atmospheric electricity

atmospheric electricity The atmospheric electrical system is sustained by naturally occurring processes which exchange and release electric charge. Atmospheric electricity is not a recent or unique phenomenon on Earth: fossilized lightning strikes (fulgurites) have been preserved from more than a hundred million years ago, and observations of lightning have been made on other planets. Large-scale natural consequences of lightning include forest fires (which contribute to global carbon dioxide) and the fixation (i.e. chemical reaction) of nitrogen in the air.

Global system

The global atmospheric electrical system consists of a few regions of intense electrical activity and a large area where only small currents flow. Thus at any one time the greater part of the Earth's surface area is only slightly affected by electrical processes. An electric field can be measured at the Earth's surface which is due ultimately to a difference in voltage between the upper atmosphere (at about 80 km) and the surface. Since electric fields dissipate in partial electrical conductors (such as air), the constant presence of this field must be due to continuous replenishment by an atmospheric electrification process. This process is charge separation during more-or-less continuous equatorial thunderstorms.

Figure 1 shows a schematic description of the global electrical processes. Charge separation occurs between the Earth's surface and the upper atmosphere by microscale cloud processes in thunderstorms. Negative charge is generally carried to the surface by leakage currents and lightning strikes beneath clouds. Positive charge carried to the upper atmosphere then spreads over the whole globe. Charge in the upper atmosphere also leaks slowly through the air to the Earth's surface, causing the upper regions of the atmosphere to discharge: the electrical resistance of the entire atmospheric air is effectively a resistor of about 230 ohms. In the absence of any atmospheric charge separation (i.e. if all global thunderstorms were to be switched off), the electric field would decay to significantly in under an hour.

Fair weather conditions

Charges are released and exchanged in air even in the absence of local thunderstorm activity. There are several sources of this free charge, or ionization. Air molecules such as nitrogen, oxygen, and water vapour are split by collisions with high-energy radioactive particles, producing charged fragments or molecular small ions. Small ions may be positive or negatively charged. Another source of ionization (particularly at high altitudes) is cosmic rays (high-energy nuclei). Near the surface, however, the radioactivity released by natural rocks is the dominant source of ionization, and in the boundary layer (the region within about a kilometre of the Earth's surface) the gas radon also causes ionization.

Air ions are highly electrically mobile, and they are therefore greatly influenced by an electric field. Atmospheric air consequently has a slight electrical conductivity, although for everyday purposes it is usually regarded as an insulator. Low conductivity usually indicates a small current flow, but when the total conductivity is calculated over the whole Earth's surface the atmospheric conductivity is quite large, and an appreciable current of about 2000 amperes flows.

Under fair weather conditions (the absence of local thunderstorms), the electric field measured near the surface is generally about 120 volts/metre, that is, the difference in voltage between the Earth's surface and a fixed point one metre above the surface is about 120 volts. At first sight it seems remarkable that most human existence occurs with such a potential between head and toe; however, only a tiny current flows because of the low conductivity of the air. Measurement of the potential difference is indeed complicated by the smallness of the current: a standard voltmeter is not able to measure the atmospheric field. Electrostatic methods are normally used, such as the field mill, which measures induced electric charge, or an ultra-high-impedance electrometer voltmeter.

The electrical conductivity of air depends on small ion concentration, which is in turn determined by the balance between ion production from radioactivity and ion removal by atmospheric particles such as aerosols (solid particles with diameters between 0.0001 mm and 0.01 mm), and raindrops. Large particle concentrations substantially reduce ion concentrations, and air conductivity is therefore highly affected by particulate pollution. Removal of ions by aerosol particles and raindrops leads to these large particles acquiring the original ions' electric charges. The large particles do not continue charging indefinitely: a raindrop will split apart if the electrical forces exceed the surface tension holding it together, at a critical level of charging known as the Rayleigh limit. Solid particles may emit charges when they become highly charged.

One curious aspect of fair weather atmospheric electricity near to the surface is the electrode effect. This occurs because of negative charge on the Earth's surface, which consequently repels negative ions. In still weather a layer of positive ions may therefore form close to the surface.

Thunderstorms

Thunderstorms offer substantial visible evidence of atmospheric electrical processes. A thundercloud (or cumulonimbus), with a distinctive upper anvil shape, results from air which is moist and unstable rising by convection. This large, deep cloud will eventually lead to lightning, hail, thunder, and heavy rain. Such clouds can exist individually, or as a group of active cells in various levels of development. The three stages of a single thundercloud are a brief cumulus stage (a strong updraft), followed by a longer mature stage (strong updrafts and downdrafts) and a final dissipating stage (weakening downdrafts) of comparable duration with the second stage. A thundercloud has a typical lifetime of about an hour. A full understanding of the electrical processes in a thundercloud has not yet been reached. To be credible, a theory of cloud electrification must explain how an electric field sufficient to produce lightning can be created within tens of minutes. It must also explain observations of an upper region of positive charge in the cloud, a lower region of negative charge, and a small amount of positive charge at the cloud base (see Fig. 1).

