US3606153A - Method for increasing visibility through fogs - Google Patents

Abstract

This disclosure related to a method for increasing visibility through warm natural or artificial fogs. It is based upon the absorption and degradation of microwave energy into heat to accelerate the evaporation of the smaller fog droplets which are the greatest hindrance to the transmission of light through fog. The method can be used within the 100 to 230,000 megahertz range, however, the most practical frequencies are those between 500 and 23,000 megahertz. Visibility can be doubled or tripled in a matter of seconds when microwave energy is emitted on both sides of a landing strip by means of a series of individual generators spaced 7 feet apart and operated at the kilowatts power level. The method comprises continuously sweeping microwave beams vertically from both sides of the runway at a maximum sweep speed of about 1 degree per second. In difficult cases, with denser fogs, the evaporation of droplets is increased by combining the microwave emission with ultrasonic waves from a number of airborne ultrasonic generators operating at intensities above 150 db and within the 16 to 100 kHz. range. The method can economically handle any kind of natural warm fogs and may thus allow continuous year round operation at certain airports.

Description

sept. 20,1971

Ks. THE SCATTERING AREA COEFFlCIENT R.M.G.BOUUCHER f 3,606,153

METHOD Fon mcREAsING vIsI'iLITY THROUGH FoGs Filed July 4ze, 1969 4 Sheets-Sheet 1 .l

vIO

IO l5 I IN MICRONS 'FJH-E RAYMOND MARCEL GUT BOUCHER ATTORNEYS sept. 2o, 1911 R. M. s. BQUCHER 3,606,163

METHOD FOR INCREASING VISIBILITY THROUGH FOGS Filed July 2 8, 1969 l 4 sheets-sheet z /T" nl INVENTOR. RAYMOND MARCEL GU ATTORNEYS R. M. G. Boum-1ERv sep1.2o, 1971 3,606,153

v METHOD FOR INCREASING VISIBILITY THROUGH FOGS 4 Sheets-Sheet 3 Filed Julyv 28, 1969 I--u--I-EC- -lNvEnrroR MOND MAmEL GUT BOUCHER m d M ATTORNEYS Sept. 20-, 1971 R. M. s. BOUCHER METHoD Fon INcREAsING VISIBILITY'THROUQH Fous K v Filed July 28,' 1969 4 Sheets-Sheet L INVENTOR. RAYMOND MAKLEL GUT BOUCHER B Y Mw W ATTORNEYS United States Patent O 3,606,153 METHOD FOR INCREASING VISIBILITY THROUGH FOGS Raymond Marcel Gut Boucher, Metuchen, NJ., assigner to Wave Energy Systems, Inc., New York, N.Y. Filed July 28, 1969, Ser. No. 845,330 Int. Cl. E01h 13/00 U.S. Cl. 239-2 12 Claims ABSTRACT F THE DISCLOSURE This disclosure relates to a method for increasing visibility through warm natural or artificial fogs. It is based upon the absorption and degradation of microwave energy into heat to accelerate the evaporation of the smaller fog droplets which are the greatest hindrance to the transmission of light through fog. The method can be used within the 100 to 230,000 megahertz range, however, the most practical frequencies are those between 500 and 23,000 megahertz. Visibility can be doubled or tripled in a matter of seconds when microwave energy is emitted on both sides of a landing strip by means of a series of individual generators spaced 7 feet apart and operated at the kilowatts power level. The method cornprises continuously sweeping microwave beams vertically from both sides of the runway at a maximum sweep speed of about 1 per second. In difficult cases, with denser fogs, the evaporation of droplets is increased by combining the microwave emission with ultrasonic waves from a number of airborne ultrasonic generators operating at intensities above 150 db and within the 16 to 100 kHz. range. The method can economically handle any kind of natural warm fogs and may thus allow continuous year round operation at certain airports.

The present invention relates to a process for dissipating natural or artificial fogs known as warm fogs i.e. with an average temperature higher that 0 C. As distinguished from supercooled fog, this type of natural fog is very difficult to dispel and therefore, today it is still the main obstruction to continuous airport use, especially during winter time. A recent survey at I ohn F. Kennedy Airport in New Yorkshowed that there was an average of 12 hours of supercooled fog per winter (November through March) and about 330 hours of Warm fog during the same period.

