the laser and its application to meteorology

564 B U L L E T I N A M E R I C A N METEOROLOGICAL SOCIETY

The Laser and its Application to Meteorology

G . G . GOYER AND R . W A T S O N

National Center for Atmospheric Research, Boulder, Colo.

1. Introduction

Since its inception, just about three years ago, the laser has stimulated the imagination of scien-tists in practically all fields of endeavor. The un-usual properties of the laser beam have opened up for research an unexplored area in the broad spec-trum of electromagnetic radiations.

The laser now makes possible the harnessing of the energy of electromagnetic radiations and the shaping of their optical properties. Consequently, the laser has already found several successful ap-plications in such diversified fields as metallurgy, chemistry and medicine. The modulation and demodulation of laser radiations are the major problems now tackled by communication engineers in the hope of reducing to rapid practical applica-tions the amazing theoretical possibilities of the laser in the field of communications. This is the major problem in space communications. Ter-restrial communications are faced with a second problem of equal magnitude: that of the transmis-sion of laser beams through the atmosphere. The latter is also of interest to meteorologists.

Meteorologists have already sensed the possi-bilities of the laser as a new powerful tool for prob-ing the atmosphere. Ideas have been suggested and there is no doubt that several are under theo-retical investigation at the present time. In this paper we intend to present a general description of the laser and of the characteristics of its beam and a descriptive, conceptual review of possible appli-cations of the laser in the field of meteorology.

2. Description

As its name implies, the laser was first de-veloped for the amplification of light through the stimulated emission of radiation. It is now used as a generator of an intense beam of coherent radia-tion. Conceived in December 1958 by Schawlow and Townes at Columbia University, the laser prin-ciple was first reduced to practice in July 1960 by Maiman at Hughes Research Laboratories. The basic principle, designated as "population inver-sion,consists of raising and storing a substantial number of atoms to an excited state and in control-

ling their emission of radiation with respect to time and wave length (Schawlow, 1961; Levine, 1963; Lengyel, 1962). There are three main kinds of lasers now in use and under intensive development: the solid-state laser, the gas laser and the injection laser.

The solid-state laser is best exemplified by the first operational type, the ruby laser. The ruby laser consists of a ruby rod, reflecting ends, and a flash lamp. The rod, 0.5 to 1 cm in diameter and 10 to 20 cm long, is a single crystal of aluminum oxide containing 0.05 per cent of chromium atoms which in their trivalent ionic state are the active species in the medium. The faces of the rod are accurately polished to a tenth or even a hundredth of a wavelength and silvered to achieve between 78 and 98 per cent reflection of the radiation back into the rod. The flash lamp, which completely sur-rounds the rod, acts as the pumping source for the active species in the rod. The ruby laser is a three energy level system having a ground state and two excited states. The broad-band radiation, 0.38 to 0.61 p from the flash lamp, is absorbed by the ruby and the chromium atoms are thereby raised to a higher energy state. It is advantageous to have a broad pump band because the highest ex-cited state is actually an absorption band consist-ing of many energy levels. In this case the broader spectral region of the incident radiation is available for pumping. In a way, the ruby focuses the energy of photons from a wide fre-quency band into a nearly monochromatic frequency which is that of the emitted radiation. If a sufficient number of atoms are excited, the population inversion, essential to the laser action, is achieved; there are in the medium more atoms at the high energy level than at the ground state. Under these conditions the probability of a trigger-ing photon colliding with an excited atom is greater than that of being absorbed and lost in a ground state atom. The interaction between the triggering photon, originating from spontaneous emission, and an excited atom stimulates the emission of an additional photon of energy identi-cal to that of the triggering photon. Amplifica-tion is then achieved. The spontaneous emission

Unauthenticated | Downloaded 05/29/22 09:25 PM UTC

V O L . 4 4 , N o . 9 , SEPTEMBER 1 9 6 3 565

arising from the return of excited atoms to the ground state supplies photons of the exact energy to trigger the laser action. Some photons are lost to the side of the rod but others travel along the axis and are reflected by the faces of the rod. Every interaction of a triggering photon with an excited chromium atom generates another photon of the same energy. Moreover, the radiation re-sulting from the interaction is exactly in phase with the triggering radiation. The photon multi-plication factor is directly related to the number of interactions between the excited chromium atoms and the photons having the same energy as the photon radiated in the process. Consequently, the triggering radiation greatly amplified by nu-merous reflections within the rod emerge as an intense beam of coherent monochromatic radiation from the partially transmitting end faces.

