Local Thunderstorms Research Articles

Variations of radio noise parameters at a tropical station

The results of a 4-year (1958–1961) period of measurement of atmospheric radio noise at chosen frequencies in the range 10 kc/s–20 Mc/s at Ibadan (7°26′N, 3°54′E) in a tropical area have been analyzed to confirm the systematic and random variations of noise parameters in the tropics. These variations are discussed and it is shown that the diurnal, monthly and seasonal variations of atmospheric noise levels are determined mainly by local thunderstorm activity and ionospheric absorption. The noise levels also show marked decrease with rise in frequency and are generally highest during the spring months. Atmospheric noise levels averaged over yearly periods tend to be reduced in the kilocycles range and increased in the megacycles range by enhanced sunspot activity. Abnormal variations which wore observed during the period, have been related to a low-altitude nuclear detonation in the Sahara Desert.

Passive electrical measurements from three Oklahoma tornados

Passive electrical measurement of three tornados in the Oklahoma area have been made and are being reported upon. The measurements made include point discharge current, 18.5-kc/s sferics, along with 10-, 50-, 100-, and 175-kc/s, and 30-Mc/s sferics. The preceding measurements of tornados are compared with a typical local thunderstorm. In this comparison, it is shown that the electrical discharges within the tornados are much more intense than in local thunderstorms, particularly in frequency of occurrence. Data also show that the normal frequency spectrum of sferics radiation is different from the frequency spectrum of the tornados measured. Furthermore, the tornado sferics appear to have a definite aperiodic structure that may be present to a limited extent in the intense portion of a thunderstorm. Similarities between the measurements made on the Oklahoma tornados and Jones's work are pointed out. Finally, it is suggested that a coupling between the electrical and mechanical energies of the tornado may be responsible for the difference noted between tornado sferics and thunderstorm sferics.

Some radio-frequency effects of the July 9, 1962, nuclear detonation

The purpose of this letter is to report radio-frequency observations of the Starfish nuclear detonation, which occurred on July 9, 1962. The observations were made at the Maipu Radio-astronomical Observatory of the University of Chile, latitude 33°30″S, longitude 70°52″W. At the time of the explosion, radio observations of the planets Jupiter and Saturn were being made at the Chilean observatory and at the University of Florida Radio Observatory. While local thunderstorm activity completely blanketed the records at the Florida station, reception at the Chilean observatory was reasonably good, and radio-frequency effects of the explosion were apparent at this station (Figure 1). The antennas being employed in Chile were dipole arrays, the characteristics of which are shown in Table 1.

Integrated field intensity of atmospherics in relation to local thunderstorms

Integrated field intensity of atmospherics in relation to local thunderstorms

The Divergence Equation as Related to Severe Thunderstorm Forecasting

The divergence form of the equations of motion is developed and the terms of the equation are related to several local thunderstorm forecast parameters. It is found that the divergence equation provides the physical basis for evaluating the validity of several forecast rules under differing synoptic situations. In order that the magnitudes of the several terms can be compared, a numerical solution for a particular severe local thunderstorm producing synoptic situation is shown.

Narrow-band atmospherics from two local thunderstorms

Atmospherics from two local storms at known distances have been recorded on magnetic tape. The frequencies of reception were 11 Mc/s for one storm and both 6 kc/s and 11 Mc/s for the other. The power bandwidths were in the range 200–300 c/s. Values of duration, peak amplitude, mean field strength and mean power flux have been deduced from the records, and estimates of the radiated power have been derived. The waveforms of the atmospherics are partly explicable in terms of known characteristics of lightning discharges, but there are many features, particularly at 11 Mc/s, for which an adequate quantitative explanation is lacking. The results conflict with some theories of the origin of high-frequency atmospheric noise which have been advanced in recent years.

Characteristics of cosmic radio radiation

Cosmic radio radiation is radiation of extraterrestrial origin in the radio‐frequency portion of the electromagnetic spectrum. Such radiation was discovered by Karl Jansky of Bell Telephone Laboratories in 1932 on a wavelength of 14.6 m. While studying the direction of arrival of the atmospheric disturbances which interfere with the transatlantic short‐wave radio communications, Jansky found that in addition to familiar atmospheric static due to local and distant thunderstorms, his directive antenna was picking up persistent strange static noise. From further observations he concluded that the static noise was coming from the general direction of the Milky Way and the maximum noise was from the Constellation of Sagittarius, the central region of the Milky Way. Jansky suggested that this cosmic radio wave might be originating either in the stars or in interstellar space.