Several attempts have been made in the past to solve the problem o-f warm fog dispersal. We shall hereafter briefly discuss some of these techniques while stressing their limitations. It will also aid in understanding the advantages of the process of the invention.

The first large scale attempt to evaporate fog droplets was made during World War II with the FIDO system (Fog Investigation and Dispersal Operation) which consisted of a series of fuel burner nozzles spaced along the runways. This technique, while generally successful under war time conditions, had to be abandoned for commercial airports because of the risk involved in mixing aircraft and fire. Moreover, it required a large capital investment and a very high operating cost. To clear a typical runway (length 7500 feet, width 300 feet, height 2.00 feet) the investment cost of a PIDO system today would be approximately $2,700,000, and the operating cost would reach $200 per minute. It must also be remembered that the burning of conventional liquid fuels cornposed of hydrocarbons produces considerable amounts of water thereby further increasing the amount of energy and fuel required. In some instances, the excess water produced during the combustion condensed in the colder ice upper fog layer thus contributing to a denser fog stratification a few hundred feet above ground level.

The second method used for the evaporation of fog droplets was radiant heat (see Nat. Bur. St. Fog Dispersal by Radiant Tubes, by E. F. Fiock and A. I. Dahl). This system involved the use of Inconel tubes, 30 feet long and 9 inches in diameter, heated at a surface temperature of 2000 F. by means of gas burners. Despite the fact that 50% of the energy radiated by a black body (within the range between 500 F. and 2000 F.) would be absorbed within 300 feet of the radiator by an atmosphere saturated with water vapor at 52 F., it was found that the power requirements for two 5000 foot banks parallel to a runway would be at least 200,000 kw. Due to the bulkiness of this system and to the high initial and operating costs, this method was never used for the practical clearance of fog at airports.

The third and oldest method was suggested by H. B. Houghton and W. H. Radford (Papers in Physical Oceanography and Meteorology. MIT and Woods Hole Ocean. Inst, VI, No. 3, 1938) who investigated the dissemination of hygroscopic nuclei such as calcium chloride, aluminium sulfate, and sodium chloride, in powder form or concentrated solution. This technique worked from the experimental viewpoint but could not be used at commercial airports for the following reasons: the installation necessary for practical dissemination of the nuclei would have to be rather large and it would constitute a serious obstruction to air traffic; with light winds the system would not work; and the fallout of corrosive chemicals in the neighborhood of planes would also create a serious problem.

More reecntly (Science, 1319-20, vol. 157, 196.7) it was reported that visibility through artificially produced, warm fogs could be increased by factors of 3 to 10 'by seeding them with carefully sized, sodium chloride of the particles between 2 and 10 microns). This method would indeed be very difficult to extrapolate to the situation at airports since it necessitates the dispersion of a carefully sized suspension (whose characteristics may vary for cleach type of fog) throughout a large volume with no win A fourth method (G. A. Elton, Chemistry and Industry, 219, Mar. 7, 1953) which was suggested was the use of surfactants sprayed into the fog. The idea was to increase the collision rate and coalescence of droplets through selective surface charging of the droplets. It was also thought (D.P. Benton et al., Int. I. Air Poll, Pergamon Press, l, 44, 1958) that an accumulation of electrical charges at the droplets interface would affect the diffusional growth rate of the water droplets. Unfortunately, careful laboratory experiments conducted in a chemical gradient, cloud-diffusion chamber by I. E. Iiusto (Nat. Aer., and Space Adm. Report CR-72, Contract No. NA Sr-l56, July 1964) showed that no significant effect on fog drops could be observed with anionic or cationic rfactants.