The laser action is only maintained as long as the population of excited atoms in the medium is large. Consequently, it is limited by the pulse length of the flash lamp ( usually around one milli-second), which produces pulsed radiation at the rate of up to 60 pulses per minute. The wave-length of the radiation is a function of the nature and of the temperature of the active medium. The ruby laser radiates at 0.6943 /x (microns). The intensity of the pulsed beam is in the kilowatt to megawatt range. The conversion efficiency of the ruby laser is extremely small.

A four level solid-state laser system, such as the trivalent neodymium in calcium tungstate, is a more efficient system. Population inversion in this system is achieved with much less energy in-put than in the ruby system because an additional, normally empty energy level lies intermediate be-tween the ground state and the two highest energy levels. Population inversion is achieved with respect to this intermediate state, designated as the terminal state for the laser action. Consequently, inversion is relatively easier to achieve with re-spect to the normally unpopulated state, than with respect to the very heavily populated ground state. As a result, power requirements are often a thousandth that of three level systems, and makes constant wave operation possible with a high in-tensity continuous light source for pumping. The power output is in the watt range. In this case neodymium atoms excited into the highest energy level fall back to a lower relatively long-life level and are stimulated into an avalanche to the termi-nal state by the first few photons resulting from spontaneous emission. This transition results in a strong coherent radiation at 1.06 /x. Several solid-state lasers have now been developed from at

least twelve ions in various host crystals. Solid-state lasers are now operational at 80 wavelengths extending from 0.254 /x to 2.698 jx.

A totally different way to obtain excited atoms for laser action employs gas atoms in an electric glow discharge. This is the gas laser. In this case the pumping is achieved by radio frequency or direct current as in an ordinary neon sign. In the case of the neon-helium gas laser a sufficient amount of neon atoms are maintained in the ex-cited state by electric excitation to permit stimu-lated emission. The triggering photons are gener-ated by spontaneous emission. The standard gas laser thus consists of a quartz enclosure about 1 cm diameter by 1 meter long, exciting electrodes and two end-face mirrors inside or outside the cavity. The neon-helium laser emits at three distinct wavelengths: 0.6328 /x, 1.15 /x and 3.39 /x. The helium atoms are excited by electrical stimulation and transfer, through collisions, their energy to the neon atoms. These, in turn, radiate in falling through to lower energy levels. The desired wavelength is obtained by amplifying radiation at this wavelength only. This is accomplished by using mirrors of highest reflectivity at the de-sired wavelength. Unlike the solid-state laser, the gas laser emits a continuous wave of much reduced intensity in the milliwatt to watt range. Laser action has been achieved in a large number of single or mixed gases. Gas lasers now operate from 0.2896 to 18.506 /x in 81 specific wavelengths.

Finally, semi-conductor or injection lasers pro-duce stimulated radiation triggered by direct cur-rents of high magnitude at the p-n junction. Their peak pulse power is much smaller than that of the solid-state lasers, but their conversion efficiency is much larger. Their main advantages rest in their mechanical stability and their small sizes. Still at the first stage of their development, the injection lasers hold great promise of becom-ing a very useful tool.

Semi-conductor lasers, liquid lasers, Raman lasers and optical mixing add 23 other wave-lengths, ranging from 0.231 to 1.6 /x to the list of available wavelengths. As of 25 March 1963, (Hayler, 1963) a total of 184 monochromatic radiations were available within the range extend-ing from the ultra-violet (0.231 /x) to the infra-red (18.506 /x). The list is growing rapidly and in the near future laser radiation should be avail-able at any wavelength desired.

This description of lasers is by no means com-plete and exhaustive. We have reviewed the basic principles and properties of operational lasers at the present time. This field is in a period of in-



tensive research with important developments in the design, operation and characteristics of new systems being continuously reported. Laser ac-tion at 28 /x has been reported by Bell Telephone Laboratories (Faust, 1963). The 75 /x line is under intensive research at Raytheon (Tang, 1963). Research in this field promises rapid progress in the development of systems applicable to meteorology.

