Cloud electrification and lightning

Two general hypotheses help to account for thunderstorm electrification. One involves induction in the presence of an electric field, the other is non-inductive charge transfer. The ionosphere at 30 to 40 km altitude is positively charged (owing to the action of cosmic and solar ultraviolet radiation in ionization) and the earth's surface is negatively charged during fine weather. Thus cloud droplets can acquire an induced positive charge on their lower side and negative charge on their upper side. Non-inductive charge transfer requires contact between cloud and precipitation particles. According to J. Latham (1966), the major factor in cloud electrification is non-inductive charge transfer involving collisions between splintered ice crystals and warmer pellets of soft hail (graupel). The accretion of

10 km

Noninducive Charging Thunderstorm
Surface
(i)

Figure 5.18 Classic view of the vertical distribution of electrostatic charges in a thundercloud and at the ground. (A) shows the common transfer of negative charge to the surface in a lightning stroke; (B) shows other cases: (i) when positive charge from the upper part of the cloud is transferred towards a locally induced area of negative charge at the surface; (ii) when positive charge transfer is from a summit or surface structure towards the cloud base.

supercooled droplets (riming) on hail pellets produces an irregular surface, which is warmed as the droplets release latent heat on freezing. The impacts of ice crystals on this irregular surface generate negative charge, while the crystals acquire positive charge. Negative charge is usually concentrated between about -10° and -25°C in a thundercloud, where ice crystal concentrations are large, and due to splintering at about the 0° to -5°C level and the ascent of the crystals in upcurrents. The separation of electrical charges of opposite signs may involve several mechanisms: one is the differential movement of particles under gravity and convective updrafts; another is the splintering of ice crystals during the freezing of cloud droplets. This operates as follows: a supercooled droplet freezes inward from its surface and this leads to a negatively charged warmer core (OH-ions) and a positively charged colder surface due to the migration of H+ ions outward down the temperature gradient. When this soft hailstone ruptures during freezing, small ice splinters carrying a positive charge are ejected by the ice shell and preferentially lifted to the upper part of the convection cell in updrafts (see Figure 5.18). However, the ice-splintering mechanism appears to work only for a narrow range of temperature conditions, and the charge transfer is small.

The vertical distribution of charges in a thundercloud, based on balloon soundings, is shown in Figure 5.19. This general scheme applies to airmass thunderstorms in the southwestern USA, as well as to supercell storms and mesoscale convective systems described in Chapter 9I. There are four alternating bands of positive and negative charges in the updraft and six outside the updraft area. The lower three bands of the four in the updraft are attributed to collision processes. Ice crystals carried upward may explain why the upper part of the cloud (above the -25°C isotherm) is positively charged. Negatively charged graupel accounts for the main region of negative charge. There is a temperature threshold of around -10° to -20°C (depending on the cloud liquid water content and the rate of accretion on the graupel) where charge-sign reversal takes place. Above/below the altitude of this threshold, graupel pellets charge negatively/positively. The lower area of positive charge represents larger precipitation particles acquiring positive charge at temperatures higher than this threshold. The origin of the uppermost zone of negative charge is uncertain, but may involve induction (so-called 'screening layer' formation) since it is near the upper cloud boundary and the ionosphere is positively charged. The non-updraft structure may represent spatial variations or a temporal evolution of the storm system. The origin of the positive area at the cloud base outside the updraft is uncertain, but it may be a screening layer.

Radar studies show that lightning is associated both with ice particles in clouds and rising air currents

Cloud Charge Particle

Figure 5.19 The electric charge structure in airmass storms in New Mexico, supercell storms and the convective elements of mesoscale convective systems (see Chapter 9), based on balloon soundings of the electric field - 33 in updrafts and 16 outside them. There are four vertical zones in the updraft region and six in the downdraft region, but the size, strength and relative positions of the up- and downdrafts vary, as do the heights and temperatures shown.

Figure 5.19 The electric charge structure in airmass storms in New Mexico, supercell storms and the convective elements of mesoscale convective systems (see Chapter 9), based on balloon soundings of the electric field - 33 in updrafts and 16 outside them. There are four vertical zones in the updraft region and six in the downdraft region, but the size, strength and relative positions of the up- and downdrafts vary, as do the heights and temperatures shown.