In a fifth category, are found several inventions (U.S. Pat. 2,835,530', and Chem. Eng. News, 31, 5377, 1953) which are said to be based upon the dispersion of surface active agents with a view toward lowering surface tension at the droplets interface and thus favor coagulation by impaction. These techniques unfortunately never gave consistent positive results which could be related specifically to the use of the surface active agents. This is understandable when one realizes that the normal collision rate of fog drops is so small that, even if every condition resulted if every collision resulted in a nonelastic shock, no effect on fog dispersal could be expected. Even if it were partially successful, techniques involving the dispersion of small droplets (the maximum tracheobronchial retention in lungs is between 1 and 15 microns) of surface active chemicals would be highly objectionable from the health and safety viewpoints due to possible further dissemination of the treatment materials in the densely populated, airport area.

Techniques other than those involving the use of heat or dispersed chemicals have also been suggested. None of them has been fully satisfactory in practice. Among these methods is the electrical sweep-out of fog droplets by a dispersion of particles carrying a high electric charge. This approach has been very promising from the theoretical viewpoint and on a laboratory scale. M. Pauthenier (C. R. Acad. Sci., Paris, 226 (7), 587-89, 1948) calculated, for instance, the increase in the cross section of the vertical column swept out by a particle carrying the maX- imum possible charge to be by a factor of 100. R. Cochet (C. R. Acad. Scien., Paris, 233, 190, 1951) showed later that highly charged drops will polarize neutral neighboring droplets and attract them. Because there is considerable difficulty in producing the necessary quantity of charged particles of the right size and distributing them uniformly at a certain altitude, this method is regarded as impractical for airport operational use.

Another electrical process (Cottrell method) which consists of artificially charging certain water droplets by corona discharge for a subsequent electrical precipitation in a strong field has been proven impractical and dangerous by L. Demon (Gen. Chim. Paris, vol. 74, No. 4, 97-105, 1955) due to the potential gradient of the space charge.

Methods involving the use of airborne sonic or ultrasonic waves to promote droplets impingement through a superimposed oscillatory motion have been reviewed by R. M. G. Boucher (Ultrasonic News, vol. IV, No. 1, l1 14/ 19, 1960). The author showed that to achieve satisfactory results a very high intensity field was required (at least 150 db) with a low frequency emission (5 to 1.5 kHz.) and a minimum fog droplets concentration of 500 mg./m.3. Since most radiation and advection fogs have a droplets concentration of the order of between 100 and 200 mg./m.3, the technique requires high intensities which are not economically feasible in the present state of the art. Recent theoretical work by S. V. Pshenay Severin on Oseen forces in the Soviet Union has shown that successful natural fog dispersal by acoustic waves can only take place in the very low frequency range (below 5 'kHZ.). This confirms both the first field tests conducted by V. K. La Mer and D. Sinclair in the United States (Report of Tests of Sonic Dissipation of Fog in California, NDRC, 10, 2-13, 1944) and other calculations made by N. P. Tverskoy (Trans Main Geo. Obsv., No. 145, 36-48, Leningrad 1963) in the Soviet Union.

The tremendous noise problem created by a low frequency, high intensity, sound wave7 field along an airport runway would indeed make this technique impractical even if one disregarded the economical disadvantages of this method. With a View to decreasing the required power output of an acoustic system, it was suggested in 1956 (French Patent No. 1,163,399) that hygroscopic nuclei be combined with acoustic agglomeration. The idea behind this approach was to take advantage of a faster droplets agglomeration by applying sound waves as soon as drops grown around the hygroscopic nuclei reached a 10 to 20 microns size. It was thought that another advantage would lie in the fact that it would not be necessary to project the hygroscopic nuclei at a height of several hundred feet to insure a proper droplets growth.

This method however did not seem to provide the ideal solution since it only minimized the disadvantages of the two techniques (noise problem, corrosive fall out, etc.). Another diiculty common to all spraying processes was also encountered, in that, each type of fog seemed to necessitate a particular type of hygroscopic particle dispersion (particles spectrum and concentration) to give consistent results.

The numerous drawbacks and impracticality of the above described processes led to the development of a new method which is an important object of the present invention.