3. Characteristics of a laser beam

The most drastically different and important property of a laser beam is its coherence with respect to time and space. From that single prop-erty stem practically all other valuable character-istics which make the laser such a major break-through in the field of optics. A wave is coherent with respect to time when a fixed phase relation-ship exists between that portion of the wave emitted at one instant and the wave emitted at a fixed time interval later. A wave is coherent with respect to space if the phase of the plane wave is uniform over a plane of observation perpendicular to the direction of the wave at any given instant: in other words, when there is a fixed correlation be-tween the phases of monochromatic radiations emanating at two different points on the source. Classical light sources being incoherent, emit sepa-rate waves in time and space that cancel or rein-force each other in a random fashion. The wave front varies from point to point and changes from instant to instant. Thus, only a small fraction of the radiation can be converted into a plane wave. Moreover, these sources spread their output over a wide spectral band without supplying much power at any particular frequency. Powerful monochromatic classical sources do not exist. The radiated energy is generally poorly collimated and the collimation can only be improved at the ex-pense of the available intensity. Consequently, the radiation from an extended source cannot be imaged with an increase in brightness. Finally, radiation from classical light sources cannot be compressed, with respect to time, in pulses as short as is now possible with lasers.

The coherence length of laser beams is several orders of magnitude larger than that of classical light sources. The coherence length is the maxi-mum distance the light can travel while its source preserves the phase of the radiation and is a function of the monochromacity of the source. The coherence length of white light is 3 /x, that of cadmium or mercury is 30 cm and that of the pulsed ruby laser 3000 cm (Berkley, 1962) and of the gas laser 10,000 miles (Jaseja, 1963). Con-

sequently, the interference phenomenon can be ob-served over much larger optical path differences with laser radiation than with classical light sources.

The radiation flux, or intensity, of the laser beam is enormous in the pulsed laser where the duration of the emission is of the order of lOOths of microseconds. The peak power output is the total energy emitted divided by the pulse duration. A 10 joule burst over a period of 0.5 millisecond yields a power output of 20,000 watts. Mega-watt outputs are possible now in bursts of one microsecond, due to the newly developed "Q-switching" technique (McClung, 1963). This technique reduces the loss of photons in the early stage of the excitation until that time when the maximum population of excited atoms is obtained and triggered.

The sun's total radiation over the whole spectral band is only 7 kilowatts per cm2 of its surface. The radiant flux from pulsed lasers extends from kilowatts to megawatts per cm2. In contrast, a 4174K black-body radiating at the wavelength of the ruby laser generates 1700 watts per cm2 over the whole spectral band. In a 0.1 A interval, the band width of an unsophisticated laser beam, the output of the blackbody is only 0.016 watt per cm2. For a blackbody to radiate energy comparable to that of the pulsed laser beam, temperatures in the order of hundreds of millions of degrees are re-quired. In addition, the laser, being an almost coherent beam, can be focused into a spot of the order of a few wavelengths to form an image much brighter than the original source.

The intensity or power output of the "con-tinuous wave" gas lasers is, however, not as spec-tacular. It ranges from 0.1 milliwatt to watts. Since the population of excited atoms is being con-tinuously depleted by the continuous wave, no bursts of very large magnitude can be triggered. However, a substantial amount of research is being expended on this problem at the present time. The prime objective is continuous operation with watts of coherent optical output. Pulsed gas lasers of higher output have already been devel-oped (McMahan, 1963). The laser produces the most perfect monochromatic radiation in the opti-cal range. Spectral widths of 0.1 A and better are easily achieved. In gas lasers monochromacity of 10"7 A has been reported.

As a result of the numerous reflections within the optical cavity of the laser, the radiation emerges as a highly parallel and directional beam. Under certain conditions its angle of divergence is of the order of 2 seconds of arc, or 10~5 radians


V O L . 4 4 , N o . 9 , SEPTEMBER 1 9 6 3 567

(Sirons, 1962). The diameter of a %-inch laser beam would increase to 1 ft 18 miles away from the source. In contrast, an arc-lamp at the focus of a 6 ft diameter parabolic mirror results in a one degree divergence angle and a diameter of the beam of 1760 ft at the same distance. This prop-erty is extremely valuable in the field of communi-cation and ranging.

4. Applications to meteorology

Lasers provide new sources of monochromatic radiation in a totally new wavelength range. While the optical characteristics of lasers are well known, their transmission properties through the atmosphere are not yet known with sufficient accuracy, especially at the far infrared wave-lengths. The advantages of using a laser beam for probing the atmosphere stem from its drastically different optical properties, coherence, mono-chromacity, power and parallelism. The disad-vantages result mostly from the wavelength range in which they operate.

