Severe storms, especially hurricanes and tornadoes, are said to make up around 20 per cent of the huge annual cost of the damage caused by natural disasters.
The most common storms, however, are thunderstorms, about 45,000 of which occur every day.
Thunderstorms, which are associated with cumulonimbus clouds formed in fast-rising air, are commonly accompanied by lightning, caused by the sudden release of accumulated static electricity in the clouds.
The mechanisms by which static electricity forms in clouds is an electrical charge that is produced as a result of the freezing of supercooled droplets in clouds. The outer layers of these droplets freeze first and, in so doing, become positively charged (a phenomenon that has been observed in laboratory conditions); the warmer, still unfrozen cores acquire a negative charge. A fraction of a second later the cores freeze and expand, thus shattering the outer layers. Positively-charged fragments of the outer layers are then swept upwards to the top of the cloud while the still intact, negatively-charged cores remain in the cloud's lower levels.
Eventually the total amount of charge in the cloud builds up sufficiently to overcome the electrical resistance of the air between the cloud and the ground, and the charge in the cloud is discharged as a huge electric spark - a flash of lightning.
The violent expansion of the air molecules along the path of the lightning generates an intense supersonic sound wave, which is heard as thunder. Lightning is seen before thunder is heard because light travels faster than does sound.