The present invention first relates to a method of using, in a particular manner, high power beams of microwave energy in the to 230,000 MHZ. range with a view to quickly and selectively accelerate the evaporation of the finer droplets (in general, smaller than fifteen microns) contained in natural warm fogs.V

Other objects, advantages, features and uses will be apparent during the course of the following discussion.

In the accompanying drawings, forming a part of this application,.and in which like numerals are employed to designate like parts throughout the same:

FIG. 1 is a plot of the scattering area coefficient against u where a is directly proportional to the radius of the water droplets and inversely proportional to the wavelength of the excitation signal;

FIG. 2 illustrates plots of the frequency distribution of the water droplets by number and weight against the droplet radius;

FIG. 3 illustrates plots of the cumulative distribution of the water droplets by number and weight against the droplet radius;

FIG. 4a is a side elevational of a typical aircraft runway which is to be cleared of fog;

FIG. 4b Vis an enlarged sectional view along lines 4b- 4b of FIG. 4a, viewed in the direction of the arrows, showing one method for continued sweep of the fog arca;

FIG. 4c is a view similar to that of FIG. 4b showing another method for continued sweep of the fog area;

FIG. 4d is an enlarged, plan view showing a typical arrangement of the energy radiators along the aircraft runway of FIG. 4a;

FIGS. 5a and 5b are embodiments of apparatus used in carrying out the method of the invention by producing a parallel beam of energy for continued sweep of the fog area; and

FIG. 6 is an elevational view of a reflector which may be used to carry out the sweep method of FIG. 4c.

For a better and clearer understanding of the teachings of the invention, one must understand the influence of microwave energy on` water droplets. The invention is based on the fact that microwave energy beamed at a suspension of small water droplets will be degraded into heat inside the liquid droplets and will thus accelerate the evaporation of the droplets. This heat is produced by the microwaves which are short radio waves. They belong to the same family of electromagnetic waves that contain the energy waves already widely used in radar, television and communications. They -vary greatly in wave length and certain wave lengths have been found to be efficient and practical heat generators as shown by the numerous industrial applications in the area of food processing and drying.

Microwave heating takes place in general when a dielectric material is Vexposed to a rapidly alternating field. Under the influence of the alternating field, dipoles in the molecules of the irradiated material reverse their orientations, and the intermolecular friction produced manifests itself in the form of heat. Microwave heating, therefore, is instantaneous, bulk heating as opposed to conventional heating through radiation or convection. As a general rule, one can say that longer wavelengths penetrate more deeply than shorter wave lenghs and material having a higher dielectric constant will absorb a greater proportion of the incident energy near the surface while materials having a low dielectric constant will allow the energy to penetrate more deeply. To determine the amount o-f microwave power absorbed in heating liquids, the analogy of a parallel plate capacitor is often used. It is assumed that the material to be heated is placed between the parallel plates to which an alternating high frequency field is applied. On the basis of the capacitor analogy, the power absorbed throughout the material in watts per cubic centimeter is governed by a formula of the following type:

p=kfE2 e"/e watts per cm.3 in which p=watts developed per cm.3 of material,

f=frequency in cycles per second,

E=the magnitude of the field strength expressed in volts/ e/e=dielectric loss factor which is a product of the di electric loss tan and the dielectric constant e/e,

The above equation indicates the power developed in the material is directly proportional to the frequency, the square of the electric field, and the dielectric loss factor. In the case of water droplets we have the best theoretical conditions since water is one of the liquids with the highest dielectric loss factor. Let us recall for instance that at 3000 MHZ. and C. the dielectric constant of water is 78.8, the dielectric loss tangent is 2050 (X104) and the dielectric loss factor is 161,000 (X 104).