Most lasers now operate in the near and far infrared region where atmospheric gases like water vapor, carbon dioxide, ozone, and nitrous oxide have a large absorption. Consequently, infrared laser radiation is seriously attenuated by these con-stituents. There are, however, several good win-dows in the near infrared (0.7 to 15 /x) spectrum of water vapor, the most important constituent, through which the attenuation of the laser radia-tions would be no greater than that of microwaves. In the near infrared, good windows, where the transmission through 1.37 cm precipitable water over a 3.4 mile path is above 70 per cent, exist at wavelengths around 2.06, 2.1 to 2.3, 3.5 to 4, 4.65 and 9 to 12.5 /x (Holter, 1962). From 15 to 20.2 /x there are 11 bands with 50 per cent trans-mission through 1000 ft path and 2.2 mm pre-cipitable water (Yates, 1960). The absorption in the far infrared, beyond 20 fx, is not known well enough to permit further speculation. However, it is indicated that water vapor almost totally absorbs radiations in the region from 20 fx to 1000 /x.

Consequently, a judicious selection from the ultraviolet, through the visible, to the infrared, of the wavelength used to probe the atmosphere is essential to obtain sufficient useful range.

In addition to the attenuation of the transmitted beam by atmospheric gases, the laser beam will suffer considerable attenuation by atmospheric particulates, water droplets, dust particulates, etc.

As shown by Van de Hulst (1957), the attenua-tion decreases as the wavelength increases. At

the laser wavelengths it will seriously limit the cloud penetration of the beam.

Another problem, arising from the operational wavelength range of most lasers, is due to the high intensity of the solar radiations in this region. The sun's spectrum extending from 0.29 /x to about 13 /x completely overlaps the spectral range of most operational lasers. Unfortunately, laser wavelengths outside the known solar spectrum fall into the high absorption region of water vapor. Consequently, in order to make possible daytime atmospheric probing with laser beams, the wave-length has to be chosen where the sun's radiation intensity is at a minimum. This fact restricts further the number of wavelengths suitable for daytime probing of the atmosphere, unless dielec-tric filters tuned to the laser wavelength are used.

The properties of the atmosphere as now known appear to set an upper limit to the useful range of laser radiations at about 20 /x. The problems associated with the optical materials and the infra-red detectors in this region are not critical. Op-tical materials of high transmission are available for wavelengths up to 50 /x and suitable photo-conductive or thermal detectors are available for radiations up to 40 /x (Hackforth, 1960). At wavelengths suffering great attenuation their sen-sitivity will be marginal. In this case, however, the laser might be used on the receiving end, as a light amplifier (Geusic, 1962).

In conclusion, the choice of wavelength of the laser radiation will be restricted not only by atmos-pheric and particulate absorption and the solar background radiation, but also in the far infrared by the suitability of the optics, the light amplifiers and the detectors available.

The study of the phenomenon of radiation scat-tering in the atmosphere involves the analysis of intensity patterns produced either by discrete or aggregate particles such as molecular gas, dust, haze, fog, and cloud droplets. These particles will vary in size over a wide range, from 0.01 to 100's of microns, as in the case of rain drops.

When the particle size is small compared to the wavelength of the incident radiation, the amount of energy scattered is proportional to the ratio of the volume squared over the wavelength raised to the fourth power, or, in the case of spherical par-ticles, to r6/A4. This type of scattering is denoted as Rayleigh scattering. Also, in this case, scat-tering is independent of diffraction, and scattering alone is said to occur. However, when the ratio is of the order of unity, diffraction effects appear. For large ratios of particle size to wavelength, the geometrical optics region is approached, in which


5 6 8 B U L L E T I N A M E R I C A N METEOROLOGICAL SOCIETY

reflections and geometric shadowing become im-portant. In this region scattering is proportional to the radius of the particle raised to the second power. The rigorous general theory of scattering as developed by Gustave Mie (Stratton, 1951) yields the scattered intensity for all three of the above regions, and is the only theory applicable to the case where the ratio of the wavelength to particle size is nearly unity.

It is worth noting that the intensity of scattering is dependent not only on the wavelength and the scattering angle but also on the index of refraction, both the real and imaginary terms, which become an important factor in the near and far infrared wavelength regions.

