Search Results

You are looking at 1 - 10 of 20 items for

  • Author or Editor: Ross Gunn x
  • Refine by Access: All Content x
Clear All Modify Search
Ross Gunn

Abstract

Thunderstorm-electrification processes that develop in clouds which are everywhere above the freezing temperature have been analyzed. Nearly equal numbers of highly electrified positive and negative raindrops are formed by the association of large numbers of cloud droplets, and these themselves are statistically electrified by ionic diffusion. The observed charges carried by both cloud droplets and rain are therefore a manifestation of the ion pairs produced in the free atmosphere primarily by cosmic rays and radioactivity. The discharge characteristics of nearly equal numbers of oppositely electrified drops are determined for drops falling through an atmosphere wherein the positive and negative ionic conductivities are different. It is found that the differential discharge of highly charged rain falling in such an environment systematically separates positive and negative electricity and establishes a distribution that accounts quantitatively for thunderstorms of low to moderate intensity. Useful expressions are developed for the convected free-charge current density established by the process, The analysis leads to a quantitative description of the principal observed features of thunderstorm electrification.

Full access
Ross Gunn

Abstract

The role of atmospheric electric fields in selectively charging raindrops is analyzed and compared with direct measurements made inside an active thunderstorm. Expressions are developed for the free charge transferred to conducting spheres polarized by an electric field and immersed in an environment in which the positive and negative ionic conductivities are different. Measurements in a wind tunnel operating with ionized air confirm the analysis.

Because the electrical conductivities inside a cloud and in the clear air immediately outside are so very different, electric fields acting across such a cloud boundary selectively concentrate ions of a single sign in a space-charge layer. Extensive regions are thereby established wherein the positive and negative ionic conductivities are notably different. Cloud and raindrops in this transition layer become hyperelectrified. Expressions for the mean space charge in the transition layer are developed. The estimated drop charges and space-charge densities are found to agree with earlier measurements made at various levels inside an active thunderstorm.

It is found that a large fraction of the free space charge inside a thundercloud is carried by the precipitation and is roughly proportional to the electric field intensity within the cloud-boundary transition layer. Since transition layers near the cloud boundaries play an important part in the electrification process, precipitating cumulus-type clouds developing in a clear-air environment are most likely to produce active lightning.

Full access
Ross Gunn

Abstract

No Abstract Available.

Full access
Ross Gunn

Abstract

The magnitude and character of the electrification of natural cloud-droplets have been investigated. Measurements of the electric field at the surface of non-precipitating clouds show that, as a whole, they are electrically neutral to a good approximation. New airborne apparatus was designed and built to reveal the detailed electrical distribution by separating the electricity carried by the larger droplets from that on the surrounding small particles or air. Measurements in different types of clouds show that the electric charges carried by the cloud elements are large. In warm swelling cumuli at 4,500 feet, the measurements show the following distribution of charge:

  • a. On large cloud-drops centrifuged out and estimated greater than 10 microns diameter Charge usually positive, approximately +3.8 × 10−6 e.s.u. cm−3.

  • b. On air and tiny drops collected by ion collector, drops estimated less than 10−2 microns diameter Charge usually negative, approximately −6.1 × 10−6 e.s.u. cm−3.

  • c. On small cloud-drops missed by both centrifuge and ion collector, estimated to lie between 10−2 and 10 microns diameter. Charge taken as algebraic sum of items (a) and (b), since electric field measurements show cloud is neutral; +2.3 × 1O−6 e.s.u. cm−3.

From these data, the average charge on each of the larger cloud-droplets was estimated to approximate 32 elementary positive units.

The centrifuge and the ion filter of the new cloud-analyzer usually capture charges of opposite sign. The measurements emphasize the fundamental electrical character of natural clouds and suggest their stability may be related to electrification.

Full access
Ross Gunn

Abstract

No Abstract Available.

Full access
Ross Gunn

Abstract

Whenever the number of collisions experienced by raindrops growing by coalescence with cloud droplets is limited either by a finite distance of fall or by the drop size, then the accumulated electrical charge is similarly limited. It is shown that under these non-equilibrium conditions the accumulated drop charges, averaged without regard to sign, are proportional to the charge on the parent-cloud droplets and to the square root of the number of charged transferring collisions; whereas the accumulated net charges are directly proportional to the number of collisions. When the parent-cloud droplets are weakly electrified, there are usually too few collisions to accumulate random equilibrium charges on a typical raindrop. On the other hand, equilibrium may be established on raindrops formed from randomly charged cloud droplets that are, themselves, highly electrified by atmospheric electric fields. Whenever the population density of the positively and negatively charged parent cloud droplets are notably different, drop charges as large as those usually observed may be established even when the number of collisions is limited. The analysis shows that the non-equilibrium regime is important in the electrification of rain.