Many mechanisms for cloud electrification have been proposed. One major hypothesis depends on transport of external ions leading to regions of different charge in the cloud. The other major hypothesis invokes particle charging interactions of either an inductive or microphysical nature.

The inductive mechanism depends on cloud particles becoming polarized by existing electric fields. If a falling soft-hail (graupel) pellet were to acquire a top negative charge and a bottom positive charge by polarization, collisions beneath the pellet with rising water droplets would carry positive charge aloft on the droplets; negative charge remaining on the hail would be carried downwards. Although this mechanism can account for rapid and substantial cloud electrification, observations have found charges on some hail pellets rather greater than would be expected from polarization alone. These observations have lead to microphysical investigations of the electrical properties of ice. Laboratory studies have found that the sign of the charge exchanged between colliding ice crystals and soft hail pellets depends on temperature. If a riming hail pellet is made to collide with ice crystals, the hail pellet will acquire a positive charge if the temperature is below about −18 °C, and a negative charge if the system is at a higher temperature. The precise reversal temperature varies with many factors, such as the speed at which the pellet falls and the liquid water content. However, because the temperature variations within a thundercloud certainly include the range of laboratory reversal temperatures, this phenomenon does offer an explanation of cloud electrification.

In the colder, upper part of a cloud, (−18 °C typically occurs in about the middle of a cloud, at about 7 km above the surface), falling hail pellets acquire a negative charge, and rising ice crystals are carried upwards, to produce a positive region in the anvil, as observed. The falling hail generates the lower negative region of charge. Additional ice–hail interactions in the lower part of the cloud (which is above the critical reversal temperature) lead to the small positively charged region at the base of the cloud, above the freezing level. Calculations have shown that this process is able to produce electrification at rates comparable with those observed, although some observations remain unexplained.

Thunder and lightning

When sufficient charge has been separated by a storm, the intense local electric fields will eventually cause a lightning discharge to occur. Lightning is a transient electrical discharge several kilometres in length, which may occur within clouds (intracloud), between clouds (intercloud), or between the cloud and the ground. Some discharges can also occur between a cloud and surrounding air, and some can occur above a cloud. The most frequent lightning discharge is intracloud. The most dramatic is cloud-to-ground, often seen as forked lightning, which accounts for about 20 per cent of discharges and typically transfers tens of coulombs of negative charge from the cloud.

Forked lightning occurs rapidly, and the structure of an entire path from cloud to ground cannot be resolved by the naked eye. However photographic techniques have been used to investigate the separate stages. A lightning discharge begins with a weak initial discharge or stepped leader, which pursues a tortuous path to the surface, followed by a luminous return stroke.

The stepped leader proceeds in steps of typically 50 m at about 100 000 m s⁻¹, with a channel diameter of approximately 5 m. When the leader is close to the surface, streamers move upwards to the leader from several points. If the upward-moving charge reaches the leader, then there is a low-resistance path from the surface to the cloud, and a vigorous return stroke occurs. In a return stroke, currents of tens of thousands of amperes will flow within tens of microseconds, falling to hundreds of amperes sustained for several milliseconds. The lightning discharge may end at this point, or, if there is additional charge in the cloud, an additional dart leader can generate further return strokes using the same ionized channel. There are typically three or four strokes per flash. Lightning discharges also generate electromagnetic energy heard on radio receivers as spherics (crackles with very low frequencies), which may be used to locate distant thunderstorms.

The energy in the return stroke channel gives it a temperature and pressure higher than the surrounding air, so that the channel expands supersonically, generating a cylindrical shock wave which is heard as thunder. Thunder is heard later than its initiating lightning is seen, because the speed of sound in air is slow, compared with the speed of light. It is likely that the base of the channel is the strongest sound source, and is the probable origin of the initial loud bang heard by an observer close to a cloud; at greater distances an observer is more likely to hear a low rumble caused by refraction of the sound. A clap of thunder usually lasts between about one-tenth of a second and two seconds.

R. Giles Harrison

Bibliography

Chalmers, J. A. (1967) Atmospheric electricity (2nd edn). Pergamon Press, Oxford.
Saunders, C. P. R. (1988) Thunderstorm electrification. Weather, 43, (9), 318–24.
Williams, E. R. (1988) The electrification of thunderstorms. Scientific American, November 1988, 48–65.