Radiation scattering is a fundamental tool in the study of the atmosphere since it provides a method of obtaining information in regions of the atmos-phere which are otherwise inaccessible or where other methods of measurement disturb the en-vironment in which information is desired. Scat-tering at 180°, or back-scattering, is the basis for both the microwave radar and the lidar (laser radar). In a comparison of the two, it can be said that the radar is the more sensitive system for investigating scattering from a distribution of large particles. This is the Rayleigh region for the radar where the wavelength is at least a factor of 10 greater than the particle radius and where the intensity of the return signal is proportional to kr6/A4, where k is a constant for a given system. A small change in particle radius r will bring about a notable change in intensity. For the same particle distribution, however, the lidar will be operating in the far Mie or geometrical optics region where the particle size is at least a factor of 10 greater than the corresponding lidar wave-length. In this region, scattering is nearly inde-pendent of the wavelength and proportional to kr2

(Kruse, 196Z). Thus for a small change in r, the intensity of the lidar return will change only slightly as compared to the radar return. A change in particle size from 100 to 200 /x will increase the scattered intensity by a factor of 64 in the Rayleigh region but only by a factor of 4 in the geometrical optics region. It should be noted that, in this example, the intensity scattered in the geometrical optics region is much greater than in the Ray-leigh region. The lidar would, thus, receive the strongest backscattering return, provided that the two systems had the same initial intensity, and the attenuation was not an important factor. Radar scattering from small particles is quite in-efficient unless particle number concentrations are very large. In contrast, lidar backscattering can

be sufficiently intense with only a moderate concen-tration of small particles. Moreover, in the region of very small particles, where the lidar wavelength is at least a factor of 10 greater than the particle radius, the lidar will be operating in the Rayleigh region and changes in the droplet radius will result in intensity changes of the same order of magni-tude as those mentioned previously for radar. Consequently, this reflectivity characteristic of the lidar would provide measurements of the in-ternal structure of non-precipitating clouds. This would be a valuable contribution since such a measurement is not possible with radar techniques.

Often the data obtained in radar sounding at a single frequency can be seriously misinterpreted because of small errors in calibration, particularly when attenuation is important. This has been demonstrated in measuring precipitation rates at a single frequency (Hitschfeld, 1954). This prob-lem, however, can often be overcome by making relative measurements at two or more wavelengths (Rogers, 1963). One of the most recent devel-opments in the laser field is the excitation of the Raman spectra in organic liquids such as nitro-benzene, in which the conversion efficiency from the laser intensity into the various Raman lines is from 1 to 10 per cent or more, and increases with increasing laser intensity. One recent experi-ment, in which the emission from a "Q-switched" ruby laser was directed into an absorption cell, demonstrated the possibility of exciting the Raman spectra in many different liquids (Eckhardt, 1962). The Raman emission was approximately of the same beam collimation as the ruby laser light, while some of the lines were as narrow as 0.3 A some as broad as 5 A. In this case, the largest transfer of energy was approximately 10 per cent. The wavelengths available from nitrobenzene, for example, were 0.7658, 0.8539, and 0.9832 /x. In the future, no doubt, an increasing list of liquids and possibly solids will be made to demonstrate this phenomenon. Such a remarkable development could lead to a multiple wavelength lidar with which spacial information on clouds could be obtained.

The application of Doppler radar to the science of meteorology has already proven of value in the determination of such parameters as wind velocity, particle-fall speeds and growth, turbulent motion, etc.

If the Doppler shift is very large in comparison to the pulsed spectrum, the observed spectrum of a single pulse can yield a large amount of informa-tion. This case is adequately represented by the coherent pulse of a gas lidar since the Doppler


V O L . 4 4 , N o . 9 , SEPTEMBER 1 9 6 3 569

spectrum is much greater than the pulse spectrum. For example, if the short term frequency stability of a gas laser is of the order of 100 cps, then, since the return signal which shifted by Af — 2f Vr/c — 2Vr/\, where Vr = the relative velocity between the source and particles in motion, the Doppler spectrum will be 1 mc wide for a relative velocity of 0.5 m sec-1 and a l-fx wavelength. The corre-sponding radar shift for a 1-cm wavelength will only be 100 cycles. The smallest particle velocity detectable with a l-/x wavelength gas lidar, under the best of laboratory conditions, is 10~3 cm sec-1. The minimum velocity resolution for radar is not limited by frequency stability but by the minimum detectable particle size, which is dependent on the characteristics of the radar set. For a mini-mum detectable diameter of 300 jx the smallest detectable velocity is 1 m sec-1. Consequently, the Doppler lidar would permit the determination of droplet size distribution in clouds with much increased accuracy.

