Lake of Thun

Now, 28 coulombs are rather less than the average quantity of electricity in a lightning discharge. Further, it is easily proved that the force due to the 28 coulombs in a sphere with diameter 500 metres is, at the surface of the sphere, equivalent to 4X volts per metre, and therefore sufficient to break down the rc sistance of the air and initiate a lightning flash. It is supposed that the lightning does not discharge the whole cloud.

The theory accounts for the flashes which convey positive electricity downwards from the lower part of the cloud towards the earth or upwards inside the cloud.

The lightning flashes by which positive electricity is conveyed upwards to the cloud, are thought by Simpson to be rather rare.

Criticisms of Simpson's Theory.

The theory is based on the observed facts that water drops which are uncharged or slightly charged, acquire positive charges when broken up, and that charged drops, when they collide, will combine. There is, however, no experimental evidence that the processes of breakage and combination can be so active simultaneously that all the water passes through the two processes each second. Such a rate seems to be necessary to account for the frequency of lightning flashes. Moreover, it is difficult to believe that the persistence for a quarter-of-an-hour of a vertical current localized in one small part of the cloud is an essential feature of a thunderstorm.

The theory has been attacked more generally on the ground that the natural interpretation of observations of the changes in the electric field in a storm is that the upper part of a thunder cloud is usually the seat of a positive charge. Simpson's reply to this criticism is that the observations in question can be ex plained on the assumption that the most frequent discharges are those in which positive electricity is displaced from the region of separation inside the cloud to the air between the cloud and the ground. To the criticism that observations such as those of Wormell and Schonland imply a great transfer of positive elec tricity from the ground upwards during a thunderstorm, Simp son's reply is that the observations are not representative, being taken under such circumstances that positive fields were excep tionally infrequent. More observations are needed, especially in parts of the world where tropical storms occur.

The literature of thunderstorms is extensive. The following list is almost confined to works quoted in the foregoing article.