To better understand the influence of small droplets evaporation upon the overall light transmission through a fog blanket, we have to consider a few fundamental facts which deal with light scattering by water droplets. This problem has been treated from the theoretical viewpoint by G. Mie (Ann Phys, 4th series, 25, 377-445 1908) and several other physicists. Confirmation of the theory from the experimental viewpoint has been made by several authors such as J. M. Waldram (Trans Illum Eng Soc, London, vol. 10, 14787, 1945) and Uno and Yosida (Low. Tenn. Sc. vol. 2, Hokkaido Univ. 130, 1949). In short, a given sphere or water droplet behaves as though it had different cross-sectional areas for different wavelengths of light. The factor by which the geometrical cross-sectional area of a sphere must be multiplied to give the effective cross-sectional area (i.e., effective in intercepting light) is called the scattering area coefficient Ks. H. G. Houghton and W. R. Chalker have shown (FIG. l) that the scattering area coefiicient Ks is a periodical decreasing function of the radius of the water sphere. In other words the maximum scattering is due to the smaller particles. Therefore, a selective evaporation of the finer droplets is most necessary to the improvement of visibility through a water droplet fog. This can be understood once it is realized (Air Pollution Handbook ed. by P. L. Magill and col, 6-24, McGraw-Hill editor, 1956) that in a 2.8 mg/m.3 fog made of 0.72 micron monodispersed droplets, the visual range will be around 0.1 mile while it will be 1.7 miles in a fog of identical concentration but made of 7.2 micron droplets.

Therefore an important object of the present invention consists of accelerating the evaporation of the smaller droplets contained in a natural warm fog since they constitute the main obstacle to light transmission through intense scattering. FIG. 2 shows the freqency distribution in number (solid line) and weight (dashed line) of a typical radiation fog with respect to droplets size. From the cumulative droplets size distribution curves of FIG. 3 (in number-solid line and in weight-dashed line), it can be seen that although the droplets smaller than 10 microns constitute 75% of the total number of drops they represent only 35% of the liquid water content in the fog. By evaporoting only 35% of the water liquid content of the fog, calculations show that one can more than double the visibility through the fog. To selectively evaporate the smaller droplets contained in a warm fog will mean an important decrease in the total amount of wave energy required for practical fog clearance. Other past thermal methods, as previously mentioned, were aimed at total evaporation of the dispersed liquid phase contained in the fog. To achieve a reasonable coupling of the microwave energy to the liquid contained in the small droplets (thus inducing strong dipole agitation) Rayleigh theory extended to the short wave length region tells us that we have to use the highest possible frequencies. From experience with centimeter radar waves, and according to the theory, it is known that the amount of electromagnetic energy absorbed and diffused by fog droplets (d=1 to 50 microns) is far smaller than for rain drops (d 500 microns). Therefore, a compromise can be achieved, in practice, by emitting energy at the most economical higher frequencies while using a minimum amount of radiated energy to sufficiently increase the vapor pressure gradient at the interface of those droplets which most affect visibility through warm fogs. Through experimentation, we have found that a suitable frequency range to achieve such results lies between and 230,000 MHZ., more specifically, practical results to double or triple visibility through Warm fogs at airports can be obtained at the following nominal frequencies 915 MHLZS; 2450 MHz. *-50; 5800 MHZ75 and 22.500 MHz.i125. It has also been observed that short pulsed emissions of the order of microseconds, as used in radar technology with large instantaneous peak power values, are unable to provide an efficient coupling of microwave energy to obtain a strong skin effect at the droplets interface. Irradiation with longer pulse duration at least greater than one microsecond or continuous wave emissions produced with standard Klystron, Magnetron or Amplitron tube generators are the best techniques to apply the process object of the present invention efficiently.

In some cases, especially with denser advection fogs, it is also advisable to combine the action of the microwave beams with high intensity airborne ultrasonic fields. The combined action of these two force fields of a different nature will considerably speed up the evaporation rate of the liquid droplets thereby decreasing, in some cases, the amount of energy per unit of time needed to clear a kown volume of fog.

