(b) Storms Produced in Air TV hich Has Travelled from a Colder Region to aWarmer.—When air flows over warm ground or warm water, heat may be communicated to the air sufficiently rapidly for the stratification to become unstable. Under these conditions, thunder showers develop. They are usually of small extent.
(c) Storms Produced by the Interaction of Converging Air Cur rents.—When a current of cold air meets a current of warm air, the warmer air, being lighter, is forced upwards. As the warm air reaches the condensation level, cloud is formed, and if conditions are favourable the clouds tower upwards so that the passage of the "cold front" is marked by a brief thunderstorm.
(d) Storms Produced by the Interaction of Parallel Currents.— If the general distribution of pressure is such that a current of polar air flows alongside a current of warmer air, a travelling area of instability is produced. Thunderstorms of great intensity de velop and may extend for a hundred miles or more in the direction of the currents.
The storms of class (b) are characteristic of the rear of baro metric depressions, and occur over the sea, in winter, when air from high latitudes is blowing over comparatively warm water. On the western coasts of the British Isles such storms are not infrequent. The storms which occur at night on certain coasts in the Tropics, when the off-shore breezes set in, may also be included in class (b).
the other hand, in the desert countries thunderstorms are almost unknown. They are rare in high latitudes. For numerous land sta tions in the Northern Hemisphere, Brooks has computed the annual average of 25;• for the land of the Southern Hemisphere (excluding Antarctica), the average is 44. His estimate for the whole globe, land and sea, is i6. As in all such statistics, the num ber is intended to refer to the number of days on which thunder is audible at any time.
In many parts of the world thunderstorms are not most fre quent in summer because that is a dry season. This is the case in Mediterranean countries. At Barcelona and at Rome, thunder storms are most common in September. Similar influences are felt in Switzerland, where thunderstorms are less frequent at the end of June than at its beginning, or in the last part of July.
To the eye, a lightning flash is instantaneous, and it cannot be seen whether it begins at one end or the other. It is clear, however, that the resistance of the air to the electric stress must break down, in the first instance, at one definite point. The air at this point is ionized and becomes a conductor, and the greatest electric stress is transferred to a neighbouring point. The dis charge is therefore progressive. From theoretical considerations and from laboratory experiments, G. C. Simpson has proved that such a progressive discharge always proceeds from a positively charged body towards a negatively charged body. The spark is frequently branched, and it is the rule for the branches to lead away from the positively charged body. When a branched light ning flash is seen, the direction in which electricity is passing can, therefore, be stated.
The absence of branches indicates that the flash has passed from a positive charge spread over a wide area to a more con centrated negative charge. Simpson classified numerous photo graphs of lightning, amongst which were 418 flashes which ap peared to be approximately vertical. In 242 of these flashes there were branches pointing downwards ; in only three were branches pointing upwards. In the former cases the clouds from which the lightning proceeded must have been positively charged, in the latter cases the cloud must have been negatively charged. Even if negatively charged clouds preponderated in the cases in which the flashes were unbranched, the positively charged clouds were in the majority.