Global Electricity

There are 1,000-2,000 thunderstorms occurring at any moment around the world, and about a hundred main flashes of lightning from the ground every second. The flashes conduct positive charge to the mainly negative base of each cumulonimbus cloud. Then the charge is carried to the cloud top in the internal updraught, where it leaks upwards to the ionosphere above 60 km (Figure 1.10). This layer is highly ionised by cosmic rays (Section 1.7), so that it easily conducts electricity from one part of the world to another, and collects charge from storms everywhere. For this reason it is called the equalisation layer, maintaining a voltage around 500 kV above that of the ground. Lower levels of the atmosphere are at correspondingly lower voltages, the gradient near the ground being of the order of 100 V/m in fair weather (Note 9.H).

The voltage gradient pulls a steady stream of negative ions from the ground to the equalisation layer, flowing in all the clear sky between storms. Therefore, there is a sort of electrical circuit, shown in Figure 9.15. The clear-sky current density is modest (e.g. only about 3X 10-12 amperes from each square metre of the ground), but is sufficient to cancel the voltage of the equalisation layer in about ten minutes if it were not for continual replenishment by the world's thunderstorms.

Leakage of electricity between the ground and the equalisation layer is carried by two sizes of ions. Small ions each consist of a few molecules of water vapour, nitrogen or oxygen, with an excess electron. Large ions consist of aerosols, which are much larger and thus less mobile, so that they carry electricity less rapidly. The larger ones absorb the smaller when both kinds of ion are present, and reduce the leakage,

+ + + + + +

+

S-

JT + /+ +/

storm cloud jf <

circuit of negative charge /

lightning Vfy

r ground

j

storm conditions

fine weather

Figure 9.15 The circuit of electricity within the atmosphere. The ground is negatively charged and the equalisation layer is positive, so there is a small but steady flow of negative ions upwards and positive ions down. Flows are in the opposite directions during thunderstorms, i.e. there is a net flux of negative charge in cloud-to-ground lightning strikes, whilst positive ions from the top of storm clouds maintain the positive charge in the equalisation layer.

Figure 9.15 The circuit of electricity within the atmosphere. The ground is negatively charged and the equalisation layer is positive, so there is a small but steady flow of negative ions upwards and positive ions down. Flows are in the opposite directions during thunderstorms, i.e. there is a net flux of negative charge in cloud-to-ground lightning strikes, whilst positive ions from the top of storm clouds maintain the positive charge in the equalisation layer.

so that there is an increase of the voltage gradient. Thus air pollution in Samoa resulting from fires on Sundays raises the gradient from the weekday values of 240 to 315 V/m.

9.8 HAIL

Hail is ice, in any of three forms. There may be (i) pellets of frozen rain up to 6 mm in diameter, essentially large sleet, (ii) soft hail, i.e. graupel, consisting of small, slushy, frozen cloud droplets, found in parts of coastal cloud which are just below 0°C, and (iii) true hail, which is larger, opaque and hard. True hail arises from thunderstorms, and we will focus on this.

One mechanism for the formation of hail involves ice crystals being carried to cloud top, as their gravitational fallspeed is less than the speed of the updraught. At the top they fall outside the main updraught, to be re-entrained near cloud base and carried up once more, completing a cycle which is repeated many times. Each time round, the embryonic hailstone is heavier and therefore falls faster (i.e. is carried aloft more slowly), so that it spends more time accreting other crystals. Such a cycle is suggested by the onion-like structure of concentric layers of hard and soft ice in a hailstone's cross-section, probably due to alternations of the wet conditions (inside the cloud) and dry (outside), within which the hailstone has grown. Air spaces between the accretions make the hailstone opaque.

Occurrence

Temperatures tend to be too high for hail at low latitudes, except on high ground. But thirty-two haildays occur annually near sea-level at Invercargill (NZ, 46°S). At the even higher latitude of Campbell Island (53°S), there are about sixty-nine haildays each year. But there are fewer at the highest latitudes because of insufficient atmospheric moisture or heating of the ground for convection to create the tall clouds that produce hail.

