Forty thunder events from intracloud and cloud‐to‐ground lightning were analyzed by analog and digital power spectrum methods. Thunder exhibits a low Q(0.5 to 2) spectrum with peak power observed at frequencies from <4 to 125 Hz. Significant differences are noted between thunder from intracloud and cloud‐to‐ground lightning. Intracloud thunder spectrums show a mean peak value of power at 28 Hz with a mean total acoustic energy of 1.9×1013 ergs. Cloud‐to‐ground spectrums show a mean peak value at 50 Hz with a mean total acoustic energy of 6.3×1013 ergs. The mean total acoustic efficiency for discharges to ground is calculated to be 0.18%. The thunder power spectrum is time varying. The mechanism of thunder production by a thermally driven expanding channel as described by A. A. Few appears to account for the dominant frequencies observed from many cloud‐to‐ground flashes, but cannot explain the high‐energy low‐frequency peaks of some cloud‐to‐ground and most intracloud discharges. The electrostatic mechanism of C. T. R. Wilson and of S. A. Colgate is proposed to explain the large energy peaks at frequencies less than 10 Hz.
The electric field along the path of an instrumented balloon was closely coupled to the wind profile and to the radar echo structure of a weak thunderstorm over Langmuir Laboratory on July 16, 1975. The balloon ascended at 3.5 m/s into the southern part of the storm, where a stable layer had stopped the cloud's vertical convection at 6.4 km above sea level. At lower altitudes, near the cloud base, the balloon rose past two nearby oppositely charged regions which were associated with a precipitation echo and with an outflow of air from the storm. When the balloon ascended into clear air through the top of the lower cloud at 6.4‐km altitude, its motion indicated a sharp change in wind direction, and its electric field meter showed an abrupt decrease in field intensity, probably from a screening layer at the cloud boundary. Above 7.5 km the balloon encountered a slanted and charged downdraft just before entering the northern part of the storm under its anvil cloud. This downdraft, which had a velocity of about 6 m/s, was a prominent and persistent feature of the cloud's circulation. It held the balloon at a nearly constant altitude of 7.7 km for 10 min while carrying it 3 km toward the center of the storm. When the electric field meter descended, after release from the balloon, it encountered the downdraft a second time, 24 min after its first encounter. Electric field measurements suggest that the downdraft was carrying a negative charge. Our measurements on this storm also contain evidence for vertical transport of horizontal momentum and for a net positive charge in the upper part of the storm.
A balloon carrying an electric field meter and a standard meteorological radiosonde rose into a relatively small, isolated thunderstorm in central New Mexico on July 16, 1977. The electric field, E, versus altitude between 3200 m (surface) and 7000 m above sea level can be explained reasonably well by the following charge distribution: (1) a 130 m thick layer of positive charge just above the surface with a density of 1.7 × 10−9 C/m3; (2) a localized 1 C of positive charge moving downward on rain with a velocity of 8 m/s at a horizontal distance of 420 m from the balloon; (3) a 1‐km thick ‘layer’ of negative charge between 4800 and 5800 m above sea level with a density of about −5 × 10−9 C/m3. This negative charge spanned a temperature range of about −2 to −5°C. Between 7000 and 10,000 m above sea level, the balloon was near the edge of the cloud where E was low and principally horizontal. Above 10,000 m the balloon ascended into an anvil cloud that was apparently positively charged.
SUMMARYFlights through the central regions of thunderstorms were made over New Mexico on 6 and 15 August I977 with the ONR/NMIMT Schweitzer aeroplane which carried equipment designed to measure all three components of the electric field, and the charge, Q, and diameter, d, of individual precipitation elements. On the earlier day, information was also obtained with : a rain-gaugenetworksurrounding Langmuir Laboratory; a 3 cm radar; an acoustic system for locating lightning channels; a ground-based field-change meter.The first cell on 6 August produced precipitation at the ground but no lightning. Vertical fields, E,, of up to about 50 kVm-' and precipitation charge densities pp of up to -0.5Cwere recorded within the cloud. The second cell, which grew as the first one decayed, produced 7 lightning strokes in 9 minutes during which time the radar revealed vigorous vertical growth in a narrow zone containing precipitation.Thunder reconstructions showed the acoustic sources for the first flash of this cell to be very near the top of the cloud at an altitude of 10 km a d . The subsequent flashes produced acoustic signals from progressively lower in the cloud. When the radar echo reached its maximum height lightning activity ceased. E. values of up to about SOkVm-' and pp values of down to -1 C km-3 were measured.pp was consistently negative, individual charges being less than f40pC. Q values were within the inductive limit for a thundercloud at breakdown but no systematic relation between Q and d was found.Six penetrations were made through the thundercloud of 15 August, which produced only two lightning strokes. The E, records were indicative of a (k) dipole located near the cloud top, at around -13°C. Fields of up to about lOOkVm-' and p,, values (positive and negative) of around 5Ckm-3 were measured. Q values of up to i250pC were recorded, with charges around i~50 pC being commonly found. No systematic Q-drelation was revealed, and smaller precipitation particles frequently carried charges (positive or negative) in excess of the inductive limit. On both days estimated precipitation rates were of order IOmmh-' and on most occasions the pilot reported precipitation particles to be either 'ice' or 'mixed liquid water and ice'.
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