Cloud Electricity

The precipitation from cumulonimbus cloud is often accompanied by lightning and thunder, which will now be considered.

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Figure 9.12 Variation with season of the distribution of thunderdays in Africa.

Lucretius, a Roman poet of the first century BC, thought that lightning consists of sparks from the collision of large clouds. But it is usually explained nowadays as due to charge separation within the updraught of a tall convective cloud. This process consists of the detachment of electrons from some drops and crystals within the cloud, and their attachment to others, creating equal numbers of negative and positive ions out of material that was initially electrically neutral (Figure 1.4 and Note 9.G). The result is that enormous differences of voltage are created between the top and bottom of a cloud, between adjacent clouds, between cloud tops and the stratosphere, and between clouds and the ground. The consequent occasional flashovers are lightning. Similar lightning occurs in volcano clouds, dust storms and snow storms.

Less than 20 per cent of lightning flashes strike the ground: most occur at 3-10 km above the surface, between parts of the same cloud or across to other clouds. This is especially true in low latitudes, where the tropopause is higher (Section 1.8, Chapter 12) and clouds can grow further upwards. Inter-cloud flashes are known as 'sheet lightning', whilst those from cloud to the ground are 'forked lightning'.

Lightning strikes are pulsatory and take place in stages. In the first stage of a cloud-to-ground strike, the negative base of the cloud attracts positive charge to the ground beneath. As a consequence, electrons flow from the cloud base towards the ground, blazing a trail through the air at a speed of 100 m/s or so. The trail consists of steps between changes of direction, hence the strike's name of step-leader or dart-leader. An extremely high temperature is generated by the current, which ionises the air in its path, creating a much better conductor of electricity. As a consequence, there is a clear channel for a

Figure 9.13 Annual frequencies of thunderdays in New Zealand during the period 1955-74.

Figure 9.13 Annual frequencies of thunderdays in New Zealand during the period 1955-74.

bright flash of positive ions up from the ground, once the leader reaches the surface. That main stroke typically peaks at 80,000 amperes, which may be compared with the 0.4 amperes to a 100W light-bulb. The amount of electricity transferred in a flash of 10 microseconds about equals that consumed by the bulb in a day, and generates a momentary temperature above 15,000°C. Next, the from its base, down the same path. There follows a series of alternate strokes downwards and return strokes up.

Lightning results from every thunderstorm containing cold clouds (as defined in Section 9.2). The amount of lightning is proportional to

Figure 9.14 The annual variation of thunderstorm frequency at three places. The maximum in Australia and New Zealand coincides with the month when the noon Sun is highest in the sky, whereas the maximum lags one to two months behind at places closer to the equator.

the speed of the updraughts and downcloud unloads the largely negative charge draughts. Consequently, electrically active storms tend to be severe in terms of hail, downpours and strong wind gusts too.

There are over a hundred occurrences of lightning annually per square kilometre in equatorial Africa. The number of days a year when lightning is visible in Australia decreases southwards, because the atmosphere is less unstable than in the hot and damp conditions of low latitudes. Lightning is seen on about ninety-six days each year at Darwin (12°S), with almost daily displays during the summertime period called 'The Wet' (Chapter 10). There are about seventy-four days of lightning at Brisbane

(at 27°S), forty-four at Sydney (34°S) and eight at Hobart (43°S).

Two-thirds of all lightning flashes occur between the Tropics. There seem to be more lightning strikes above and downwind of a major city than just upwind, presumably because of the plume of warm polluted air.

Hazards

Lightning is dangerous. Hollow trees filled with damp termite nests act as lightning conductors, and explode when struck, as the moisture is instantaneously evaporated by the current. Many bushfires are ignited by lightning; over half those in the Australian state of Victoria are attributed to lightning. Fortunately, some of these fires are put out by the accompanying rain. A different hazard is an electric-power failure, due to lightning striking overhead cables. Radio communication is interrupted by the 'spherics'.

In addition, lightning can stun or kill people. A party of bushwalkers on a track in the Blue Mountains near Sydney were affected by a flash behind them; those at the rear suffered immediate painful leg cramps, whilst those 15 m ahead were unaffected except by the noise. More seriously, there were 1.5 deaths annually per million of the population in South Africa during the period 1950-70, 0.6 in the USA (more than were killed by tornadoes or other weather events), 0.4 in Australia (80 per cent of them men, because more work outdoors) and 0.2 in Britain. The rate for Queensland is ten times what it is for South Australia, whose population is overall less rural. Globally, 5,000 people are estimated killed every year by lightning, most in developing countries. About equal numbers were killed in the USA, (a) on foot in the open, (b) sheltering under trees, or (c) riding on horseback or open vehicle. Fortunately, lightning fatalities are becoming fewer, e.g. 21 per million in Australia in 1825, 4 in 1880, 2 at the turn of the century, half that in 1950, and only about 0.3 since 1970.

It is unwise to be in an open high area, by a wire fence, using electrical equipment like a telephone or electric razor, to be swimming, in a small boat or on horseback. One should avoid isolated trees or the edge of a forest, but you are well protected whilst indoors or in a car. There is no safety in places struck already; lightning can strike twice.

Low-flying aircraft are often struck, but harmlessly. Tall buildings are also hit, roughly in proportion to the square of the height, i.e. four times as often if twice as high. Protection is given by a projecting conductor rod leading to the ground, shielding an area with a radius equal to the rod's height.

A beneficial consequence of lightning is the chemical combination of some of the air's oxygen with nitrogen, called 'fixation'. The compounds formed eventually reach the soil and improve its fertility. But lightning also produces ozone at low levels, which is harmful to people.

Thunder

Lightning is often accompanied by thunder (Figure 9.9), caused by shaking of the air when there is first an explosion of air along the lightning trail, due to immediate heating by the electric current, and then, secondly, an equally rapid implosion back into the track after the extremely rapid cooling due to the radiation of light energy from the stroke. High frequencies in the sound of thunder are quickly attentuated, so one hears only a rumble from any distant storm.

The million-times difference between the speeds of light (about 3X10 m/s) and sound (about 330 m/s) means that lightning at a distance of D kilometres is heard 3D seconds after it has been seen. (As a result, you can find D by counting the number of seconds between flash and thunder, and dividing by three.) But lightning is often hidden by cloud, or the thunder made inaudible by distance (Note 7.M).

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