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William E. Cobb

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William E. Cobb

Abstract

The measurement of atmospheric electrical conductivity, ion density or mobility, by the aspiration method, may result in the undesired removal of ions within the air-intake system.

A field experiment has been carried out to determine the loss of small ions when an air sample is aspirated through a 10-cm diameter intake pipe of different lengths and for different flow velocities. The percentage of small ions removed from the original air sample appeared to depend upon the ion transit time within the intake pipe and ranged from zero to 80% for sampling times of 0.15 to 10 sec, respectively.

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William E. Cobb

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The final analysis is presented of a one-year measurement of the atmospheric electric elements at all isolated mountain site in Hawaii, where the environmental electrical properties are generally representative of the mid-Pacific troposphere at 3.4 km. The importance of the atmospheric electric climate as an indication of the global suspended fine-particle pollution is discussed. Data are presented showing the influence of local meteorological parameters on the electric elements in both fair and disturbed weather at Mauna Loa.

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William E. Cobb

Abstract

The electrical conductivity of the atmosphere, monitored at sites remote from sources of anthropogenic aerosols, may be used to provide an index of the level of suspended particulates for the area representative of the sampling site. Conductivity measurements taken at Mauna Loa, Hawaii, and from ocean research vessels, indicate that most of the oceanic regions of the world are maintaining a natural aerosol level unchanged by the activities of mankind. Significant exceptions are the paths of aerosol pollution extending eastward from the United States in the North Atlantic, from Japan in the North Pacific, and southward from Asia in the Northern Indian Ocean. These regions are discussed with respect to the lifetime of the suspended particulates and the influence of large-scale atmospheric circulation. Anthropogenic aerosols are largely produced from the burning of fossil fuels such as coal and oil. The present trend is toward control of these particulate emissions and an inevitable switch to other forms of energy production. It is concluded that the anthropogenic aerosols now detectable in some oceanic regions will begin to decline by the end of the century and that any global climatic changes due to the current increase will be insignificant.

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WILLIAM E. COBB

Abstract

The monitored atmospheric electric elements at Mauna Loa Observatory, Hawaii, have provided good evidence of a direct solar influence on some of the electric elements recorded at the mountain observatory. An analysis of the data has shown that following a solar flare eruption, both the air-earth conduction current and the electric field, measured during fair weather at Mauna Loa, usually exceeded their established normal values.

For the 1-year measurement period from September 1960 to September 1961, nearly one-third of the days were considered as “disturbed” solar days due to solar flare activity. The mean value of the air-earth conduction current and the electric field on “disturbed” solar days exceeded that of the “quiet” solar days by about 10 percent.

During the month of July 1961, a period of spectacular solar activity, the highest sustained values of the year for the air-earth current and the electric field were recorded with the normal 24-hr. values being exceeded by as much as 35 percent, and for one 6-hr. period following a multiple flare burst, by 7.5 percent.

The influence of corpuscular solar radiation on the earth-ionosphere electric circuit and upon the global thunderstorm activity are discussed.

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WILLIAM E. COBB and F. JAMES HOLITZA

Abstract

A DC-6 research aircraft was struck by lightning on three occasions during a thunderstorm research project at Flagstaff, Ariz., in July 1967. Electric fields and meteorological parameters were measured and recorded.

Similar conditions existed at the time of the lightning strikes. Each event occurred in a dissipating cumulonimbus, near the freezing level and in a region containing both ice and water. Corona discharge from the aircraft occurred prior to each strike.

The possibility exists that one or more of the lightning strikes were triggered by the aircraft.

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William E. Cobb and Howard J. Wells

Abstract

The atmospheric electrical conductivity was recorded during the 1967 global expedition of the research vessel Oceanographer. Seventy-five complete days of fair weather conductivity observations were obtained and compared to earlier observations of the Carnegie Institution and others.

Significant results show that the atmospheric conductivity in the remote South Pacific has remained fairly constant over the past half century but has decreased by at least 20% in the North Atlantic. The secular conductivity decrease in the North Atlantic is attributed to an increase in the fine-particle aerosol pollution suspended in the atmosphere of the Northern Hemisphere. The influence of atmospheric aerosols, primarily in the form of condensation nuclei, on the conductivity is discussed.

The observations of the conductivity in the two hemispheres are discussed with respect to the sources of pollution, the tropospheric lifetime of the suspended aerosols, and the influence of the primary atmospheric circulation.

It is urged that the Carnegie measurements be repeated.

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Gilbert D. Kinzer and William E. Cobb

Abstract

Water droplets initially 2 µ in radius were held stationary for observation in an upward streaming cloud and were found to grow by accretion. The rate of increase in the radius of individual droplets was measured from 4 to 65 µ. Experimental collection efficiencies obtained from the data agree within limits of experimental error with those calculated from a theory that is developed from intuitive physical arguments which are based on a superimposed random motion observed in the fall of droplets in the supporting cloud. The theory implies that a small-scale eddy diffusion transfers smaller droplets from the parent cloud to the growing droplet. The coefficient of droplet transport calculated from the theory agrees well with one evaluated experimentally.

Past measurements of drops growing by accretion from 100 µ to 1.5 mm in radius have been combined with the results of the present investigation in order to compare the efficiencies calculated by the new theory with measured values over a more extensive range.

The free water content and the temperature of the parent cloud were approximately 3.5 gm m−3 and 22C, respectively. Two types of distributions were employed: the first with 80 per cent of the free water contained in droplets 5.5 to 8.0 µ in radius; and the second with the free water distributed more widely among droplets 3.9 to 10.4 µ in radius.

The droplets of the parent cloud were produced by atomization of water and were randomly electrified. The electrification was found to be of a magnitude similar to that measured recently on droplets in some types of natural cumuliform clouds. It is shown that the electrification was too small to produce dynamic effects of more than one-tenth of the order of magnitude of those associated with the eddy transfer, although coalescence may be promoted by electrical forces whenever eddy diffusion transports droplets to within 2 or 3 µ of the growing droplet.

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Gilbert D. Kinzer and William E. Cobb

Abstract

Determinations were made of the rate of growth of a drop of water supported by an upward flow of a cloud whose free water content and droplet-radius distribution were measured. The water content was varied through a range of 5.5 to 10.9 grams per cubic meter, but the droplet-radius distribution was kept constant with a single maximum at a radius of 5.2 microns and with no radii exceeding 18µ. Collection efficiencies are evaluated directly from the data and are compared to those calculated from Langmuir's theory. The measured values of the efficiencies are found to be significantly greater than the theoretical values when the radius of the growing drop lies within the range from 150µ to 0.5 mm. When the radius of the drop lies outside this range, the measured efficiency is much smaller than theory predicts.

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