So far we have considered only the round-topped types of cumulus--those which mark the tops of ascending currents whose ascent has been stopped at a comparatively early stage, or those whose ascent is still in that early stage, though the upward movement has not yet come to an end. The full story of the growth of a cumulus is identical with that of the youth of a cumulo-nimbus, the later stages of which we will consider in another chapter. CHAPTER VII CUMULO-NIMBUS GRANDEST of all clouds are the huge mountains of vapour which are the parents of summer thunder-storms. They are at once distinguished from ordinary cumulus by their upper parts, which sometimes reach beyond the region of the alto clouds high into the realm of cirrus, and extend outwards as a broad disc, which is occasionally indistinguishable from the cirro-nebula and cirro-stratus which form the van of a cyclone cloud canopy. Indeed, there seems to be no essential dividing line between a large cumulo-nimbus and the cloud pile of a small cyclone, and no real difference between them except their size. As a matter of fact, the term cumulo-nimbus would only be given to the cloud when a large fraction of the whole can be seen at once. In dealing with common cumulus, it has been pointed out that the cessation of the uprising convection currents which determines the maximum height to which the cloud will grow is due to the rate of cooling within the ascending column being greater than the rate of cooling outside it. It follows that when the ascending current has reached a certain height it will, as a whole, be just as heavy as an equal column outside. Ascent must then cease. The equilibrium of the air in such a case is said to be stable, and the condition of such stability is simply that the general rate of fall of temperature per 100 metres of ascent is less than the rate of cooling dynamically produced in an ascending current. If, however, the general rate of fall of temperature is greater than that produced dynamically, the consequence will be that the upward tendency of the rising air will increase as it moves upward, and the taller the column becomes the greater will be the difference of weight between the inside and outside columns. In such a case the equilibrium is said to be unstable, and the result will be the production of cumulo-nimbus.