The use of the laser in radar-type instruments would, in addition, lead to a higher precision in the determination of the structures of clouds. Indeed, the radar return signal is the average of the light scattered by droplets in the solid angle of the probing beam. Because of the much smaller divergence of the laser beam, the lidar would pick up intensity differences corresponding to differ-ences in size or number densities of cloud droplets at much closer spacial separations than the radar could. In other words, the spacial resolution of the lidar would be much increased and may permit the accurate location of internal currents and tur-bulence in clouds and possibly the determination of a reliable relationship between return signal and rate of rainfall.

The coherence of the laser radiation makes in-terference measurements possible over much larger distances than previously feasible. The coherence length of a light beam increases with its mono-chromacity. Interference phenomena between two coherent gas laser beams can be observed for opti-cal path differences of the order of miles. Conse-quently, the measurement of the liquid water con-tent of clouds, of the temperature, or of the refrac-tive index of clear air might be possible from inter-ference measurements. Indeed, the optical path is the product of the geometrical path and the refrac-tive index of the medium through which the light beam travels. The refractive index is a function of the temperature, and the pressure of clear air, and of the concentration of water vapor. Conse-quently, the water vapor concentration could be measured over considerable distances by inter-

ference measurements at given altitudes, where temperatures and pressures are known.

In clouds, the measurement implies the use of a rather long wavelength which would locate the cloud droplet sizes far into the Rayleigh region of the scattering diagram. A wavelength of the order of hundredths of a micron is necessary to assimilate cloud droplets suspended in air to solute molecules suspended in a solvent. In the latter case, the re-fractive index of the solution varies with the con-centration of the solute at a given temperature. In the air-water droplet system, the refractive index of the system would also vary with the concentra-tion of liquid water in the system. Consequently, if a laser beam is split into two beams which are brought back together after travelling the same geometrical path—one through a cloud and the other through pure air, at the same temperature, pressure, and humidity—the observed interference fringe displacement would be a direct measure-ment of the liquid water content in the geometri-cal path. This development requires the availabil-ity of laser radiation, optical materials and de-tectors suitable above the 100 /x wavelength range.

Several other concepts may be suggested to make use of the unusual properties of laser radia-tions. Some properties may be disadvantageous in a given application but may serve as the basis for some other applications of equal usefulness. The strong attenuation of infrared radiations by water droplets, detrimental for the probing of thick clouds, may serve to measure the height of cloud bases. Cloud base height could be accu-rately measured by reflecting a pulsed laser beam on it and measuring the time of arrival of the re-turn signal in the same way as present day track-ing radars operate.

The radiant flux or output intensity of pulsed lasers is enormous. In a laser beam focused in an image diameter of a few wavelengths, the temperature of a carbon target can be raised to around 8000C in a few milliseconds. What would be the effect of such energies on cloud droplets? In an exploratory experiment, no visual effects could be detected when a collimated beam of 0.6943 jx wavelength radiation and about 1 jx sec duration was incident on a water droplet of a few millimeters diameter. However, it would be in-teresting to evaluate the possibility of dissipating fogs with a continuous wave laser beam of high intensity in a wavelength region totally absorbed by liquid water.

Other applications may have been proposed and surely will be as the characteristics of the laser beam become better known. As with every new



technological breakthrough, fantastic claims have been made. The theoretical possibilities are pres-ently being explored for the laser in several areas of modern technology. We have reviewed con-cepts. The reduction to practical applications is the major and more difficult step. The laser will only live up to its theoretical promises if the problems of their reduction to practice are tackled diligently in the field of meteorology.

REFERENCES Berkley, D. A., and G. J. Wolga, 1962: Coherence studies

of emission from a pulsed ruby laser. Phy. Rev. Letters, 9, 479.

Eckhardt, G., et al., 1962: Stimulated Raman scattering from organic liquids, Phy. Rev. Letters, 19, 455.

Faust, W. L., R. A. McFarlane and C. K. N. Patel, 1963: New infrared maser lines of the noble gases. Bull. Amer. phys. Soc., 8, 299.

Geusic, J. E. and H. E. D. Scovil, 1962: A unidirectional traveling wave optical maser. B.S.T.J., XLI, 1371.

Hackforth, R. L., 1960: Infrared radiation. New York, McGraw-Hill Book Co., Inc. 303 pp.

