

companying release of latent heat might alter the storm in some man-

ner. Results of the experiment could not be evaluated, however, owing



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