As is well known, the evaporation of drops is made up of two elementary processes: (a) removal of molecules from the liquid surface to form a layer of saturated vapor, and (b) molecular diffusion from the saturated layer into the surrounding ambient atmosphere. As previously eX- plained, microwave energy will mainly affect the first process (formation of a layer of saturated vapor) through the continuous agitation of dipolar molecules which produce friction heat, while the airborne ultrasonic emission will favor the second mechanism (diffusion from the saturated layer). A high intensity, airborne, ultrasonic field will put in motion the gas layers surrounding the fog droplets. By so doing, the diffusion boundary layer will be partially blown off so that the evaporation rate of drops will be higher than in a stationary medium. The theory of orthokinetic coagulation (O. Brandt, H. Freund, E. Hiedmann, Kolloid Zeit, 77, 103, 1936) tells us that to achive the most favorable particle to gas amplitude ratio (ie. 0.5 when dealing with the finer fog droplets, we have to irradiate at frequencies well in the ultrasonic range with an intensity of at least decibels (the particle velocity in a free traveling plane wave will then be greater than 100 cm./sec.). E. P. Mednikov (Acoustic Coagulation and Precipitation of Aerosols, page 158, translatedv and edited by Consult. Bur. N.Y. 1965) recalls, for instance, that the critical velocity above which fast fog droplets evaporation is observed corresponds to 100 cm./sec. for drops which are 15 microns in diameter and 500 cm./ sec. for drops which are 3 microns in diameter. Theoretical relationships have also been worked out by R. M. Fand (l. Aust Soc. Am., 34, 1887, 1962) and P. J. Westerwelt to calculate the critical sound pressure level (SpL) above which the heat transfer rate becomes a function of sound pressure. For a ratio of particle displacement amplitude to acoustic boundary layer thickness greater than one, the formula is:

where F is in kHz.

Beside acoustic wave interaction with boundary layers at the interface of the warm fog droplets other secondary phenomena peculiar to high intensity fields (such as Andrade vortices) may also play a role in accelerating droplet evaporation. All this has been experimentally proven with liquid droplets in a 35.5 kHz. acoustic field within the low Reynolds number region by W. Mirsky and J. A. Bolt (The Effect of Ultrasonic Energy upon Evaporation of Fuel Drops, ASME meeting, N.Y. Dec. 1-5, 1958). These authors showed that increased evaporation rates of more than 50% could be obtained by creating the proper acoustic field around suspended liquid droplets. To create high power, airborne, acoustic ields with intensity levels above 150 decibels (0.1 watt/cm2) at frequencies higher than 16 kHz. is well `within todays state of the art. To carry out the objects of the present invention, the use of Electro-Acoustic Horns of the type described by R. M. G. Boucher (Textile Research Journal, vol. 37, No. 8, 635- 43, August 1967) or the Ling-Temco-Vought Electrostatic transducers of the LTV-700 series is recommended. However, it should be understood that the combined use of airborne ultrasonics with microwave irradiation is justified only when the overall energy consumption of the two systems shows a substantial economical gain over the microwave process alone.

Having described the influence of microwave and ultrasonic energy on warm fogs, we shall now describe, by way of a nonlimiting example, one embodiment of the apparatus of the present invention, as shown in the accompanying drawings.

FIG. 4a shows a horizontal cross section of a landing strip. The cross hatched area is the area in which the visibility has been increased through continuous irradiation by microwave generators placed alongside the runway. The dimensions of the tunnel in which the visibility has been improved are: overall length (L) 7500 feet, maximum height (H) 300 feet, minimum height (h) 200 feet, maximum width (D) 130 feet, minimum width (w) 100 feet. The plane 6V lands along path 5 as shown by the dashed line. The microwave generators 1 (1 kw. output) are spaced alongside the runway at a distance of about fifteen lfeet from the edge of the runway. The distance x between adjacent microwave radiators (sources of microwave energy)` on the same side of the runway is of the order of 7 or 8 feet. If auxiliary, airborne, ultrasonic radiators 2 are used, they can either be placed between the microwave radiators or alternate with the microwave units at the 7 or 8 feet distance. FIG. 4a shows jan arrangement which uses radiators (microwave or ultrasonic) connected at distance through coaxial cable 3 and 4 with their respective signal generators (details not shown). The driving units, however, due to their small dimensions can be placed close to the radiators and be controlled from a distant location through well-known, electrical switching systems in any manner used in the art. One can see in FIG. 4a that a large volume of fog will be cleared in the approach area where the plane begins to flare out (1:300 to 500 feet). This will be achieved either by an increase in the power output of the signal generators (2 to 2.5 kilowatts) or by an increased number of standard units placed at shorter distances from each other.

