Now a thunderstorm is accompanied by strong upward movements of the air; so strong that small drops cannot fall to the ground, while large drops, which would be heavy enough to fall through such rising currents if they could retain their integrity, are broken up by the blast of air and carried aloft, where they tend to accumulate, recombine, and fall again. This process may be repeated many times, so that the positive charge of the drops is continually increasing, and at the same time negative ions are being set free and carried by the ascending air to the upper part of the clouds. Here they unite with the cloud particles and give them a strong negative charge. Thus eventually there is formed a heavily charged positive layer of cloud between a heavily charged negative layer above and the negatively charged earth beneath. When the differences of potential thus brought about become great enough, disruptive discharges of electricity will occur, and these may be either between the upper and lower layers of cloud or between the clouds and the earth, or, sometimes, between two different clouds. Probably much the most frequent lightning flashes are those that occur within a single thundercloud and do not reach the earth. However, it will often happen that the negatively charged upper layer of cloud is either carried very high or drifted away by the wind, and then the discharges that occur will be chiefly between cloud and earth. Such conditions are likely to prevail in the case of cyclonic thunderstorms, in which there is often great difference in the direction and force of the winds at different levels. On the other hand, heat thunderstorms usually occur when the general winds are light at all levels, and it is probable that such storms are relatively free from cloud-to-earth discharges. We seem to have here an explanation of the paradox that tropical thunderstorms, which are nearly always of the noncyclonic type, though notoriously violent, are generally harmless.