Its highly character- istic shape is often compared with the profile of an anvil of which it displays the upper and lower horizontal planes. The existence of extreme temperature gradients in a cumulo-nim- bus (the temperature can drop to -65 ☌ at the top) generates very rapid ascending air currents, and results in the electrical energisation of the water particles. In a typical storm cloud, the upper part, consisting of ice crystals, is normally positively charged, whilst the lower part, consisting of water droplets, is negatively charged. Conse- quently, the lower part of the cloud causes the development of electrically opposite charges (i.e. positive over the part of the ground nearby). Thus the cumulo-nimbus formation constitutes a sort of huge plate /ground capacitor whose median distance can often reach 1 to 2 km. The atmospheric electrical field on the ground, about 600 V/m in fine weather is reversed and can reach an absolute value of 15 to 20 kV/m when a ground discharge is imminent (the lightning stroke). Before and during the appearance of the lightning stroke, discharges can be seen both within the cloud and between clouds.
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