AFMAG—AIRBORNE AND GROUND

The existence of natural magnetic fields in the audio and subaudio frequency range has been known for some time. The primary source of energy for these fields is usually considered to be distant and local thunderstorms. Because of this origin, the fields are quasi‐random with both amplitudes and directions changing drastically over short periods of time. Hence, use of these fields in geophysical prospecting has been extremely limited. A new development, AFMAG, however, essentially eliminates the time variance in recording these fields without any sacrifice of the intelligence of their space variance. Since the space variance can be correlated with geologic features, AFMAG provides a new method of exploration with particular application to prospecting for conductive mineral deposits. Instrumentation of the AFMAG method currently is available for both ground and airborne operation; the tilt of the plane of polarization of the natural magnetic fields is recorded simultaneously at two frequencies. Examples drawn from airborne and ground surveys show that the method has a much greater depth of exploration than its conventional cousin, the induction electromagnetic method. Numerous other advantages, such as the possibility of choosing discrete operating frequencies over a broad band from 1 cps to 1,000 cps, are discussed. The chief disadvantage of the method lies in a sometimes restricted daily measuring period during which the fields are of an amplitude too low to permit measurement with current instrumentation; this is not a serious problem and is being minimized as the technology improves.

Lapse rate change due to advection at lower levels and occurrence of pre-monsoon thunderstorms

Using the method of vertical wind shear vector difference, the change in lapse rate brought about by advection alone (upto a height of 5000-7000 ft) was estimated for Calcutta from the upper wind data of 0200 GMT ascent for every day, spread over a period of one month of the usual peak activity of nor'westers which (violent thunderstorms). This change applied to the morning tephigram is found to provide a moderately successful tool for prediction of local afternoon thunderstorms. The percentage of success for the period studied is nearly 80 per cent.

Current direction-finding practice

Direction-finding practice in Britain is reviewed over the frequency band 10kc/s–400Mc/s; the survey is restricted to land-based systems. Current applications are outlined, the factors affecting accuracy are discussed with particular reference to those concerned with propagation, and equipment is described.The applications are numerous, ranging from location of thunderstorms at the lowest frequencies to widespread aeronautical uses at the highest frequencies. Over the lower parts of the frequency range the effects of the ionosphere on accuracy have to be borne in mind in many of the applications. In the v.h.f. and u.h.f. bands these limitations are absent for short-range working and the accuracy is determined largely by site and instrumental properties.The majority of direction-finders in current use have fixed aerial systems, the most common being the Adcock type. At the lower frequencies, however, loops are used, and wide-aperture systems having circular arrays of aerial elements are rinding application at the higher frequencies. Regarding the display of bearing information, although some use is still made of the aural-null method, automatic presentation of the bearing either on a meter or on a cathode-ray tube is generally employed. For aeronautical purposes in the v.h.f. and u.h.f. bands, where development has been particularly marked, remote control of the equipment and remote display of the bearings (on a number of frequencies simultaneously, if required) are of increasing importance. The paper also includes a description of automatic position-fixing systems for which these remotely operated instruments are especially adapted.

Radio Noise from Lightning Discharges

IN studies of atmospheric radio noise from thunderstorms, and of its interference with radio systems, it is useful to know the wave-forms of the atmospherics received in different frequency bands from a single lightning discharge. Although measurements of the electromagnetic field changes are the basis of much of our knowledge of the discharge processes, they have been mainly restricted to frequencies less than 100 kc./s. Norinder1 has recorded sample of noise at 500 kc./s., but there are few records of the waveforms of atmospherics in the high-frequency band which is important for long-distance communication. Records obtained at the Radio Research Station, Slough, during two local thunderstorms are therefore of interest both in the study of radio interference and also as a possible source of further information about the discharge processes.

A Practical Way to Make Air Mass Thunderstorm Forecasting Easier and More Reliable

This study introduces a plastic device for mechanically progging soundings rapidly. It also shows how this device can be used with the Bailey local thunderstorm forecasting graph to make local thunderstorm forecasting easy and more reliable.