Source: Stolzenburg et al. (1998) Fig. 3, by permission of the American Geophysical Union.

carrying small hail aloft. Lightning commonly begins The sound travels at about 300 m s-1. Less commonly, more or less simultaneously with precipitation downpours and rainfall yield appears to be correlated with flash density. The most common form of lightning (about two-thirds of all flashes) occurs within a cloud and is visible as sheet lightning. More significant are cloud-ground (CG) strokes. These are frequently between the lower part of the cloud and the ground which locally has an induced positive charge. The first (leader) stage of the flash bringing down negative charge from the cloud is met about 30 m above the ground by a return stroke, which rapidly takes positive charge upward along the already formed channel of ionized air. Just as the leader is neutralized by the return stroke, so the cloud neutralizes the latter in turn. Subsequent leaders and return strokes drain higher regions of the cloud until its supply of negative charge is temporarily exhausted. The total flash, with about eight return strokes, typically lasts for only about 0.5 seconds (Plate 12). The extreme heating and explosive expansion of air immediately around the path of the lightning sets up intense sound-waves, causing thunder to be heard.

positive CG flashes occur from the upper positive region (Figure 5.18B, case (i)), and they predominate in the stratiform cloud sector of a travelling convective storm (Chapter 9I). Positive charge can also be transferred from a mountaintop or high structure towards the cloud base (case (ii)). In the United States, over 20 per cent of flashes are positive in the Midwest, along the Gulf Coast and in Florida. Figure 5.18A represents a simple dipole model of cloud electricity; schemes to address the complexity shown in Figure 5.19 remain to be developed.

Lightning is only one aspect of the atmospheric electricity cycle. During fine weather, the earth's surface is negatively charged, the ionosphere positively charged. The potential gradient of this vertical electrical field in fine weather is about 100 V m-1 near the surface, decreasing to about 1 Vm -1 at 25 km, whereas beneath a thundercloud it reaches 10,000 V m-1 immediately before a discharge. The 'breakdown potential' for lightning to occur in dry air is 3 X106 V m-1, but this is ten times the largest observed potential in thunderclouds. Hence the necessity for localized cloud droplet/ice crystal charging processes, as already described, to initiate flash leaders. Atmospheric ions conduct electricity from the ionosphere down to the earth, and hence a return supply must be forthcoming to maintain the observed electrical field. A major source is the slow point discharge, from objects such as buildings and trees, of ions carrying positive charge (electrons) induced by the negative thundercloud base.

Upward currents have recently been discovered high above the stratiform regions of large convective storm systems with positive CG lightning. Brief luminous emissions, due to electrical discharges, appear in the mesosphere and extend downward to 30 to 40 km. These so-called sprites are red in the upper part, with blue tendrils below. The red colour is from neutral nitrogen molecules excited by free electrons. In the ionosphere above, a luminous expanding ring (termed elve) may occur. High above the lightning storm, a discharge takes place because the imposed electric field of a vertical dipole exceeds the breakdown potential of the low-density air. The electrically conductive ionosphere prevents sprites from extending above 90-km altitude.

The other source of a return supply (estimated to be smaller in its effect over the earth as a whole than point discharges) is the instantaneous upward transfer of positive charge by lightning strokes, leaving the earth negatively charged. The joint operation of these supply currents, with approximately 50 flashes/sec over the globe at any time, is thought to be sufficient to balance the air-earth leakage, and this number matches reasonably well with observations.

Globally, thunderstorms are most frequent between 12:00 and 21:00 local time, with a minimum around 03:00. An analysis of lightning on satellite imaging systems for 1995 to 2002 shows a predominance of flashes over tropical land areas between 15°N and 30°S (Plate C). In the austral summer, lightning signatures are along the equatorial trough and south to about 30°S over the Congo, South Africa, Brazil, Indonesia and northern Australia, with activity along cyclone paths in the northern hemisphere. In the boreal summer, activity is concentrated in central and northern South America, West Africa - the Congo, northern India and Southeast Asia and the southeastern United States. The North American lightning detection network recorded 28 to 31 million flashes per year for 1998 to 2000. The median peak current was about 16 kA. In Florida and along the Gulf Coast there were nine flashes/km2. Lightning is a significant environmental hazard. In the United States alone there are 100 to 150 deaths per year on average, as a result of lightning accidents.

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Responses

  • Yohannes Yohannes
    What is cloud electrification?
    10 months ago
  • teresio
    How thundercloud induced positive charge on the earth?
    5 months ago

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