FIG. 4b and 4c are cross-sectional views of the area of the cleared tunnel. They show two different techniques for continuously sweeping fog from the landing strip. The irst method shown in FIG. 4b indicates that the microwave (or ultrasonic) radiators are slowly rotated around the axis parallel to the ground which allows continuous sweeping through an angle a from to 70'o in an upward motion followed later by a downward course and so on.

The motion is electrically controlled in such a way that when the beam 7 of radiator 1 (left) is at a 0 angle the beam 7 of radiator 1 on the opposite (right) side is at the maximum 70 angle. This results in the best fog clearance inside a zone of trapezoidal shape (Vl/:100 feet and D=l feet).

FIG. 4c shows another construction which makes use of a fixed position, radiator 1 radiating at its lower positionwhile a reflecting plate 9 placed opposite radiator 1 slowly rotates to produce a reflected beam 8 which sweeps the fogged area over the same angle or.

FIG. 4d is a top plan View of the runway 10 which shows a typical radiator arrangement on both sides of the landing strip. VThe runway proper 10i has a width of about 100 feet. At a distance of l5 feet on each side of the runway two lines of radiators 1 and 1' are installed. They are connected to their signalV generators through coaxial cables 3 and 3', respectively. The distance between adjacent radiators on either side is 7 or 8 feet. The high intensity radiation area is in the areas indicated by the numbers 10` and 11. Servicing during operation can be done through the areas numbered 12.

FIGS. 5w and 5b show an example of the type of radiators which Vcan be used to carry out the teachings of the present invention. One can see that the apparatus illustrated in FIG. 5a comprises a U-shaped frame support 21 which permits radiator 25 to rotate around an axis zz. This radiator 25 is placed, for instance, at

the focus of a parabolic reflector 23 (or at any convenient location in front of a reflector of suitable shape). It enables the production of a powerful parallel beam of energy 27. The radiator 25 is fastened by means of a spider type support whose three legs 31 are hollow in order to bring the connecting cable 28 from the signal generator unit 24, which is powered from the main line 30, to the radiator. The shafts 26 around which the unit rotates through an angle of 70 in an upwards and downwards motion is driven by a motor 22 connected to the main power line by cable 29.

In FIG. 5b ultrasonic radiators (Electro Acoustic Horns or LTV Electrostatic transducers) replace the microwave radiator arrangement `shown in FIG. 5a. The electro-acousic horn 41 replaces the microwave radiator 25 of FIG. 5a. This electro-acoustic horn 41 is yfastened in the same manner as described for the microwave and will produce a parallel beam of ultrasonic waves 43. The horn is connected by a coaxial cable 44 to an ultrasonic signal generator (Microsonics type 150 LF, for instance, details not shown) fed from the main line 46 (115 volts, 60 cycles) ina manner well-known in the art.

FIG. 6 is an elevational view of a device which can be used as the reliector previously shown in FIG. 4c. This device consists of a U-shaped frame-support 51, a microwave reflecting plate 53 which can rotate around the axis y-y from a vertical position to a position -corresponding to a beam angle a of 70. The shaft V54 is rotated by a motor 52 electrically fed from the main line With the system above described, we successfully processed a warm radiation fog with the following characteristics:

Average drop size: l0 microns Drop size range: l-35 microns Liquid water content: mg./m.3 Droplet concentration. 20G/cm3 Vertical depth of fog: 30() feet Horizontal visibility: 300 feet.

In a matter of seconds we were able to increase the visibility over 800 feet through the entire runway length (7500` feet) by using one thousand radiators (each one rated at 1 kw. continuous output) on each side of the runway, operated at a frequency of 2450 MHz., continuous wave. The running cost of the entire system (controls plus radiators) was only a few cents per minute.

