Before developing our own picture of a thunderstorm let us recognize that science has found it necessary to reverse the explanation so long in Vogue. Whereas it was formerly taken for granted - and the assumption was supposed to rest upon experimental proof - that the condensing of atmospheric vapour which accompanied lightning was the consequence of a release of electrical tension by the lightning, the view now held is that the electrical tension responsible for the occurrence of lightning is itself the effect of a sudden condensing process of atmospheric moisture.