Full access
Ross Gunn

Abstract

The increase in the world-wide fine Particle pollution of the atmosphere between 1914 and 1962 is inferred from measurements of the electrical conductivity of the atmosphere over the surface of the North Atlantic. Systematic instrumental errors have been discovered in some of the early measurements of conductivity made on the research ship Carnegie which have long been considered as “standard.” Measurements for the seventh cruise of the Carnegie should be multiplied by 1.39 to correct for errors introduced by eddy diffusion.

Notable improvements in the design of the electrical conductivity apparatus and an electric field meter permit the continuous collection and recording of data at sea even under adverse conditions such as heavy fog and light rain. More than 12 days of continuous new measurements in all kinds of weather during May–June 1962 are summarized.

A comparison of the new measurements with the corrected values of the early Carnegie observations suggests that the electrical conductivity of the atmosphere over the North Atlantic has decreased by about 5 per cent between 1929 and 1962. This implies a measurable and parallel increase in the average fine particle pollution of the atmosphere that is doubtless world-wide and represents an increasing hazard to the health of all terrestrial life.

Our measurements fail to confirm G.R. Wait's (1946) forecast of a very great and alarming increase in world-wide air pollution.

Full access
Ross Gunn

Abstract

A season's.record of the electric-field intensity measured on the earth during active thunderstorms is evaluated.

The records show that two basic types of storms exist. The most common, or positive, type of active storm systematically induces positive electricity on the earth. The normal systematic increase in electric field is interrupted and partly neutralized or over-neutralized by two types of lightning discontinuity. The most frequent electrical discharges reduce the field only by a small fraction. After four to ten fractional decreases in field, a large change is observed that frequently reverses the field.

Negative storms that systematically induce negative charge on the earth occurred infrequently, as did storms of the mixed type.

The fundamental cloud-charging mechanism is formulated in quantitative terms. The analysis suggests that two basic time intervals related to the observed field discontinuities are associated with thunderstorm electricity. The first is the regeneration time necessary for the falling rain to raise the electric field to the critical breakdown field for air. The second is the relaxation time required to distribute free electrical charge through the thundercloud by conduction. The regeneration time in active storms is always a fraction of the relaxation time.

The quantitative formulation is compared with observed measurements made inside an active thunder-storm, and it is concluded that electrified falling rain is just adequate to describe the observed phenomena.

Full access
Ross Gunn

Abstract

The fundamental electromechanics of droplet electrification and coagulation within stable and unstable clouds is investigated. The analysis shows that atmospheric ions formed by cosmic rays or other means normally diffuse onto cloud droplets and electrify them. A nearly Gaussian distribution is established in which about half of the droplets in any selected volume of a stable cloud acquire a positive charge that is typically eleven electronic units, while the other half is negative. More than 9.5 per cent of the droplets of a typical cloud are electrified. When droplet association is negligible, an equipartition is established between the thermal kinetic energy of the droplets and their electrical potential energy.

In an unstable cloud, these electrified droplets mechanically associate by relative motion in the gravitational field. Thus, the growing droplets accumulate charge and a statistical distribution of highly charged droplets is established. Expressions are derived for the distribution and for the mean statistical charge on the associated droplets. Equal numbers of positive and negative droplets are normally produced, but systematic charging due to unequal ionic conductivities sometimes results in marked electrification of a single sign.

The influence of droplet electrification upon cloud stability is considered. The droplet charges normally have a detectable, but not important, direct effect upon the coagulation rate of the cloud. The indirect effects, however, may be large. Electric fields of appreciable magnitude always accompany precipitation as a result of drop-charge separation. Expressions are derived for the electrical coagulation of clouds immersed in an electrical field. It is shown that electrical coagulation may exceed that due to gravitation whenever the environmental electric field is 2 statvolts per centimeter or larger. Since fields this large are commonly observed in thunderstorms, the resulting precipitation rates are correspondingly large.

Full access
Ross Gunn

Abstract

Light ions generated in the atmosphere by cosmic rays and radioactivity normally diffuse onto cloud droplets and establish a nearly symmetrical Gaussian distribution of positively and negatively charged droplets. When the cloud becomes unstable, these cloud elements grow by association and the charges accumulate to establish a new equilibrium consisting of nearly equal numbers of larger and highly charged positive and negative cloud droplets. Growing raindrops or graupel falling through such a cloud are accordingly bombarded by both positive and negative droplets, and this establishes a statistical accumulated charge on the various drops. The distribution of the number of drops in relation to the sign and magnitude of the free charge is worked out from basic principles and shown to agree remarkably well with the magnitude and distribution of drop charges measured inside precipitating clouds. The mean drop charge, irrespective of sign, depends on the square root of the drop size and kinetic energy of the small droplets relative to the larger moving drops. An equipartition is established between the electrical potential energy carried by the larger drops and the relative kinetic energy of the smaller drops. The estimated drop charges on warm rain or graupel are sufficiently large to account for thunderstorm phenomena, provided only that some unspecified process systematically separates the positively and negatively charged drops. The charges produced at the rain forming level commonly approximate 50 electrostatic units per gram. The observed electrification of quietly falling rain is primarily a manifestation of the ionization produced in the atmosphere by radioactivity and cosmic rays.

Full access