The location of thunderstorms by a single-station high-frequency ranging technique

By observing the signal radiated by a lightning discharge on a number of frequencies in the high-frequency band, and measuring the maximum usable frequency, the range of the source may be found from a knowledge of the ionospheric conditions. By measuring simultaneously the direction of arrival of the signal, using a low-frequency cathode-ray direction finder operating on 10 kc/s, the position of the storm is determined. A preliminary investigation of the technique, reported in a previous paper, has now been extended by a more thorough investigation of the influence of ionospheric conditions and by improvements in the method of observation. It is necessary to know which layer is controlling the propagation and, in the case of F2, to know also the height and thickness of the layer. Outside the sunset and sunrise periods, the E and F1 conditions may be calculated sufficiently accurately from the zenithal angle of the sun; the F2 conditions are derived from the predicted critical frequencies and MUF factors. Range measurement appears to be impossible when propagation is controlled by sporadic E ionization. Observations made during the summer of 1954 are compared with the thunderstorm positions given by the Meteorological Office network of direction finders. Some of the measurements wore made in conjunction with backscatter observations for determining the ionospheric conditions. It is concluded that the system provides a possible technique for the location of thunderstorms within 500–2000 km of a single observing station, for about 70% of the time. Apart from four months around midwinter, it is essential to have backscatter equipment to provide continual monitoring for Es propagation. An accuracy of 10% in range should be obtained at 1000 km under good conditions.

Some waveforms of atmospherics and their use in the location of thunderstorms

Illustrations are given of the main types of waveforms which have been recorded in southern England, using the equipment described in an earlier paper. Particular emphasis is given to echo-type waveforms comprising a series of echoes arriving by different modes of reflection between the earth and the ionosphere. Practical difficulties arising in the use of this type for estimating the distance of the source are discussed. The range is estimated from the measurement of the time-intervals between echoes, which under good conditions may be determined to an accuracy of two microseconds. Waveforms usually show discrepancies considerably greater than this figure when the time-intervals are compared with what would be expected from simple reflection theory and a graphical method of analysis is used which averages the errors on individual echoes. The effect of these discrepancies on the accuracy of distance estimation is discussed. The frequency of occurrence of suitable types of waveform is derived from an analysis of routine recordings over a period of one year. Approximately three per cent of the waveforms recorded in daylight and five per cent at night were suitable for deriving an estimate of distance, but more detailed analysis of a proportion of the waveforms indicated that by changes in technique, the night-time figure might have been increased by a factor of about 2. With experience it is not difficult to select by cursory inspection those waveforms which are likely, on analysis, to yield a useful distance estimate. Many of them, however, are rather complex, and considerable time is required to determine from these the most probable value of distance. Most night-time storms within 2,000 km give rise to some waveforms from which a reliable distance estimate can be deduced; by day the corresponding range is about 500 km in summer and 1,000 km in winter. Within these ranges errors can be expected to be within ± 30 per cent. It is concluded that waveform analysis would be of some value for thunderstorm location as an adjunct to a network of cathode-ray direction finders, but that the reliability and convenience of the technique are not yet sufficient for use as the sole method of estimating distance.

The accuracy of the location of sources of atmospherics by radio direction-finding

In recent years, increasing importance has been attached to the use of radio direction-finders for the location of thunderstorms. A large number of stations for this purpose are now in operation, and for the planning of further networks it is necessary to assess the accuracy to which storms may at present be located, and to delimit the areas in which location is satisfactory with the existing networks. An investigation of the accuracy of the United Kingdom network of twin-channel cathode-ray direction-finders, operating at frequencies near 10 kc/s, has shown that instrumental errors other than polarization errors are small, but that at particular stations, errors of several degrees have been caused by the hilly nature of the terrain and by buried cables. When the equipment is on a level site, free from obstructions, polarization errors are likely to be the main limitation to accuracy, particularly at distances of a few hundred kilometres and at night. Maximum errors of 2? or 3? may be expected in summer day-time and rather more in winter day-time. The magnitudes of polarization errors at night are not accurately known. Over a limited range of distance, around 400km, observations on continuous-wave stations exhibit, at times, r.m.s. errors of more than 20?, but there is evidence that errors of this magnitude do not normally occur with atmospherics. At distances greater than 1 000km, polarization errors may be expected to be no greater than 2? or 3?, even at night. At these distances there are also significant errors caused by interference between the atmospheric under observation and others arriving almost simultaneously. Although knowledge of the errors is far from complete, preliminary estimates of their magnitudes have been made and an estimate of the accuracy of storm location has been derived. It is not possible to express the potential accuracy in simple terms, since it depends not only on the bearing errors, the spacing of the stations and the distance of the storm, but also on the number of observations made and the manner in which they are interpreted. As an indication of the order of magnitude, it is estimated that with the present network and observation technique the probable error in position of a storm centre at a distance of 1 000km is about 20km by summer day, 50km by winter day and 100km by night. These preliminary estimates, particularly that for night-time conditions, must be regarded as approximate because knowledge of the bearing errors is still lacking in some important respects. In particular, a more complete knowledge is required of their amplitude distribution and of the degree of correlation, if any, between polarization errors at the different stations. The types of error discussed are liable to occur with all atmospherics direction-finders in current use; polarization errors, in particular, may be expected with any instrument with an aerial system incorporating either a single rotating loop or a pair of fixed crossed-loops.

