Fmamj J Asond

Figure 9.9 The connection between the frequencies of thunder and lightning at Brisbane at various times of the year.

as particularly cold spots because of their elevation.

Finally, networks of radio receivers for detecting lightning can produce detailed maps of lighting flashes over a large area. Each receiver can detect radio 'noise' from a thunderstorm up to about 1,000 km away. This noise, called atmospherics (or 'spherics'), is due to radio waves (Figure 2.1) generated by the lightning.

The global distribution is shown in Figure 9.11. They are most common within 30 degrees of the equator. For instance, there are 225 thunderdays annually at some places on Java. Also, there are regions with 100-180 thunderstorms each year in the high land of northern South America, and many places in central Africa, e.g. 240/a at Kampala on the equator. There are few in arid areas, e.g. less than 10/a in the south-west corner of Africa (Figure 9.12), though these few supply most of what rainfall there is. There are between 520 annually at places in New Zealand (Figure 9.13), mostly among the mountains on the west coast of the South Island.

Mountains increase the chance of thunderstorms by enhancing atmospheric instability (Section 7.3). So most storms in South Africa occur over the mountains of Lesotho in the south-east. On the other hand, thunderstorms are less common over the Andes between 10-30°S than over the adjacent Amazonian low-lands, because the chain of high mountains blocks the flow of low-level moisture from the east.

Thunderstorms are generally less common over the sea, though more than forty occur each year over the Atlantic east of Uruguay. Shipboard records show that thunderstorms at sea occur mostly at 1,000-3,000 km from the tropical and subtropical land masses.

Their frequency in Australia is shown in Figure 9.11. There are relatively few in the south and away from the coast. In Sydney, on the east coast, they occur on about thirty days each year, and the coastal area of New South Wales regularly experiences squall-line thunderstorms

Figure 9.10 The development of a storm on 5 November 1995, indicated by radar reflections from the rain, to a measuring station south-west of Sydney. The diagram shows the respective areas at 1100Z over which rainfall exceeded 2 mm/h (light shading) and 40 mm/h (dark), and also the areas with over 40 mm/h at half-hour intervals beforehand. The storm can be seen to travel to the south-east at a speed of about 60 km/h.

Figure 9.10 The development of a storm on 5 November 1995, indicated by radar reflections from the rain, to a measuring station south-west of Sydney. The diagram shows the respective areas at 1100Z over which rainfall exceeded 2 mm/h (light shading) and 40 mm/h (dark), and also the areas with over 40 mm/h at half-hour intervals beforehand. The storm can be seen to travel to the south-east at a speed of about 60 km/h.

ahead of southerly changes in spring and summer. These thunderstorms tend to form over the coastal mountains of the Dividing Range when the lower air has been made unstable by surface heating inland, and then they move offshore. Most thunderstorms along the Queensland coast are overgrown cumulus clouds carried onshore by the prevailing southeasterly winds over the Pacific, whereas the storms at the northern edge of Australia are mostly air-mass thunderstorms. Thunderstorms in Perth, Adelaide and Melbourne tend to develop ahead of cold fronts.

Variations in Time

Midlatitude thunderstorms mostly occur in the warmer months, e.g. October to January in Australia. Between the tropics, the region of most storms follows the Sun's movements across the

ESQ greater than 30 days 1 0 I—I 10 to 30 days □ less than 10 days

Figure 9.11 Global distribution of the annual number of days with thunderstorms.

ESQ greater than 30 days 1 0 I—I 10 to 30 days □ less than 10 days

Figure 9.11 Global distribution of the annual number of days with thunderstorms.

equator (Figs 9.9 and 9.14), for reasons considered in Chapter 12.

The shorter-lived air-mass storms (the first of the four kinds mentioned earlier) tend to happen during the afternoon. For instance, 34 of 93 storms in Brisbane occurred between 3-6 p.m. The longer-lived squall-line storms may occur at any time of the day or night. Mesoscale convective systems typically form near midnight and dissipate around 10 a.m., both in South America and Australia.

There is a pronounced maximum of mountain and coastal thunderstorms in the afternoon, due respectively to 'anabatic flow' up hillsides and uplift caused by sea breezes (Chapter 14). For example, the sky is normally clear at night on Mt Wilhelm (which reaches 3,480 m in Papua New Guinea) but clouds begin to form at about 2,000 m around 8 a.m., and then they grow until there are intermittent showers from 11 a.m. until sunset.

In contrast, thunderstorms are more common during the late night over the waters of tropical archipelagos like Indonesia, because of uplift initiated by low-level convergence of nocturnal land breezes (Chapter 14) from nearby islands. Even over open tropical oceans there is a slight preference for thunderstorms around midnight, on account of instability induced in the lower atmosphere by nocturnal radiative cooling from the top of the moist marine air.

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