Hayler, D. A., 1963: Douglas Aircraft. Private commu-nication.

Hitschfelt, W. and J. Bordan, 1954: Errors inherent in the radar measurements of rainfall at attenuating wavelengths. J. Meteor., 11, 58.

Holter, M. R. et al., 1962: Fundamentals of infrared tech-nology. New York, McMillan Co., 442 pp.

Jaseja, T. S., A. Javan and C. H. Townes, 1963: Fre-quency stability of He-Ne masers and measurements of length. Phy. Rev. Letters, 10, 165.

Kruse, P. W., et al., 1962: Elements of infrared technol-ogy. New York, Wiley, 448 pp.

Lengyel, B. A., 1962: Lasers, generation of light by stimu-lated emission, New York, Wiley, 125 pp.

Levine, A. K., 1963: Lasers. Amer. Scientist, 51, 14. Lhermitte, R. M., 1963: Weather echos in Doppler and

conventional radars. Proc. 10th Weather Radar Conf., 323.

McClung/ F. et al., 1963: Characteristics of giant optical pulsations from ruby. Proc. IEEE, 51, 46.

McMahan, W. H., 1963: Observations of increased power output from He-Ne optical maser by means of externally applied high voltage. Proc. IEEE, 51,360.

Rogers, C. W. C. and R. Wexler, 1963: Rainfall deter-mination from 0.86 and 1.82 cm. radar measurements. Proc. 10th Weather Radar Conf., 260.

Schawlow, A. L., 1961: Optical masers. Sci. Amer., 204, 52.

Sirons, J., 1962: Lasers for aerospace weaponry. A.S.D.-TDR-62, 440.

Stratton, J. A., 1941: Electromagnetic theory. New York, McGraw-Hill, 615 pp.

Tang, C. L., 1963: Relative probabilities for the xenon laser transitions. Proc. IEEE, 51, 219.

Van de Hulst, H. C., 1957: Light scattering by small particles. New York, John Wiley & Sons, Inc. 470 pp.

Yates, H. W. and J. H. Taylor, 1960: Infrared transmis-sion of the atmosphere. N.R.L. Report 5453, Wash-ington, D. C.

(Continued from N E W S AND NOTES, page 548)

Plan for International Hydrologic Decade

Specialists from 48 countries attended a meeting on scientific hydrology held at UNESCO headquarters, Paris, on 20-29 May 1963. The purpose of the meeting was to draw up a plan for an extensive, international study of the water resources of the world and of the problems posed by the use of these resources.

The conferees began by documenting the extreme seri-ousness of these problems. In a draft report of the meet-ing they stated: "With increased water use, shortages have become more frequent and constitute a danger to human welfare through the disruption of production and through health hazards. The growing intensity of land use by human populations more and more interferes with the natural balance of nature and has led to soil erosion, increased liability of flooding, and increased devastation by floods in productive and densely populated river valleys."

The meeting agreed, therefore, that it is necessary to organize a Hydrologic Decade, which should be launched by the General Conference of UNESCO in 1965. The program for such a decade should be centered upon ter-restrial waters and should include seven basic operations:

1. Careful appraisal of the state of our knowledge of the hydrology of the whole world; •

2. Standardization of observations, techniques and ter-minologies for collection, compilation and recording of data;

3. Establishment of basic networks and the improve-ment of existing networks to provide fundamental data on hydrological systems;

4. Research on certain hydrological systems in selected geological, climatic, etc., regions constituting what may be called "representative basins;"

5. Research on specific hydrological problems whose urgency and special nature call for international effort;

6. Education and training in hydrology and related domains ;

7. Systematic exchanges of information. For the purpose of basic data and observation net-

works, the experts recommended the creation of Hydro-logic Decade Stations. For these stations, States should undertake to comply with certain quality standards so as to provide thoroughly reliable data for hydrologists. In addition, simple forms of small experimental basins would be of great value in providing basic data on surface runoff and seepage.

As a means to carry out the program, the conferees stressed that national committees for the decade should be set up as soon as possible. They also approved in principle the creation by the next session of the UNESCO General Conference of an International Decade Board, composed of 15 to 18 members and representatives of the UN, UNESCO, WMO, W H O , FAO, IAEA, and ICSU.

The meeting elected as its president, E. S. Hills, profes-sor of geology, University of Melbourne, Australia.

(More N E W S AND NOTES on page 575)


the laser and its application to meteorology

Documents