Measurement of atmospheric noise interference to broadcasting

The paper gives a general survey of the problem of atmospheric noise measurement and establishes, by a subjective method, a criterion for assessing the annoyance value of the noise to broadcast reception. There follows the development of an objective method of measurement and a comparison is made between these measurements and the subjective observations, etc. Ten impulses per minute are estimated to have an annoyance value to the listener of broadcast programmes. Noise is classified into three types: ( a) type A noise giving the impression of continuous noise, ( b) type B noise coming as distinct impulses, and ( c) type C noise, a special form of type B noise arising from local thunderstorms. The importance of bringing statistical considerations into the field of assessment of atmospheric noise interference is discussed and a procedure for the collection of data and its assessment for obtaining monthly values for a specific service, broadcasting, is described.

Recent Developments in Radio Location of Thunderstorm Centers *

The location of thunderstorms through observations of the directions of arrival and the instantaneous intensity variations of the electromagnetic radiations produced by the associated lightning discharges is described. The method of triangulation (θ1, θ2), employed by the Air Forces during World War II, and the proposed single-station (R, θ) technique currently under investigation at the University of Florida are discussed.

The variation with frequency of the signal range of f.m. and television broadcasting stations

It is the purpose of this report to determine the effect of the carrier frequency on the expected maximum signal ranges of F.M. and television broadcasting stations as affected by the transmitting and receiving antenna gains and as limited by internal receiver noise and the ever-present effects of cosmic noise received on the very high frequencies from inter-stellar space. The signal ranges shown on the attached charts are thus applicable only in the absence of local thunderstorms or man-made noise.

Location of Thunderstorms by Radio

THE study of atmosphere disturbances in radio reception began at or before the time when electromagnetic waves were being applied to practical communication ; find from the beginning of the present country, various investigators were exploring the possibility of using radio methods for forecasting the arrival of thunderstorm conditions. For more than twenty years, the radio direction-finder with cathode-ray tube indication, first described by R. A. Watson-Watt and J. F. Herd1, has been used in the study of the direction of arrival of atmospherics. By the use of a number of such direction-finding stations suitably situated, simultaneous observations can be made on individual atmospherics, and from the bearings obtained the source of the disturbance can be determined. Such a storm-location network has been built up by the Meteorological Office in the British Isles during the past few years, and the regular daily information obtained therefrom has for a long time formed a substantial contribution to the knowledge used in weather forecasting. The general organisation and arrangement of this ‘Sferics' service, as it is called, have been described by C. V. Ockenden2 ; and in a paper read before Section A of the British Association at Dundee, a general review hf the present and possible future techniques for location of thunderstorms by radio was given R. L. Smith-Rose8.

The location of thunderstorms by radio direction-finding

The nature of the radiation from a lightning flash is discussed, and it is shown that the main source of inaccuracy in the crossed-loop cathode-ray types of direction-finder at present in use by the Meteorological Office is due to polarization errors. Various methods are then considered for improving the accuracy of thunderstorm location, and the conclusion is drawn that a cathode-ray direction-finder, using a spaced-aerial system and with brilliance modulation applied to the display tube, is likely to be more successful than other systems. A description is then given of a direction-finder, having greatly improved instrumental accuracy, which has been designed as an experimental tool for the development of the improved method of location.