It was discovered that the most favorable operating conditions corresponded to a sweep rate of 1 for a period of time of the order of between 1 and 2 seconds. Each one kilowatt microwave radiator was excited by an individual signal generator. Standard electronic control systems with circuit breaking devices enabled us to switch on r shut off at any time any desired number of signal generators from a remote control point. It is also possible to obtain equally effective results using pulsed emissions similar to radar.

It was calculated that to achieve a significant increase in visibility the high energy microwave beam needed between 1 and 2 seconds to evaporate the 7 gr. (20X 0.35) of small droplets liquid water contained in a beam of 300 feet length having a cross sectional area of one square meter.

In short, one can say that the process object of the present invention has overcome the two main objections made in the past to the use of microwaves for fog dispersal. The first one was the cost of power systems using specially designed high power Klystron or Amplitron tubes and the second the health hazards created by permanent high power, high level elds on the runway. The discovery of satisfactory energy coupling at medium intensity levels and high frequencies for the evaporation of the finer droplets opens the door to the use of standard low cost, low power systems (1 to 2.5 kw. output). The concept of highly directive microwave beams sweeping the runway in a downwards and upwards manner eliminates the health hazard which could be created by high power, stationary, permanent fields. As a matter of fact, the level of radiation will be quite safe (below milliwatts per square centimeter) one foot away on the back side of a parabolic reflector. Passengers in the planes will not even be subjected to this radiation level since they are shielded by the plane walls and will always disembark in an area where the system is not operating.

It must be well understood that the present invention can be applied to variable volumes and types of warm fogs at different temperatures or wind velocities and that, still without departing from the scope of the invention, the structural details of the described devices, the dimensions and the shapes of their members (such as the type of microwave or ultrasonic radiators) and their arrangement (alternation of microwave, ultrasonic generators and reflectors, for instance) may be modified, and that certain members may be replaced by other equivalent means (parabolic reflectors replaced by ellipsoidal or others).

While the invention has been described by means of specific examples and in a specific embodiment, obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

The embodiments of the invention in which an eX- clusive property or privilege is claimed are defined as follows:

-1. The method of increasing visibility through warm natural or artificial fogs which comprises selectively 10 evaporating the finer fog droplets by subjecting them to microwave beams from at least one source of microwave energy having a frequency between and 230,000 MHZ.

2. The method of claim 1 including applying airborne ultrasonic waves to the fog, said beams having an intensity above decibels and a frequency between 16 and 100 kHz.

3. The method of claim 2 wherein the ultrasonic waves are produced by a plurality of ultrasonic wave sources paced along the periphery of the area to be cleared o fog.

4. The method of claim 1 wherein the microwave beams are produced by a plurality of microwave energy sources placed along the periphery of the area to be cleared of fog.

5. The method of claim 1 wherein the microwave beams are pulsed at a pulse duration greater than l microsecond.

6. The method of claim 1 wherein the source of microwave energy is directed to sweep through vertical angle of about 70.

7. The method of claim 6 wherein the sweep rate of the beam of microwave energy is no more than 1 per second and the power radiated by the microwave energy source is at least about l kilowatt.

8. The method of claim 1 wherein the microwave beams are produced by a plurality of microwave energy sources placed along one edge of the area to be cleared of fog and including a plurality of rotatable reflectors placed along the opposite edge which rotate so as to reflect the energy beam through a vertical angle of `about 70.

9. The method of claim 1 wherein the frequency of the source of microwave energy is of the order of be- 'tween 100 and 915 MHZ. f

10. The method of claim 1 wherein the frequency of the source of microwave energy is of the order of 915 MHz.

11. The method of claim 1 wherein the frequency of the source of microwave energy is of the order of 2450 MHz and each source of microwave energy has a power of the order of at least one kilowatt.

12. The method of claim 1 wherein the frequency of the source of microwave energy is of the order of 225,000 MHz. and each source of microwave energy has a power of the order of at least one kilowatt.

References Cited UNITED STATES PATENTS 1,284,042 11/ 1918 Balsillie 239--2 2,960,693 11/1960 Fry 343-882 M. HENSON WOOD, JR., Primary Examiner E. D. GRANT, Assistant Examiner U.S. C1. X.R.

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