The chance of hail is greater in high country; the number of haildays in New South Wales ranges from about 0.7 annually between 50-200 m above sea-level, to about 2.2 between 5001,000 m. This is partly because thunderstorms are more common over hills (compare Figure 9.13 with a contour map), but also because the freezing level over elevated terrain is closer to the ground, so that there is insufficient time for precipitation to melt before reaching the ground. That is particularly important at lower latitudes. For instance, the area of most hail in southern Africa is Lesotho, which is over 2 km above sea-level (Figure 9.16), with more than eight haildays annually at any point. Similarly, there are more than five in central Madagascar at over 1.5 km. However, the ground's elevation is of little importance for very large hailstones, given their high speed of falling: a hailstone of 50 mm diameter drops from 2 km to the ground within a minute, for example.

Figure 9.16 Map of the annual frequency of haildays in South Africa. This matches Figure 9.12, since thunderstorms are prerequisite for hail.

Observations in Kansas indicate that hail is most likely downwind of terrain which rises smoothly for several kilometres, with a light-coloured soil, and downwind of cities. There appear to be fewer hailstorms over forests.

Most hail in Sydney is triggered by convection, and therefore occurs during spring afternoons. In other places, such as Adelaide (Australia) and parts of New Zealand, there is most hail in winter when cold fronts are more frequent (Chapter 13). In these cases, hail is due to showers or shallow thunderstorms in the cold air behind mobile cold fronts, and the frequency varies with the temperature (Note 9.I).

Figure 9.17 Map of the risk from hail in eastern New South Wales, in terms of the approximate number of tonnes of grain lost through hail as a percentage of the number insured. Contours at 200 m, 500 m and 1,000 m indicate the Dividing Range.

Hazards

Hailstones can be dangerous, especially in combination with strong winds. Deaths due to hail are mentioned in the Old Testament (Joshua 10:11). The largest hailstones are the most lethal: a stone the size of a tennis ball weighs about 150 grams and falls at a speed of 40 m/s. The biggest hailstones recorded weighed over a kilogram, killing 92 people in Bangladesh in 1986. Earlier, in 1888, there was a storm in India where 246 people were knocked down by hail and then frozen to death beneath drifts of hailstones. Even in Sydney there have been hailstones as big as 45 mm, enough to damage cars outdoors for instance. Fortunately, such cases are rare and any particular storm usually affects an area only a few kilometres across. The average area of hail in American storms is 20 km2.

Damage to crops depends on the stage the plants have reached. For instance, most hail forms in Iowa in May when maize is still emerging from the ground and so is hardly affected, whilst most harm results from hail in July when the crop is more vulnerable. Hail does millions of dollars worth of harm to fruit crops in New Zealand each year, but much more in North America. A farmer in Alberta may expect to lose his entire crop about three times in the course of his working life.

The frequency of damaging hail can be gauged by the history of insurance claims by farmers. The regional variation of the fraction of grains (oats, barley, and mainly wheat) lost annually to hail damage in New South Wales is shown in Figure 9.17. This reveals more risk at lower latitudes and higher elevation.

Efforts have been made to prevent hail damage. One unsuccessful idea was to create a bang to shatter the hailstones (Note 9J). More recently, rockets or ground-based burners have been used in Russia and elsewhere to inject active nuclei into clouds in order to create many small crystals rather than a few large ones (Section 9.3). As a result, the crystals would melt en route to the ground, yielding beneficial rain instead of harmful hail. In fact, hail was reduced in this way by about 42 per cent in sixteen studies in various countries during the period 1956-85.

So we have considered hail formation and several other aspects of clouds. The most important is the creating of rain, the subject of Chapter 10.

NOTES

9.A Monthly mean cloudiness and rainfall in Australia

9.B The rainfall rate from stratiform cloud

9.C The Bergeron-Findeisen process

9.D Rainfall intensity and raindrop size

9.E The early history of rain-making

9.F The effectiveness of cloud seeding

9.G Electrification within cumulonimbus cloud

9.H The gradient of electrical potential in the lower atmosphere 9.I Temperature and the frequency of hail 9.J Hail cannon

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